Strain partitioning and interplate coupling along the
-
Upload
others
-
View
3
-
Download
0
Embed Size (px)
Citation preview
535Nishimura et al. | Strain partitioning and interplate
coupling along the northern margin of the Philippine Sea
plateGEOSPHERE | Volume 14 | Number 2
Strain partitioning and interplate coupling along the northern
margin of the Philippine Sea plate, estimated from Global
Navigation Satellite System and Global Positioning SystemAcoustic
data Takuya Nishimura1,*, Yusuke Yokota2,*, Keiichi Tadokoro3,*,
and Tadafumi Ochi4,* 1Disaster Prevention Research Institute, Kyoto
University, Gokasho, Uji, Kyoto 611-0011, Japan 2Hydrographic and
Oceanographic Department, Japan Coast Guard, 3-1-1, Kasumigaseki,
Chiyoda-ku, Tokyo 100-8932, Japan 3Graduate School of Environmental
Studies, Nagoya University, D2-1(510), Furo-cho, Chikusa-ku, Nagoya
464-8601, Japan 4Geological Survey of Japan, National Institute of
Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba,
Ibaraki 305-8560, Japan
ABSTRACT
Southwest Japan is located in the subduction margin between the
continental Amurian and oceanic Philippine Sea plates. Recent land
GNSS (Global Navigation Satellite System) and offshore Global
Positioning System- Acoustic geodetic measurements were used to
clarify the defor- mation in and around these plate margins. We
examined strain partitioning and interplate coupling using a block
modeling approach on the observed velocities. Although the main
plate boundaries are the Nankai Trough and Sagami trough, our
results suggest that one-third of the relative plate mo- tion
between the two plates is accommodated by several block boundaries
in the southeastern margin of the Amurian plate. The most active
bound- aries, with a slip rate of ≥8 mm/yr, cross southwest Japan
from the Okinawa Trough through the Median Tectonic Line and
Niigata Kobe tectonic zone, to the eastern margin of the Japan Sea.
A subparallel boundary with a slip rate of 4–5 mm/yr is along the
coastline of Japan. These two boundaries have a right-lateral shear
motion that accommodates part of the interplate motion, with a
boundary across the Korean Peninsula and Japan Sea. The slip parti-
tioning results in an eastward decrease of relative block motion
from 78 to 4 mm/yr along the Nankai Trough and Suruga trough.
Interplate coupling is moderate to strong at 10–25 km depth
along the Nankai Trough, but it is lower at ~132°E, ~136°E,
and ~137°E than in the surrounding regions, corre- sponding to the
segment boundaries of past megathrust earthquakes, sug- gesting
that regions of insufficient strain accumulation act as a barrier
for earthquake rupture.
INTRODUCTION
Southwest Japan is situated in a subduction zone, where the
northern margin of the Philippine Sea plate subducts beneath the
southeastern mar- gin of the continental Amurian plate. Large M ≥ 8
megathrust earthquakes have ruptured shallow parts of the plate
interface with a recurrence interval of 100–200 yr along the
Nankai Trough, the main boundary between these plates (Ando, 1975;
Ishibashi, 2004). Many slow earthquakes, including slow slip events
(SSEs), low-frequency tremors, and very low frequency earthquakes
(VLFEs), also occur around the rupture area of the megathrust
earthquakes on the plate interface (cf. Obara, 2010). However,
previous studies suggest that the relative plate motion between the
Amurian and Philippine Sea plates is not accommodated by a single
boundary. Numerous active faults and high seis- micity, including M
~7 earthquakes on the islands of Japan, indicate strain par-
titioning in the continental margin (e.g., Shen-Tu et al.,
1995). The most famous feature of the strain partitioning is the
fault system of the Median Tectonic Line (MTL), which accommodates
part of the margin-parallel component of the oblique relative
motion between the Amurian and Philippine Sea plates (i.e., Fitch,
1972). Its right-lateral strike-slip motion was proposed based on
geologi- cal studies (Kanaori, 1990; Tsutsumi et al., 1991),
and has been confirmed by geodetic observations (Miyazaki and Heki,
2001; Tabei et al., 2003). Another dextral strike-slip fault
system subparallel to the MTL has been proposed 400 km from
the Nankai Trough (Gutscher and Lallemand, 1999; Gutscher, 2001).
Contemporary deformation in southwest Japan has been monitored by
dense, continuous GNSS (Global Navigation Satellite System)
networks since the mid-1990s. The largest network is the GEONET
(GNSS Earth Observation Network System), operated by the Geospatial
Information Authority of Japan (Sagiya, 2004). The GEONET showed
that high strain rates are found not only
GEOSPHERE
doi:10.1130/GES01529.1
11 figures; 3 tables; 1 supplemental file
CORRESPONDENCE: nishimura .takuya .4s@ kyoto -u .ac.jp
CITATION: Nishimura, T., Yokota, Y., Tadokoro, K., and Ochi, T.,
2018, Strain partitioning and interplate coupling along the
northern margin of the Philippine Sea plate, estimated from Global
Navigation Satellite System and Global Positioning System-Acoustic
data: Geosphere, v. 14, no. 2, p. 535–551, doi:
10 .1130 /GES01529.1.
Science Editor: Shanaka de Silva Guest Associate Editor: Laura M.
Wallace
Received 15 March 2017 Revision received 16 November 2017 Accepted
19 January 2018 Published online 16 February 2018
OPEN ACCESS
GOLD
This paper is published under the terms of the CC-BY-NC
license.
© 2018 The Authors
*E-mail: nishimura .takuya .4s@ kyoto -u .ac.jp (Nishimura), eisei@
jodc .go .jp (Yokota), tad@ seis .nagoya -u .ac .jp (Tadokoro),
tadafumi .ochi@ aist .go.jp (Ochi)
THEMED ISSUE: Subduction Top to Bottom 2
536Nishimura et al. | Strain partitioning and interplate
coupling along the northern margin of the Philippine Sea
plateGEOSPHERE | Volume 14 | Number 2
in areas close to the main subduction margin along the Nankai
Trough and the Sagami trough, but also the middle of the island arc
and backarc. One such area is the Niigata-Kobe tectonic zone (NKTZ,
Fig. 1) (Sagiya et al., 2000). In re- cent years, the geodetic
network has been expanded offshore, and more than 20 GPS-A (Global
Positioning System-Acoustic) seafloor stations have been deployed
along the northern margin of the Philippine Sea plate (e.g., Yokota
et al., 2015; Tadokoro et al., 2012; Nishimura
et al., 2014).
Using these dense geodetic data, many previous studies have focused
on interplate coupling along the Nankai Trough (i.e., Liu
et al., 2010; Yoshioka and Matsuoka, 2013). Yokota et al.
(2016) first used GPS-A data to estimate inter- plate coupling;
their result suggests heterogeneous interplate coupling along the
Nankai Trough, which cannot be resolved only by land GNSS data.
How- ever, most previous studies have attributed deformation in the
overriding plate only to interplate coupling on the megathrust
interface, and their estimated coupling may be biased because they
ignore the strain partitioning, as pointed
out by Nishimura and Hashimoto (2006). In order to overcome this
problem, several studies have modeled GNSS velocities in southwest
Japan using the block model approach, incorporating strain
partitioning in the plates as well as elastic deformation.
Hashimoto et al. (2000) first used the block model for GNSS
velocity data in the islands of Japan; they did not estimate a
detailed distribution on the plate interface because their model
approximated the plate interface as several rectangular faults.
Nishimura and Hashimoto (2006) and Wallace et al. (2009)
divided southwest Japan into three and five blocks, re- spectively,
but they both only modeled the region west of 135°E. Loveless and
Meade (2010) modeled all the islands of Japan, and their result is
often referred to as the standard result of block motion and
interplate coupling. However, none of these studies used offshore
GPS-A data or GNSS data from after 2002.
We also model crustal velocities in southwest and central Japan
using the block model approach. Our study is distinct because we
use all available geo- detic data, including GPS-A data. We use
data from GEONET stations con-
126° 127° 128° 129° 130° 131° 132° 133° 134° 135° 136° 137° 138°
139° 140° 141° 30°
31°
32°
33°
34°
35°
36°
37°
HonshuKorean Peninsula
Izu Islands
Kyushu-Palau Ridge
Japan Sea
Okinawa Trough
Figure 1. Interseismic secular velocity at land GNSS (Global
Navigation Satellite System) and offshore GPS-A (Global Posi-
tioning System-Acoustic) stations in and around southwest Japan.
Vectors repre- sent velocities relative to stable Amurian plate
with 95% confidence limit ellipsoids. The shaded purple region and
pink line are the Niigata-Kobe tectonic zone (NKTZ) (Sagiya
et al., 2000) and the Median Tec- tonic Line (MTL),
respectively.
537Nishimura et al. | Strain partitioning and interplate
coupling along the northern margin of the Philippine Sea
plateGEOSPHERE | Volume 14 | Number 2
structed after 2002, as well as GNSS stations operated by several
institutions, so that the average density of GNSS stations used is
almost double that of previous studies. Using GNSS stations in the
Korean Peninsula that belong to the stable Amurian plate allows us
to clarify strain partitioning in the continen- tal margin. We
obtain a high-resolution estimate of the coupling distribution
along the entire Nankai Trough and Sagami trough owing to the use
of off- shore geodetic data. We then discuss the estimated
interplate coupling and block rotation from the viewpoint of
seismotectonics in Japan.
DATA AND METHOD
GNSS Velocity Data
We estimated site velocities at permanent GNSS stations in Japan
and South Korea for a stable interseismic period, free of
earthquakes and transient events during 2005–2009. Daily
coordinates at GNSS stations in Japan are esti- mated from raw
RINEX (Receiver Independent Exchange Format) data pro- vided by the
Geospatial Information Authority of Japan, Japan Coast Guard,
National Institute of Advanced Industrial Science and Technology,
Kyoto Uni- versity, Nagoya University, and International GNSS
Service. The coordinates were estimated by precise point
positioning with ambiguity resolution ( Bertiger et al., 2010)
using the GIPSY-OASIS ver. 6.2 software (https:// gipsy -oasis .jpl
.nasa .gov/), and were transformed into the IGb08 reference frame
(http:// acc .igs .org /igs -frames .html) using the transformation
parameters provided by the National Aeronautics and Space
Administration Jet Propulsion Laboratory. Daily coordinates for
stations on the Korean Peninsula were downloaded from the website
of the Nevada Geodetic Laboratory (http:// geodesy .unr .edu/). The
strategy and procedure for estimating daily coordinates at the
Nevada Geo- detic Laboratory is almost the same as ours. Therefore,
we regarded all daily coordinates used in this study as being in
the ITRF2008 (International Terres- trial Reference System)
reference frame (Altamimi et al., 2011). We estimated site
velocities at all the GNSS stations by fitting a function with
linear, annual, and semiannual terms for the period from April 2005
to December 2009; we chose this period because it spans only a few
tectonic events, including the 2006 earthquake swarm off the
eastern coast of the Izu Peninsula, and the 2007 moment magnitude,
Mw 6.6 Noto Peninsula (Ozawa et al., 2008) and Mw 6.6
Niigataken Chuetsu-oki (Nishimura et al., 2010) earthquakes.
We removed their coseismic displacements by adding a step term in
the function for stations in the affected area. The velocities
shown in Figure 1 were transformed to the Eurasian plate reference
frame using the Euler vectors of the ITRF2008 plate motion model
(Altamimi et al., 2012).
The primary feature of the velocity vectors (Fig. 1) is the
west-northwest- ward movement along the Nankai Trough. The velocity
in most regions is al- most parallel to the motion of the
subducting Philippine Sea plate, which sug- gests strong interplate
coupling at the Nankai subduction zone (Liu et al., 2010;
Yoshioka and Matsuoka, 2013). However, the velocity direction in
some areas deviates systematically from that of the Philippine Sea
plate. For example, the
southern part of Kyushu and the northernmost part of the Izu
Islands move southward and westward, respectively.
Although the GNSS velocity vectors (Fig. 1) are intuitively easy to
under- stand in terms of the tectonic deformation, their direction
and magnitude de- pend on the reference frame. In contrast, the
strain rate distribution is inde- pendent of the reference frame,
and useful for identifying where high strain rates are concentrated
in the subduction margin. We calculated the distribu- tion of
strain rates in southwest Japan using the method of Shen
et al. (1996) and Sagiya et al. (2000). The distribution
of areal strain rates and maximum shear strain rates is plotted in
Figures 2A and 2B, respectively; both plots show high strain rates
along the Pacific coast, particularly in Shikoku. The direction of
contraction there is consistent with that of the Philippine Sea
plate, and consistent with the interplate coupling along the Nankai
Trough and Sagami trough shown by previous studies (e.g., Nishimura
et al., 2007; Liu et al., 2010; Yoshioka and Matsuoka,
2013). However, the principal strain in central Kyushu and central
Honshu around (136°E, 35°N) shows east-west contraction, which
deviates from the direction of plate convergence. We also note that
the strain rate distribution in the inland regions is very
heterogeneous. Several zones of strain concentration, including the
NKTZ (Sagiya et al., 2000), are clearly recognized in both the
areal (region a in Fig. 2A) and maximum shear (region a in Fig. 2B)
strain rates. Although most zones of high strain rates correspond
to surface traces of major active faults, we note zones of high
shear strain rates in southern Kyushu (region b in Fig. 2B) and
along the northern coast of western Honshu (region c in Fig. 2B),
where there are no major active faults. These zones have been
identified geodetically by previous GNSS studies in Kyushu (e.g.,
Takayama and Yoshida, 2007; Wallace et al., 2009) and western
Honshu (Nishimura and Takada, 2017). Some zones of strain
concentration are far from the main plate boundary (the Nankai
Trough and Sagami trough) and cannot be explained by simple elastic
deformation due to interplate coupling on the plate boundary,
because elastic strain decreases with cubic distance from the
source in a homogeneous medium. Therefore, using a block model, we
sug- gest that the inland strain concentration zones provide
evidence of strain par- titioning in the overriding continental
plate. Figure 2A shows inflation around some volcanoes, including
Mount Fuji and Sakurajima, Oshima, and Miyake- jima. In the block
model, we correct these inflations using the velocity data and
point inflation sources, which we explain in detail herein. (For a
detailed discussion on strain rates and inland earthquakes, see
Nishimura, 2017.)
GPS-A Velocity Data
We used velocity data from 23 offshore GPS-A stations operated by
the Japan Coast Guard and Nagoya University along the northern
margin of the Philippine Sea plate (e.g., Nishimura
et al., 2014). The start of measurement at these stations
varies from 2004 to 2012. Campaign GPS-A measurements are usually
conducted a few times per year. Although the 2011 Mw 9.0 Tohoku-
oki earthquake caused significant coseismic and postseismic
deformation at the offshore stations, we need a long period to
estimate the stable velocity
538Nishimura et al. | Strain partitioning and interplate
coupling along the northern margin of the Philippine Sea
plateGEOSPHERE | Volume 14 | Number 2
130° 132° 134° 136° 138° 140° 30°
32°
34°
36°
38°
200 nanostrain/yr Cont. Ext.
−300 −200 −150 −100 −50 0 50 100 150 200 300 Areal strain rate
(nanostrain/yr)
130° 132° 134° 136° 138° 140° 30°
32°
34°
36°
38°
100 km
0 10 50 100 200 500 Maximum shear strain rate (nanostrain/yr)
A
B
b
c
a
a
Figure 2. Strain rate distribution estimated from GNSS (Global
Navigation Satellite System) velocities from April 2005 to December
2009. Brown lines and red tri- angles represent surface traces of
major active faults (Headquarters for Earthquake Research
Promotion, 2011) and active volcanoes (Japan Meteorological Agency,
2011). (A) Areal strain rates; arrows repre- sent principal strain
rates. Ext—extension; Cont—contraction. (B) Maximum shear strain
rates.
539Nishimura et al. | Strain partitioning and interplate
coupling along the northern margin of the Philippine Sea
plateGEOSPHERE | Volume 14 | Number 2
because of the temporal sparseness and precision of GPS-A
measurements. The velocity from 2004 to 2016 was estimated at 19
stations along the Nankai Trough after removing the coseismic and
postseismic displacement using the model. These velocities were
updated using those published in Yokota et al. (2016) and
Tadokoro et al. (2012) to include coordinates observed by
recent campaigns conducted to 2016. The procedure for correcting
for the displace- ment caused by the Tohoku-oki earthquake and
estimating the secular veloc- ities were explained in detail in
Yokota et al. (2016) and Graduate School of Environmental
studies, Nagoya University (2016). We also used the published
velocity for two stations in the easternmost part of the Nankai
Trough ( Yasuda et al., 2014) and at two stations along the
Sagami trough (Watanabe et al., 2015 before the Tohoku- oki
earthquake. After transforming the secular veloci- ties with the
MORVEL (mid-ocean ridge velocity) Euler vectors (DeMets
et al., 2010), which were used to realize the reference frame
in Yokota et al. (2016), offshore GPS-A and on-land GNSS
velocities are plotted in Figure 1 in a com- mon reference frame
fixed to the Eurasian plate.
Block Modeling
In the block model, that surface deformation is expressed as a sum
of rigid block rotations and interseismic elastic deformation due
to locked faults on block boundaries (Matsu’ura et al.,
1986). We used the inversion program DEFNODE (McCaffrey, 2002) to
model the observed velocities with this ap- proach. Elastic
deformation is calculated as nominal slip opposite to the rela-
tive block motion, that is, back slip (Savage, 1983) or more
generally, slip deficit on the fault in an elastic half-space
(Okada, 1985). The slip-deficit rates are esti- mated at nodes on
the boundary faults under the constraint of a rate ranging between
zero and that of the relative block motion, but we did not apply a
spa- tial constraint to the slip-deficit distribution. Slip-deficit
rates between nodes are linearly interpolated on the faults. We
also estimated internal permanent strain in most continental
blocks.
We used horizontal and vertical velocities at 840 GNSS stations and
hori- zontal velocities at 23 GPS-A stations in the inversion
analysis (Supplemental File 11). The data weight in the inversion
is the reciprocal square of the product of the velocity
uncertainties and data type constants. Data type constants are
introduced to weight the vertical and offshore velocities, although
their uncer- tainties are larger than those of the horizontal GNSS
velocities. The vertical deformation is important for
distinguishing between rigid rotation and elastic deformation due
to fault locking because only elastic deformation causes ver- tical
deformation in the block model we used. Offshore GPS-A data are
also important for resolving offshore interplate coupling near the
Nankai Trough and Sagami trough. Data near the region of interplate
coupling have the most resolving power for coupling distribution,
which is our main objective, and are less sensitive to various
model assumptions including block geometries. We therefore used 1,
1/5, and 1/10 as data type constants for horizontal GNSS, vertical
GNSS, and GPS-A velocities, respectively. The velocities were cor-
rected for volcanic deformation around the Izu Oshima, Miyakejima,
Fuji, and
Sakurajima volcanoes by removing the synthetic velocities due to
point infla- tion (Mogi) sources beneath the volcanoes. The
parameters for the inflation sources (Table 1) were estimated from
the velocities at stations within 50 km of the volcanoes by
fixing the source locations estimated in previous studies
(Nishimura, 2011; Hotta et al., 2016).
Block Geometry
We constructed the block geometry mainly based on surface traces of
major active faults (Headquarters for Earthquake Research
Promotion, 2017), shallow seismicity, the strain rate distribution
from the geodetic data, previ- ous block models (Wallace
et al., 2009; Loveless and Meade, 2010; Nishimura, 2011), and
previous regional seismotectonic studies (Matsumoto et al.,
2015), as shown in Figure 3. We adjusted the block geometry to
reduce the misfit of velocities near the block boundaries by trial
and error.
Most block boundaries are derived from a simplified geometry of
major active faults (Fig. 3A). Boundaries between the Setouchi (SE)
and Shikoku (SH) blocks, and between the Chubu (CH) and Kanto (KA)
blocks, correspond to the fault systems of the MTL and the
Itoigawa-Shizuoka tectonic line, regarded as the most active
onshore fault system in Japan. The boundary around the Osaka (OS)
block corresponds to the Rokko-Awaji, Arima-Takatsuki, Ikoma, and
MTL fault systems. The boundary around the Biwako (BI) block
corresponds to the Biwako-Seigan, Hanaore, Nobi, Kizugawa, and Ise
Bay fault systems. Most part of the boundary between the Hokuriku
(HO) and Chubu (CH) blocks is identified by the Atotsugawa fault
system. The northern and southeastern boundaries of the Central
Kyushu (CK) block correspond to the northern rim of the Saga Plain–
Mizunawa–Beppu Haneyama and Futagawa-Hinagu fault systems,
respectively. Part of the latter fault system was ruptured by the
16 April 2016 Mw7.0 Kuma- moto earthquake (Figs. 3A, 3B; Fukahata
and Hashimoto, 2016). The boundary between the Amurian (AM) plate
and Japan Sea block (JS) approximates the Honam shear zone (Jin and
Park, 2007) in the Korean Peninsula.
Shallow seismicity in the crust (Fig. 3B) also gives clues as to
the block structures. Seismicity lineaments often, but do not
always, correspond to ac- tive faults. A distinctive lineament
without major active faults exists along the northern coast of
western Honshu between 131°E and 135°E, and was identi- fied as a
seismic belt by Kawanishi et al. (2009). The eastern part of
the seismic belt and the proposed San-in shear zone (Nishimura and
Takada, 2017) overlap the zone of concentrated maximum shear strain
rates (Fig. 2B). We therefore
Longitude(º) Latude(º) EW Vel. (mm/yr) EW Error (mm/yr) NS Vel.
(mm/yr) NS Error (mm/yr) UD Vel. (mm/yr) NS Error (mm/yr) Staon ID
Staon Type Crustal Block 136.3889 36.3944 -5.40 0.18 2.40 0.28
-0.72 1.30 0255 GNSS HO 136.6053 36.3700 -6.34 0.18 2.19 0.29 -0.24
1.32 0578 GNSS HO 136.1729 36.2311 -5.42 0.20 1.38 0.33 -2.46 1.35
0055 GNSS HO 136.3616 36.2295 -6.79 0.22 3.75 0.29 -1.79 1.41 0974
GNSS HO 136.6339 36.1652 -7.96 0.22 2.16 0.34 -1.01 1.41 0256 GNSS
HO 136.2789 36.1456 -6.95 0.22 2.31 0.35 -2.17 1.38 0257 GNSS HO
136.0489 36.1097 -6.66 0.26 0.95 0.37 -2.29 1.52 0975 GNSS HO
135.9897 35.9450 -6.65 0.21 1.42 0.35 -3.96 1.34 0259 GNSS HO
136.1849 35.9688 -8.88 0.22 1.62 0.34 -0.75 1.43 0579 GNSS HO
136.5049 35.9849 -10.64 0.23 1.97 0.39 -2.77 1.47 0258 GNSS HO
136.3395 35.8882 -9.95 0.23 2.18 0.39 -1.73 1.48 0977 GNSS HO
136.1974 35.7927 -9.98 0.23 2.03 0.36 -1.75 1.40 0260 GNSS HO
136.0559 35.8368 -8.99 0.21 1.48 0.34 -2.97 1.43 0580 GNSS HO
135.1732 35.7523 -5.20 0.23 2.18 0.33 -2.54 1.36 0327 GNSS JS
135.0340 35.6850 -5.13 0.20 2.57 0.31 -2.15 1.37 0640 GNSS JS
134.6773 35.6212 -4.76 0.25 3.08 0.33 -1.53 1.45 0645 GNSS JS
134.1952 35.5298 -2.00 0.21 2.53 0.31 -7.27 1.41 1014 GNSS JS
134.0466 35.4567 -4.63 0.32 1.28 0.40 2.09 1.54 0378 GNSS JS
133.8697 35.4932 -2.51 0.19 1.93 0.31 -1.72 1.38 1016 GNSS JS
133.6995 35.4904 -2.77 0.27 2.76 0.38 -2.07 1.68 1135 GNSS JS
133.4951 35.5069 -2.22 0.19 2.03 0.30 -1.59 1.33 1015 GNSS JS
133.3396 35.4360 -1.76 0.19 2.81 0.30 -1.20 1.35 0654 GNSS JS
133.1920 35.3106 -2.86 0.24 2.12 0.35 0.23 1.51 1019 GNSS SE
132.9026 35.3097 -0.40 0.22 3.10 0.32 -4.22 1.47 1020 GNSS JS
133.1378 35.5635 -3.50 0.28 4.75 0.36 -2.83 1.43 0656 GNSS JS
133.0586 35.4338 -1.97 0.19 2.32 0.31 -1.30 1.37 0074 GNSS JS
132.9219 35.4919 -1.62 0.22 2.43 0.33 -1.98 1.47 1018 GNSS JS
132.7432 35.3926 -1.59 0.21 2.84 0.31 -4.08 1.40 0384 GNSS JS
132.9026 35.3097 -0.40 0.22 3.10 0.32 -4.22 1.47 1020 GNSS JS
1Supplemental File. Velocity data at GNSS and GPS-A stations used
in this study. Please visit http:// doi .org /10 .1130 /GES01529
.S1 or the full-text article on www .gsapubs .org to view the
Supplemental File.
TABLE 1. PARAMETERS OF VOLCANIC INFLATION SOURCES
Latitude (°N)
Longitude (°E)
Depth (km)
Volcano name
35.356 138.733 15.0 5.16 Fuji 34.736 139.399 6.0 2.47 Oshima 34.067
139.503 9.5 5.99 Miyakejima 31.649 138.689 8.6 4.46
Sakurajima
540Nishimura et al. | Strain partitioning and interplate
coupling along the northern margin of the Philippine Sea
plateGEOSPHERE | Volume 14 | Number 2
assumed a block boundary between the Japan Sea (JS) and Setouchi
(SE) blocks on the eastern part of the seismic belt. The assumed
block boundary goes offshore west of 132.5°E, while the seismic
belt continues inland along the coast in order to reduce residuals
after trial and error. The boundary be- tween the Shikoku (SH) and
southern Kyushu (SK) blocks was proposed as an active shear zone by
previous GNSS studies (e.g., Wallace et al., 2009) and
is
assumed to be in an active seismic zone where Mw 6.1 and 6.0
earthquakes occurred in 1997 (e.g., Toda and Stein, 2003).
The locations of the block boundaries offshore are mostly
speculative, be- cause there is no clear supporting evidence, and
they cannot be constrained by our geodetic data because of the
sparse distribution of stations. However, the offshore block
boundaries do not significantly affect our inversion results
for
127° 128° 129° 130° 131° 132° 133° 134° 135° 136° 137° 138° 139°
140° 141° 142°
30°
31°
32°
33°
34°
35°
36°
37°
38°
OS
SH
127° 128° 129° 130° 131° 132° 133° 134° 135° 136° 137° 138° 139°
140° 141° 142°
30°
31°
32°
33°
34°
35°
36°
37°
38°
2000 Mw6.7
Figure 3. Block geometry used in this study. Abbreviations of the
tectonic blocks: IF—Izu forearc block, IM—Izu microplate,
PS—Philippine Sea plate, KA—Kanto block, HO—Hokuriku block,
CH—Chubu block, BI—Biwako block, OS—Osaka block, SE— Setouchi
block, SH—Shikoku block, JS— Japan Sea block, SK—southern Kyushu
block, CK—central Kyushu block, AM— Amurian plate. (A) Active
faults (Headquar- ters for Earthquake Research Promotion, 2017) in
the shaded relief topographic map. (B) Shallow seismicity at depth
≤20 km and magnitude ≥2, 1923–2016 (Mw is moment magnitude).
Stars with occurrence year and magnitude indicate epicenters of se-
lected large earthquakes. The orange trian- gles indicate active
volcanoes.
541Nishimura et al. | Strain partitioning and interplate
coupling along the northern margin of the Philippine Sea
plateGEOSPHERE | Volume 14 | Number 2
block rotations and slip-deficit rates. For example, separation of
the Shikoku (SH) and Chubu (CH) blocks reduces misfits in the
model, but the misfits are not highly dependent on the location of
their boundary if it is located in the Kii Channel (~135°E) between
Shikoku and Honshu. We divide the continental part of the analyzed
region into 11 blocks. The subducting plate, which is conven-
tionally regarded as the single Philippine Sea plate, is divided
into three blocks, the (stable) Philippine Sea (PS) plate, Izu
forearc (IF) block, and Izu microplate (IM), following the model of
Nishimura (2011). In total, 14 tectonic blocks are assumed in the
block model and their rigid rotational movements are esti- mated
(Figs. 3A, 3B). Internal permanent strain rates, which are uniform
in each block, are also estimated for most continental blocks,
i.e., HO, CH, BI, OS, SE, SH, JS, CK, and SK.
The three-dimensional fault geometry of the block boundaries is
mainly based on previous studies of seismic tomography and active
source seismic surveys, and seismicity. The shape of the plate
interfaces between the sub- ducting and continental plates along
the Nankai Trough is approximated to the isodepth contours of Baba
et al. (2002) and Hirose et al. (2008a). The plate inter-
face along the Sagami trough is derived from Takeda et al.
(2007) and Hirose et al. (2008b). Because GPS-A stations are
located on the seafloor at depths of 800–2900 m below sea
level, the surface displacement would be biased if we calculated
surface displacement due to slip below the stations using an
elastic half-space dislocation model (Okada, 1985). In order to
reduce this effect, we made the subsurface PS-SK, PS-SH, PS-CH, and
IM-CH interfaces at depths of 5–15 km shallower by as much as
2 km along the Nankai Trough. We also made the subsurface
PS-SH interface shallower by several kilometers near 33°N, 133°E,
between Shikoku and Kyushu, because a northwestward systematic
misfit was found with the original model derived from Hirose
et al. (2008a). The node intervals for estimating slip-deficit
rates vary from ~5 to 70 km and ~20 to 80 km along the
dip and strike directions for subduction faults along the Nankai
Trough and Sagami trough, respectively. The uniform depth interval
of the nodes along the dip direction is 5 km. The maximum
depth of the nodes is 50 km and 40 km for the Nankai
Trough and Sagami trough, respectively. The node intervals for
crustal faults are 8 km and ~10–90 km along the dip and
strike directions, respectively. Most of the updip and downdip
nodes are assigned to depths of 0.5 and 24.5 km, respectively.
We assumed a subsurface fault geom- etry only for boundaries with
slip rates that are expected to be high and can be sufficiently
constrained by neighboring geodetic stations. Block boundaries for
which subsurface fault geometries are not assigned indicate
creeping faults slipping at the rate of the relative block motion
in our model.
Resolution Tests
Geodetic inversion generally has a limited resolving power for slip
deep and offshore faults (e.g., Sagiya and Thacher, 1999; Nishimura
et al., 2004). Yokota et al. (2016) showed that offshore
geodetic data improve resolving power near the Nankai Trough. We
made assumptions about the fault geom- etry and demonstrated how
much slip is resolved with our data set. We calcu-
lated synthetic displacements predicted from the assumed stripe
distribution along the strike and dip directions (Figs. 4A, 4D).
The synthetic displacements were added; random Gaussian noises
correspond to three times the data uncer- tainties assigned in the
inversion, because data misfits in our inversion result suggest
that the assigned data uncertainties are underestimated (as
discussed in RESULTS). The synthetic displacements are inverted to
estimate slip-deficit rates only on the subduction interfaces along
the Nankai Trough and Sagami trough. The other parameters, i.e.,
slip-deficit rates on crustal faults, block rota- tion, and
internal strain of the blocks, are fixed in these resolution
tests.
The stripes along the strike are reproduced at an intermediate
depth (Fig. 4B). Those along the dip are reasonably reproduced,
except for a region near the trough (Fig. 4E). The southwestern
region with a depth of ≤20 km east of Kyushu is poorly
resolved. Figure 4 suggests that the unresolved area for the
assigned stripes approximately corresponds to the area with
estimated uncertainties of ≥20 mm (shaded regions in Fig. 4).
Comparisons between Fig- ures 4B and 4C and 4E and 4F suggest that
offshore GPS-A data are useful to resolve the offshore slip,
although the slip near the trough axis cannot be resolved despite
using the existing GPS-A data.
RESULTS
One interesting aspect of the block model is the separation of the
observed velocities into the estimated block rotation and elastic
deformation compo- nents because of fault locking, and the internal
uniform deformation com- ponent of each block. We plot the first
two components in Figure 5. Except for the offshore regions along
the troughs and southern Shikoku, the block rotation component is
greater than the elastic deformation. Because the direc- tions of
both components are similar in most areas, ignoring the block
rotation in the continental plates results in larger estimates of
interplate coupling. The Euler poles and rotational rates of the
rigid blocks are listed in Table 2. Except for the Izu microplate,
the Euler poles are far from the studied region, which means that
block motion can be approximated by translation. The block mo-
tions of the KA, CH, BI, OS, and SH blocks are similar and
westward. However, those of the HO, SE, CK, and JS blocks are
similarly westward to northwest- ward with a rate of ≤10 mm/yr.
Therefore, the boundary between these two groups, as well as the
Nankai Trough and Sagami trough, are the most impor- tant in
southwest Japan from the viewpoint of contemporary kinematics. This
characteristic is apparent in the relative block motion along the
boundaries of the tectonic blocks (Fig. 6), as discussed herein.
Estimated internal strain rates (Table 3) show that except for the
SK block, compressional strain in the continental blocks dominates
in a direction of relative plate convergence north of the Nankai
Trough (Fig. 6). The internal extension in the east-west direction
may be related with extensive volcanic activities of Sakurajima and
several calderas in SK, which is apparent on areal strain rates
shown in Figure 2A. The OK and BI blocks in the NKTZ show the
largest internal strain rates, to ~8 × 10–8 yr–1. These large
internal strain rates may be attributed to the numerous active
faults in our simplified tectonic block (Fig. 3A).
542Nishimura et al. | Strain partitioning and interplate
coupling along the northern margin of the Philippine Sea
plateGEOSPHERE | Volume 14 | Number 2
Slip-deficit rates on the subduction interfaces and crustal faults
are shown in Figures 7 and 8, respectively. The shaded regions
represent large uncertain- ties in the estimated slip-deficit
rates. Along the Nankai Trough, the distribution of slip-deficit
rates is heterogeneous in both the strike and dip directions, and
regions of high slip-deficit rates are found in offshore Shikoku,
offshore Kii Penin- sula, the Bungo Channel, and the southernmost
region east of Kyushu. Although we are not confident of the results
for this last region because of the large un- certainties there,
results for the other three regions are robust based on our trial
and error method for the block geometries. The regions of high
slip-deficit rates
off Shikoku and the Kii Peninsula are located at 10–20 km
depths, but the region in the Bungo Channel is located at
20–40 km depth. Regions of large uncertain- ties are located
near the updip and downdip edges of the assigned fault area.
Although the offshore data contribute to decreasing the width of
the updip un- certain region by ~50 km, slip near the trough
axis is still not constrained by the geodetic data. It is notable
that the uncertainties in the slip-deficit rates are large in the
most southwestern part of the Nankai Trough east off Kyushu and the
east- ern half of the Sagami trough. Slip-deficit rates in these
regions are important for assessing the size of the source region
of future megathrust earthquakes, but the
130° 131° 132° 133° 134° 135° 136° 137° 138° 139° 140° 141°
142°
30°
31°
32°
33°
34°
35°
36° 50 km
A B C
130° 131° 132° 133° 134° 135° 136° 137° 138° 139° 140° 141°
142°
30°
31°
32°
33°
34°
35°
36° 50 km
D E F
0 6 12 18 24 30 36 42 48 54 60 80
Rate of slip deficit (mm/yr)
130° 131° 132° 133° 134° 135° 136° 137° 138° 139° 140° 141°
142°
30°
31°
32°
33°
34°
35°
36° 50 km
130° 131° 132° 133° 134° 135° 136° 137° 138° 139° 140° 141°
142°
30°
31°
32°
33°
34°
35°
36° 50 km
130° 131° 132° 133° 134° 135° 136° 137° 138° 139° 140° 141°
142°
30°
31°
32°
33°
34°
35°
36° 50 km
130° 131° 132° 133° 134° 135° 136° 137° 138° 139° 140° 141°
142°
30°
31°
32°
33°
34°
35°
36° 50 km
Figure 4. Results of the resolution test for slip-deficit
distribution on subduction interfaces. (A) Assumed slip-deficit
distribution for a stripe pattern along the strike. (B) Estimated
slip-deficit distribution with offshore stations corresponding to
the distribution in A. (C) Same as B but without offshore stations.
(D) Assumed slip-deficit distribution for a stripe pattern along
the dip. (E) Estimated slip-deficit distribution with offshore
stations corresponding to the distribution in D. (F) Same as E but
without offshore stations.
543Nishimura et al. | Strain partitioning and interplate
coupling along the northern margin of the Philippine Sea
plateGEOSPHERE | Volume 14 | Number 2
geodetic data cannot constrain them. Slip-deficit rates for inland
faults (Fig. 8) are estimated to be mostly similar to their
interblock velocities, but they are also mostly within their
uncertainties. Slip-deficit rates of ≥10 mm/yr are estimated on the
SH-SK, BI-SE, BI-HO, and CH-HO boundaries. The latter three
boundaries correspond to the Biwako-Seigan, Hanaore, and Atotsugawa
faults, and they are accumulating elastic strain that may be
released by future earthquakes.
The observed velocities are reasonably reproduced by our model
(Fig. 9). Along the Nankai Trough and Sagami trough, the observed
vertical velocities show subsidence around the southern tips of the
capes and peninsulas, and uplift in the more northern region. This
systematic pattern is explained by the model (Fig. 9B). The
normalized root mean squares (nrms) for horizontal onshore,
horizontal offshore, and vertical onshore velocities for the
preferred
128° 129° 130° 131° 132° 133° 134° 135° 136° 137° 138° 139° 140°
141°
30°
31°
32°
33°
34°
35°
36°
AM
JS
OS
SH
Figure 5. Estimated rigid block motion and elastic deformation due
to fault locking. Tectonic block abbreviations as in Figure
3.
TABLE 2. EULER VECTORS WITH RESPECT TO EURASIAN PLATE
Block Latitude
(°N) Longitude
(°)
Philippine Sea plate (PS) 54.38 157.28 –1.255 ± 0.014 0.55
0.09 220 Izu forearc block (IF) 46.64 156.95 –1.331 ± 0.116
2.28 0.14 233 Izu microplate (IM) 36.77 138.91 –8.778 ± 0.311
0.09 0.02 193 Kanto block (KA) 75.61 169.77 –0.321 ± 0.070
15.69 0.39 218 Chubu block (CH) 82.38 194.39 –0.250 ± 0.043
16.87 0.45 241 Hokuriku block (HO) –5.07 112.38 0.083 ± 0.080
66.50 1.12 26 Biwako block (BI) 69.16 137.65 –0.277 ± 0.181
33.75 0.75 182 Osaka block (OS) 67.04 153.16 –0.243 ± 0.272
52.52 0.74 207 Setouchi block (SE) –22.24 95.58 0.082 ± 0.017
38.32 1.11 32 Shikoku block (SH) 24.30 130.29 0.972 ± 0.093
1.35 0.20 12 Southern Kyushu block (SK) 30.23 120.98 –0.570 ±
0.237 5.36 1.54 257 Central Kyushu block (CK) 67.79 138.87
–0.076 ± 0.101 60.28 5.82 195 Japan Sea block (JS) 37.36
173.67 –0.052 ± 0.018 17.65 1.92 99 Amurian plate (AM) 74.98
243.43 0.015 ± 0.010 71.12 5.35 303
Note: The rotation rate is indicated with one standard error. emax,
emin, and azimuth represent the maximum and the minimum axes of the
68% confidence error ellipse and azimuth of the major axis,
respectively.
544Nishimura et al. | Strain partitioning and interplate
coupling along the northern margin of the Philippine Sea
plateGEOSPHERE | Volume 14 | Number 2
model are 4.76, 15.64, and 7.75, respectively. These statistics
suggest that our block model does not explain the data within the
assigned data uncertainties, which are generally <0.5 mm/yr (as
shown in the Supplemental File [footnote 1]). We attribute these
large misfits to the underestimated velocity uncertainties assigned
in the inversion analysis. Another major source of the large
misfits is anomalous GNSS sites affected by nontectonic
disturbances such as land- slides, monument instability, and local
multipath interferences. Because the nrms are ~3 for the horizontal
onshore GNSS data of most continental blocks, we increased the
uncertainties used for the resolution test by three times, and
demonstrated that the quality of our data is sufficient to resolve
the coupling distribution on a major part of the subducting plate
interface along the Nankai Trough and Sagami trough. The residual
(i.e., observed – calculated) horizontal velocities appear to be
almost random, but there are some systematic pat- terns near the
block edges and along the Nankai Trough and Sagami trough (Fig.
10). This can probably be attributed to our simplified geometry of
blocks and boundary faults. The largest residuals are found
southeast off Kii Penin- sula, near 137°E. This may be caused by
postseismic deformation of the Mw 7.2 and Mw 7.4 earthquakes
off Kii Peninsula on 26 September 2004, which was an intraslab
event in the subducting plates (Nakano et al., 2015) (Fig.
3B). Sys- tematic southward residuals are also found at nearby
on-land GNSS stations, similar to the observed coseismic
displacements and predicted postseismic displacements due to
viscoelastic relaxation (Suito and Ozawa, 2009). There- fore,
heterogeneous slip-deficit rates along the Nankai Trough near 137°E
may be affected by the postseismic deformation due to the 2004
earthquakes.
DISCUSSION
Strain Partitioning of Interplate Motions in
and around Southwest Japan
Our results show that rigid block motions on the continental plate,
except for the SK block, have a west to northwest direction, and
that they serve as intermediate motions between those of the
subducting Philippine Sea plate and the stable Amurian plate (Fig.
5). Oblique subduction along the Nankai Trough causes strain
partitioning, as represented by the MTL (e.g., Fitch, 1972;
128° 129° 130° 131° 132° 133° 134° 135° 136° 137° 138° 139° 140°
141°
30°
31°
32°
33°
34°
35°
36°
37°
Block boundary type Convergence Divergence Shear (Right lateral)
Shear (Left lateral)
5 cm/yr Relative block motion
78
39-48
35-40
4-31
3-4
1
OS Figure 6. Rates and types of relative block motion along block
boundaries. Color of boundaries represents principal type of
relative motion. Numerals represent rate of relative block motion
in millimeters per year. Tectonic block abbreviations as in Figure
3.
TABLE 3. INTERNAL PRINCIPAL STRAIN RATE OF CRUSTAL BLOCKS
Block εmax
(nanostrain/yr) εmin
(°)
Chubu block (CH) 1.3 ± 1.4 –42.7 ± 1.0 305 Hokuriku block
(HO) –11.9 ± 1.9 –26.0 ± 2.0 303 Biwako block (BI)
–11.2 ± 4.2 –60.8 ± 4.0 309 Osaka block (OS) –52.4 ±
7.2 –82.8 ± 6.8 227 Setouchi block (SE) –4.06 ± 0.8
–34.1 ± 1.0 332 Shikoku block (SH) 10.8 ± 2.4
–33.6 ± 2.3 313 Southern Kyushu block (SK) 12.1 ± 6.3
–0.7 ± 3.1 355 Central Kyushu block (CK) –0.1 ± 2.9
–49.8 ± 2.1 298 Japan Sea block (JS) 7.2 ± 0.7
–6.0 ± 0.5 303
Note: The other blocks except above ones are assumed to have no
internal crustal strain.
545Nishimura et al. | Strain partitioning and interplate
coupling along the northern margin of the Philippine Sea
plateGEOSPHERE | Volume 14 | Number 2
130° 131° 132° 133° 134° 135° 136° 137° 138° 139° 140° 141°
142°
30°
31°
32°
33°
34°
35°
36° 50 km
0 6 12 18 24 30 36 42 48 54 60 80
Rate of slip deficit (mm/yr)
10
20
30
40
50
10
20
30
Nankai Trough
Sagami trough
Figure 7. Estimated slip-deficit rates on subduction interfaces
along the Nankai Trough and Sagami trough. Shaded region represents
standard errors ≥20 mm/yr. Con- tours on subduction interfaces
are isodepths at 10 km intervals. Black dots represent nodes
used in estimating slip-deficit rates.
130° 131° 132° 133° 134° 135° 136° 137° 138° 139°
32°
33°
34°
35°
36° 50 km
0 2 4 6 8 10 12 14 16 18 20 40
Rate of slip deficit (mm/yr)
JS
PS
SK
CK
CH
KA
HO
OS
SH
IM Figure 8. Estimated slip-deficit rates on crustal faults. Shaded
region represents standard errors ≥5 mm/yr. Tectonic block
abbreviations as in Figure 3. Other sym- bols are the same as in
Figure 6. Note that crustal faults without the plotted slip-deficit
rates are assumed to creep.
546Nishimura et al. | Strain partitioning and interplate
coupling along the northern margin of the Philippine Sea
plateGEOSPHERE | Volume 14 | Number 2
Tabei et al., 2003). The motions of our continental blocks can
be interpreted as those of forearc slivers. Gutscher and Lallemand
(1999) proposed two major right-lateral shear zones in Shikoku and
western Honshu (i.e., the MTL and North Chugoku shear zone, NCSZ),
and that part of the transcurrent motion is being transferred from
the MTL to the NCSZ based on recent and historical seismicity.
Nishimura and Takada (2017) identified a high strain rate in an
east- ern part of the NCSZ using the GNSS data and proposed to call
it the San-in shear zone; they suggested that the 2000 Mw 6.7
western Tottori (Fig. 3B) and 2016 Mw 6.2 central Tottori
earthquakes occurred in the San-in shear zone. However, Loveless
and Meade (2010) concluded, from GNSS data, that move- ment across
the NCSZ was negligible. Our model suggests that the boundary
between the JS and SE blocks, corresponding to the NCSZ, has a
significant slip rate (~5 mm/yr), which is half of that between the
SE and SH blocks, corre- sponding to the MTL (9–10 mm/yr). This
discrepancy is explained by an insuf- ficient number of stations
near the NCSZ (used in Loveless and Meade, 2010) providing velocity
data for 1996–1999. Five GNSS stations were added along the Japan
Sea coast north of the NSCZ between 132.5°E and 135°E in 2002 and
2004. We found a considerable velocity gradient, as well as high
rates of maxi- mum shear strain (Fig. 2B), across the eastern part
of the NCSZ in 2005–2009 using these new stations. Another
possibility to explain the discrepancy is the location of the shear
zone. Because we found no significant velocity gradients across the
western part of the NCSZ, we assumed a block boundary in the
offshore region west of 132.5°E, not at the NCSZ, that is identical
to the seis- micity lineament. Our model further predicts
significant relative block motion between the JS and AM blocks on
the Korean Peninsula at a rate of 1–3 mm/yr.
Movements of the forearc blocks along the Nankai Trough demonstrate
the total partitioning ratio within the continental plate. The
block motion ve- locities at (33°N, 133°E) in the SH block and at
(33.5°N, 137°E) in the CH block are 18 and 24 mm/yr with respect to
the (stable) Amurian plate, respectively. Their components that are
parallel to the Philippine Sea plate motion reach ~26% and ~36%,
respectively, of the Philippine Sea plate motion. Along the
easternmost part of the Nankai Trough, called the Suruga Trough,
partitioning within the subducting oceanic plate also appears, as
in the Izu microplate (e.g., Sagiya, 1999; Nishimura et al.,
2007). A significant part of the plate motion be- tween the
subducting Philippine Sea and stable Amurian plates is accommo-
dated by several block boundaries along the margin of both plates.
This strain partitioning along the plate margins causes a dramatic
decrease in the relative block velocity along the
Ryukyu-Nankai-Suruga trench system, from 78 to 4 mm/yr eastward in
the studied area. One interesting point is that the block
boundaries along the plate margins accommodate not only
margin-parallel motion but also margin-normal motion, which may be
attributed to crustal shortening across the plate margins.
From the viewpoint of movements relative to the Okhotsk (or North
Amer- ica) plate, which northeast Japan belongs to, many previous
studies using GNSS (e.g., Sagiya et al., 2000; Heki and
Miyazaki, 2001) suggest that the NKTZ is a major tectonic boundary
between the Okhotsk and Amurian plates (Fig. 1). Our model predicts
a high rate of interblock motion (~3–14 mm/yr) along the
128° 129° 130° 131° 132° 133° 134° 135° 136° 137° 138° 139° 140°
141°
30°
31°
32°
33°
34°
35°
36°
37°
100 km
10 mm/yr
Obs. Cal.
128° 129° 130° 131° 132° 133° 134° 135° 136° 137° 138° 139° 140°
141°
30°
31°
32°
33°
34°
35°
36°
Obs. Cal.
Figure 9. Comparison between observed (Obs.) and calculated (Cal.)
velocities. (A) Horizontal component. (B) Verti cal
component.
547Nishimura et al. | Strain partitioning and interplate
coupling along the northern margin of the Philippine Sea
plateGEOSPHERE | Volume 14 | Number 2
HO-CH, SE-BI, and SE-OS boundaries, consistent with previous
studies (Fig. 6). The boundary of high interblock velocities
extends to the SE-SH and CK-SH boundaries. This suggests that these
boundaries corresponding to the NKTZ, MTL, and Futagawa-Hinagu
fault zones are major tectonic boundaries between the Okhotsk and
Amurian plates that transect the islands of Japan. Therefore, the
KA, CH, BI, OS, and SH tectonic blocks can be interpreted as either
a forearc sliver south of the MTL related to oblique subduction of
the Philippine Sea plate or part of the Okhotsk plate. Although it
is difficult to distinguish between the motions using the observed
crustal deformation, the similarities of the KA motion to those of
CH, BI, OS, and SH favor the latter interpretation, because most
previous studies showed that KA is a part of the Okhotsk plate and
plate convergence to the south of KA (i.e., Sagami trough) is much
less oblique.
Comparison between Geodetic and Geological
Slip Rates along Major Fault Zones
The estimated relative motion along the block boundaries (Fig. 6)
is almost consistent with the slip sense of the corresponding
faults in a compilation of many geological studies (Headquarters
for Earthquake Research Promotion,
2017). For example, right-lateral strike-slip motion with a small
convergent component between the SE and SH blocks is consistent
with the slip sense of the MTL. The motion between the SE and BI
blocks has both convergent and right-lateral strike-slip
components. Two parallel faults, i.e., a right- lateral strike-slip
fault (the Mikata-Hanaore fault system) and a reverse fault (the
Biwako- seigan fault system), correspond to this boundary, and the
estimated relative motion is the sum of the slip senses on these
faults. Boundaries with our slip rates similar to geological ones
within twice the ratio are the CH-KA, SE-SH, and BI-CH boundaries,
which correspond to the major fault systems, i.e., the
Itoigawa-Shizuoka tectonic line, Nobi, and MTL fault systems; slip
rates for these boundaries are estimated to be 1–5, 1–2, and 3–9
mm/yr in this study, respectively. They are approximately equal to
~1–9, ~2, and ~5–9 mm/yr from the geological studies. However, the
slip rates estimated in this study are sev- eral times to an
order of magnitude larger than the geological rates along several
boundary faults. For example, the CK-SH, HO-CH, and CK-SE bound-
aries were estimated to have slip rates of 6–9, 13–14, and 5 mm/yr,
respectively. The geological slip rates for the corresponding fault
systems (i.e., the Beppu Haneyama–Futagawa–Hinagu, Atotsugawa, and
Mizunawa fault systems) are <4, 2–3, and 0.2 mm/yr,
respectively. Possible causes of this apparent discrep- ancy
between geodetic and geological slip rates are insufficient
assumptions
126° 127° 128° 129° 130° 131° 132° 133° 134° 135° 136° 137° 138°
139° 140° 141°
30°
31°
32°
33°
34°
35°
36°
37°
38°
39°
548Nishimura et al. | Strain partitioning and interplate
coupling along the northern margin of the Philippine Sea
plateGEOSPHERE | Volume 14 | Number 2
in the block-fault model, and fluctuations and evolution of fault
slip rates at different time scales.
We included internal permanent deformation in the crustal blocks,
as there are numerous active faults in some of the blocks (Fig.
3A). Microseismicity (Fig. 3B) is located not only on the block
boundaries, but also within many of the blocks. Using a 13 yr
catalogue of microseismicity in Kyushu, Matsumoto et al.
(2016) showed that accumulation of small earthquakes can cause de-
tectable inelastic deformation; this is evidence of permanent
internal deforma- tion in the blocks. Incorporating internal
permanent deformation in the block model decreases the relative
block motion rates along some block boundaries; this reduces the
discrepancy between the geodetic and geological slip rates. Johnson
(2013) showed that the discrepancy can be solved by incorporating
internal deformation into a block model for California; our study
demonstrates that this is also the case in southwest Japan.
Geological studies have shown that fault slip rates can vary
temporally on a scale of 1 k.y. (e.g., Knuepfer, 1992; Friedrich
et al., 2003). The discrepancy between geodetic and geological
slip rates on some faults in California is ex- plained by changes
in the activity at fault systems at difference time scales (e.g.,
Meade and Hager, 2005). The SH-SK, HI-SK, and JS-SE block
boundaries in southern Kyushu and western Honshu do not correspond
to the major active faults. However, these block boundaries are
required in order to explain the observed velocities. We speculate
that these boundaries are relatively young geologically, and
evolved in the recent past. Therefore, geomorphological fea- tures
caused by subsurface fault movements have not been well
developed.
Relationship between Interplate Coupling, Past Large
Earthquakes, SSEs, and Shallow VLFEs
The heterogeneous distribution of slip-deficit rates at depths of
5–25 km along the Nankai Trough is one of the most distinctive
findings of this study (Fig. 7). Previous studies using only
on-land GNSS data (e.g., Wallace et al., 2009; Liu
et al., 2010; Yoshioka and Matsuoka, 2013) showed more uniform
distributions in this depth range, which is suggested to be due to
a thermally controlled transition from brittle to ductile behavior
(Hyndman et al., 1995). A peak of slip-deficit rates off
Shikoku and an overall decrease eastward (to- ward the Izu Islands)
and westward (off Kyushu) are common to most previous studies and
ours. The eastward decrease in slip-deficit rates can probably be
attributed to a reduction in interblock velocities along the trough
(Fig. 7), while an increase in the degree of aseismic creep is
thought to occur on the plate boundary offshore Kyushu. Yokota
et al. (2016) used offshore GPS-A data to estimate
slip-deficit rates, and found a heterogeneous distribution along
the offshore Nankai Trough. Their results and ours are similar in
pattern, but our rates are generally smaller than theirs. We
suspect that ignoring the contribu- tion of inland block motions
biased the slip-deficit rates and caused the larger rates reported
by Yokota et al. (2016). A deep-slip peak around (136.5°E,
35°N) estimated by Yoshioka and Matsuoka (2013) and Yokota
et al. (2016) was not resolved in our study. This slip deficit
may also be an artifact of ignoring in-
land block motions. We plotted the distribution of coupling ratios,
which is the ratio between the slip-deficit rate and interblock
velocity, and source areas of large earthquakes and SSEs in Figure
11. The coupling ratios at 10–25 km depth along the Nankai
Trough are mostly >0.5, and the past and anticipated source
areas of large earthquakes along the Nankai Trough are mostly
locked and are accumulating strain that may be released by future
large earthquakes (Fig. 11). We found three areas of low coupling
ratio (<0.5) at ~132°E, ~136°E, and ~137°E for this depth range.
These areas may correspond to the segment boundaries of megathrust
earthquakes along the Nankai Trough. We speculate that insuffi-
cient strain accumulation due to low coupling may prevent a fault
rupture from propagating over these areas. No historical Nankai
earthquakes have ruptured west of 132°E (Ishibashi, 2004). The
eastern edge of the source area for the 1944 Mw 8.1 Tonankai
earthquake is located near 137°E (Sagiya and Thatcher, 1999);
~136°E bounds the source areas of the 1944 Tonankai and 1946 Nankai
earthquakes. The epicenters of these earthquakes were also located
near 136°E (Fig. 11). On 1 April 2016, an Mw 6.0 interplate
earthquake occurred there, and significant afterslip was observed
by offshore borehole observatories (Wallace et al., 2016).
This is also a region of episodic shallow SSEs recently observed by
the same borehole observatories (Araki et al., 2017). These
phenomena support low coupling near 136°E, and it may cause stress
concentration to promote the initiation of seismic rupture in the
surrounding region. In addition to these low coupled regions, we
find a locally less coupled region near 135°E, 33°N (Figs. 7 and
11). This region corresponds to a subducted seamount and was
proposed to act as a barrier to prevent a rupture during the 1946
Nankai earthquake (Kodaira et al., 2002). These observations
support the inference of Wang and Bilek (2011, 2014), i.e., that
the subducted rough surface on a megathrust interface promotes
creeping on the interface, thus stopping earth- quake rupture
propagation.
Most long-term SSEs along the Nankai Trough occur downdip of the
source region of megathrust earthquakes, and short-term SSEs occur
further down- dip (Fig. 11) (e.g., Obara, 2010; Kobayashi, 2014).
Source areas of the long-term SSEs, except those around Kyushu near
131.8°E, have high coupling ratios (≥0.5), similar to those of the
megathrust earthquakes. Because long-term SSEs have occurred only
near 131.8°E in the period of the used GNSS data (Yarai and Ozawa,
2013), high coupling is common in the source areas of the Nankai
long- term SSEs during inter-SSE periods. However, many short-term
SSEs occurred in the period spanned by the GNSS data (e.g., Obara,
2010; Nishimura et al., 2013). The coupling ratio in their
source areas is low (<~0.5) east of 135°E and moderate to high
(≥~0.5) west of 135°E, although slip accommodated by short- term
SSEs west of 135°E is larger than that east of 135°E. These suggest
that there is no relationship between the interevent coupling ratio
and the slip mode (i.e., fast or slow slip) during an event
releasing accumulated strain. VLFEs oc- cur both downdip and updip
of the source areas of megathrust earthquakes along the Nankai
Trough (Ito et al., 2009a, 2009b). Shallow VLFEs occur in a
limited region close to the trough (Fig. 11). The coupling ratio is
small in the VLFE region, except east of Kyushu. The estimated high
slip-deficit rates east of Kyushu are not well resolved by the
observations and may be ghosts due to
549Nishimura et al. | Strain partitioning and interplate
coupling along the northern margin of the Philippine Sea
plateGEOSPHERE | Volume 14 | Number 2
inland deformation related with large calderas and unmodeled block
rotation in and around southern Kyushu (cf. Wallace et al.,
2009). Offshore geodetic obser- vations are necessary to clarify
the coupling in this region. A low coupling ratio in the other VLFE
regions is concordant with a recent finding that shallow SSEs,
releasing tectonic strain, sometimes occur in the VLFE region at
the offshore Nankai Trough (Araki et al., 2017).
Along the Sagami trough, the shallow plate interface from the
trough axis to a depth of 15 km is almost fully coupled, not
only in the source area of the 1923 Kanto earthquake (Wald and
Somerville, 1995) but also in the source area of the repeating Mw
~6.6 Boso SSEs (Ozawa, 2014). Large uncertainties east of the 1923
source region (Fig. 7) indicate that our model cannot constrain the
strain accumulation there. However, 20 yr of GNSS data in
southeastern Kanto show a contraction in the direction of
interblock motion in spite of the occurrence of 5 large SSEs.
Offshore GPS-A stations on the eastern part of the Sagami
trough are required in order to assess the earthquake potential
in the Tokyo metropolitan area.
CONCLUSIONS
We combined onshore GNSS and offshore GPS-A data to estimate a
secu- lar velocity in and around southwestern Japan, where the
Philippine Sea plate is subducting beneath the Amurian plate along
the Nankai Trough and Sagami trough. The strain rate distribution
from a dense GNSS network, current seis-
micity, and many active faults support strain partitioning in both
the overriding continental plates and the subducting oceanic plate.
We modeled the contem- porary deformation using a block model
approach and estimated the rigid rotations of 12 crustal blocks, as
well as the coupling distribution on faults bounding the blocks.
Our results suggest that approximately one-third of the relative
plate motion between the Amurian and Philippine Sea plates is ac-
commodated in the southeastern margin of the Amurian plate, causing
a rapid eastward decrease in the convergence rate along the Nankai
Trough, from 78 to 4 mm/yr. Offshore geodetic data revealed a
heterogeneous coupling distri- bution at seismogenic depth, between
10 and 25 km. Weak coupling was found at ~132°E, ~136°E, and
~137°E, corresponding to the segmentation boundaries of past
megathrust earthquakes along the Nankai Trough. This suggests that
dynamic rupture during an earthquake might not propagate over low
coupled regions because of insufficient accumulated strain.
ACKNOWLEDGMENTS
We thank Takeshi Sagiya and Youichiro Takada for helping with
Global Navigation Satellite Sys- tem (GNSS) data collection. We are
grateful to the Geospatial Information Authority of Japan, Nevada
Geodetic Laboratory, Japan Meteorological Agency, and Global
Earthquake Model for providing GNSS and earthquake catalogue data.
We also appreciate constructive comments by Laura Wallace and Jack
Loveless and an anonymous reviewer that improved the quality of the
pa- per. This study was supported by the Japan Society of the
Promotion of Science (JSPS) Grants-in Aid for Scientific Research
(KAKENHI; grant JP26109007), the MEXT (Ministry of Education, Cul-
ture, Sports, Science and Technology of Japan) Earthquake and
Volcano Hazards Observation and Research Program, and the MEXT “New
Disaster Mitigation Research Project on Mega Thrust Earthquakes
Around the Nankai/Ryukyu Subduction Zones.”
130° 131° 132° 133° 134° 135° 136° 137° 138° 139° 140° 141°
142°
30°
31°
32°
33°
34°
35°
36°
2040
Coupling ratio
Source areas of past large earthquakes Source areas of anticipated
Tokai earthquake
Source areas of long-term SSEs Source areas of short-term
SSEs
Epicenters of past notable earthquakes
1968 Mw7.5
2004 Mw7.4
1946 Mw8.3
1944 Mw8.1
2016 Mw6.0
Source areas of shallow VLFEs
Figure 11. Geodetic coupling ratio on sub- duction interfaces along
the Nankai Trough and Sagami trough. Solid blue lines rep- resent
source regions of 1946 Nankai and 1944 Tonankai (Sagiya and
Thatcher, 1999) and 1923 Kanto (Wald and Somerville, 1995)
earthquakes (Mw is moment mag- nitude). Dashed blue lines represent
the suggested source region for Tokai earth- quake. Solid and
dotted green lines repre- sent source regions of long-term slow
slip events (SSEs) (Ozawa et al., 2013; Yarai and Ozawa, 2013;
Kobayashi, 2014; Takagi et al., 2016) and short-term SSEs
(Nishimura et al., 2014; Ozawa et al., 2014), respec-
tively. Dotted orange lines represent source regions of very low
frequency earthquakes (VLFEs) determined by National Research
Institute for Earth Science and Disaster Re- silience. Stars
represent epicenters of nota- ble earthquakes. Contours on
subduction interfaces are isodepths at 10 km intervals.
550Nishimura et al. | Strain partitioning and interplate
coupling along the northern margin of the Philippine Sea
plateGEOSPHERE | Volume 14 | Number 2
REFERENCES CITED
Altamimi, Z., Collilieux, X., and Métivier, L., 2011, ITRF2008: An
improved solution of the interna- tional terrestrial reference
frame: Journal of Geophysical Research, v. 85,
p. 457–473, https:// doi .org /10 .1007 /s00190 -011 -0444 -4
.
Altamimi, Z., Métivier, L., and Collilieux, X., 2012, ITRF2008
plate motion model: Journal of Geo- physical Research, v. 117,
B07402, https:// doi .org /10 .1029 /2011JB008930 .
Ando, M., 1975, Source mechanisms and tectonic significance of
historical earthquakes along the Nankai Trough, Japan:
Tectonophysics, v. 27, no. 2, p. 119–140,
https:// doi .org /10 .1016 /0040 -1951 (75)90102 -X .
Araki, E., Saffer, D.M., Kopf, A.J., Wallace, L.M., Kimura, T.,
Machida, Y., Satoshi, I., Davis, E., and IODP Expedition 365
Shipboard Scientists, 2017, Recurring and triggered slow-slip
events near the trench at the Nankai Trough subduction megathrust:
Science, v. 356, p. 1157–1160, https:// doi .org /10
.1126 /science .aan3120 .
Baba, T., Tanioka, Y., Cummins, P.R., and Uhira, K., 2002, The slip
distribution of the 1946 Nankai earthquake estimated from tsunami
inversion using a new plate model: Physics of the Earth and
Planetary Interiors, v. 132, p. 59–73, https:// doi .org
/10 .1016 /S0031 -9201 (02)00044 -4 .
Bertiger, W., Desai, S., Haines, B., Harvey, N., Moore, A.W., Owen,
S., and Weiss, J.P., 2010, Single receiver phase ambiguity
resolution with GPS data: Journal of Geodesy, v. 84,
p. 327–337, https:// doi .org /10 .1007 /s00190 -010 -0371 -9
.
DeMets, C., Gordon, R.G., and Argus, D.F., 2010, Geologically
current plate motions: Geophysical Journal International,
v. 181, p. 1–80, https:// doi .org /10 .1111 /j .1365
-246X .2009 .04491 .x .
Fitch, T.J., 1972, Plate convergence, transcurrent faults, and
internal deformation adjacent to Southeast Asia and the western
Pacific: Journal of Geophysical Research, v. 77,
p. 4432–4460, https:// doi .org /10 .1029 /JB077i023p04432
.
Friedrich, A.M., Wernicke, B.P., Niemi, N.A., Bennett, R.A., and
Davis, J.L., 2003, Comparison of geodetic and geologic data from
the Wasatch region, Utah, and implications for the spectral
character of Earth deformation at periods of 10 to 10 million
years: Journal of Geophysical Research, v. 108, 2199, https://
doi .org /10 .1029 /2001JB000682 .
Fukahata, Y., and Hashimoto, M., 2016, Simultaneous estimation of
the dip angles and slip distri- bution on the faults of the 2016
Kumamoto earthquake through a weak nonlinear inversion of InSAR
data: Earth, Planets, and Space, v. 68, p. 204, https://
doi .org /10 .1186 /s40623 -016 -0580 -4 .
Graduate School of Environmental Studies, Nagoya University, 2016,
Seafloor geodetic observa- tion along the Nankai Trough
(preliminary results): Report of Coordinate Committee of Earth-
quake Prediction, v. 96, p. 325–327 (in Japanese).
Gutscher, M.-A., 2001, An Andean model of interplate coupling and
strain partitioning applied to the flat subduction zone of SW Japan
(Nankai Trough): Tectonophysics, v. 333, no. 1,
p. 95–109.
Gutscher, M.-A., and Lallemand, S., 1999, Birth of a major
strike-slip fault in SW Japan: Terra Nova, v. 11,
p. 203–209, https:// doi .org /10 .1046 /j .1365 -3121 .1999
.00247 .x .
Hashimoto, M., Miyazaki, S., and Jackson, D.D., 2000, A block-fault
model for deformation of the Japanese Islands derived from
continuous GPS observation: Earth, Planets and Space, v. 52,
no. 11, p. 1095–1100, https:// doi .org /10 .1186
/BF03352337 .
Headquarters for Earthquake Research Promotion, 2017, Evaluations
of active faults: http:// www .jishin .go .jp /evaluation /long
_term _evaluation /major _active _fault/ (in Japanese, January
2017).
Heki, K., and Miyazaki, S., 2001, Plate convergence and long-term
crustal deformation in cen- tral Japan: Geophysical Research
Letters, v. 28, no. 12, p. 2313–2316, https:// doi
.org /10 .1029 /2000GL012537.
Hirose, F., Nakajima, J., and Hasegawa, A., 2008a,
Three-dimensional seismic velocity structure and configuration of
the Philippine Sea slab in southwestern Japan estimated by
double-dif- ference tomography: Journal of Geophysical Research,
v. 113, B09315, https:// doi .org /10 .1029 /2007JB005274
.
Hirose, F., Nakajima, J., and Hasegawa, A., 2008b,
Three-dimensional velocity structure and configuration of the
Philippine Sea slab beneath Kanto District, central Japan,
estimated by double-difference tomography: Zisin, v. 60,
p. 128–138, https:// doi .org /10 .4294 /zisin .60 .123 (in
Japanese with English abstract) .
Hotta, K., Iguchi, M., Ohkura, T., and Yamamoto, K., 2016,
Multiple-pressure-source model for ground inflation during the
period of high explosivity at Sakurajima volcano, Japan—Combi-
nation analysis of continuous GNSS, tilt and strain data: Journal
of Volcanology and Geother- mal Research, v. 310,
p. 12–25, https:// doi .org /10 .1016 /j .jvolgeores .2015 .11
.017 .
Hyndman, R.D., Wang, K., and Yamano, M., 1995, Thermal constraints
on the seismogenic por- tion of the southwestern Japan subduction
thrust: Journal of Geophysical Research, v. 100,
p. 15,373–15,392, https:// doi .org /10 .1029 /95JB00153
.
Ishibashi, K., 2004, Status of historical seismology in Japan:
Annals of Geophysics, v. 47, p. 339– 368, http:// dx .doi
.org /10 .4401 /ag -3305 .
Ito, Y., Asano, Y., and Obara, K., 2009a, Very-low-frequency
earthquakes indicate a transpressional stress regime in the Nankai
accretionary prism: Geophysical Research Letters,
v. 36, L20309, https:// doi .org /10 .1029 /2009GL039332
.
Ito, Y., Obara, K., Matsuzawa, T., and Maeda, T., 2009b, Very low
frequency earthquakes related to small asperities on the plate
boundary interface at the locked to aseismic transition: Journal of
Geophysical Research, v. 114, B00A13, https:// doi .org
/10 .1029 /2008JB006036 .
Japan Meteorological Agency, 2013, National catalogue of the active
volcanoes in Japan (fourth edition: English version): Tokyo, Japan
Meteorological Agency, http:// www .data .jma .go .jp /svd /vois
/data /tokyo /STOCK /souran _eng /menu .htm (January 2017).
Jin, S., and Park, P.H., 2007, Tectonic activities and deformation
in South Korea constrained by GPS observations: International
Journal of Geology, v. 2, p. 11–15, http:// www .naun
.org /main /NAUN /geology /ijgeo -03 .pdf.
Johnson, K.M., 2013, Slip rates and off-fault deformation in
southern California inferred from GPS data and models: Journal of
Geophysical Research, v. 118, p. 5643–5664, https://
doi .org /10 .1002 /jgrb .50365 .
Kanaori, Y., 1990, Late Mesozoic–Cenozoic strike-slip and block
rotation in the inner belt of southwest Japan: Tectonophysics,
v. 177, p. 381–399, https:// doi .org /10 .1016 /0040
-1951 (90)90397 -Q .
Kawanishi, R., Iio, Y., Yukutake, Y., Shibutani, T., and Katao, H.,
2009, Local stress concentration in the seismic belt along the
Japan Sea coast inferred from precise focal mechanisms: Implica-
tions for the stress accumulation process on intraplate earthquake
faults: Journal of Geophysi- cal Research, v. 114, B01309,
https:// doi .org /10 .1029 /2008JB005765 .
Knuepfer, P.L., 1992, Temporal variations in latest Quaternary slip
across the Australian Pacific plate boundary, northeastern South
Island, New Zealand: Tectonics, v. 11, p. 449–464,
https:// doi .org /10 .1029 /91TC02890 .
Kobayashi, A., 2014, A long-term slow slip event from 1996 to 1997
in the Kii Channel, Japan: Earth, Planets, and Space, v. 66,
p. 9, https:// doi .org /10 .1186 /1880 -5981 -66 -9 .
Kodaira, S., Kurashimo, E., Park, J. O., Takahashi, N., Nakanishi,
A., Miura, S., Iwasaki, T., Hirata, N., Ito, K., and Kaneda, Y.,
2002, Structural factors controlling the rupture process of a
megathrust earthquake at the Nankai trough seismogenic zone:
Geophysical Journal International, v. 149, no. 3,
p. 815–835, https:// dx .doi .org /10 .1046 /j .1365 -246X
.2002 .01691 .x .
Liu, Z., Owen, S., Dong, D., Lundgren, P., Webb, F., Hetland, E.,
and Simons, M., 2010, Estimation of interplate coupling in the
Nankai Trough, Japan using GPS data from 1996 to 2006: Geophysi-
cal Journal International, v. 181, p. 1313–1328, https://
doi .org /10 .1111 /j .1365 -246X .2010 .04600 .x .
Loveless, J.P., and Meade, B.J., 2010, Geodetic imaging of plate
motions, slip rates, and partition- ing of deformation in Japan:
Journal of Geophysical Research, v. 115, B02410, https://
doi .org /10 .1029 /2008JB006248 .
Matsumoto, S., Nakao, S., Ohkura, T., Miyazaki, M., Shimizu, H.,
Abe, Y., Inoue, H., Nakamoto, M., Yoshikawa, S., and Yamashita, Y.,
2015, Spatial heterogeneities in tectonic stress in Kyushu, Japan
and their relation to a major shear zone: Earth, Planets, and
Space, v. 67, p. 172, https:// doi .org /10 .1186 /s40623
-015 -0342 -8 .
Matsumoto, S., Nishimura, T., and Ohkura, T., 2016, Inelastic
strain rate in the seismogenic layer of Kyushu Island, Japan:
Earth, Planets, and Space, v. 68, p. 207, https:// doi
.org /10 .1186 /s40623 -016 -0584 -0 .
Matsu’ura, M., Jackson, D.D., and Cheng, A., 1986, Dislocation
model for aseismic crustal defor- mation at Hollister, California:
Journal of Geophysical Research, v. 91, p. 12661–12674,
https:// doi .org /10 .1029 /JB091iB12p12661 .
McCaffrey, R., 2002, Crustal block rotations and plate coupling, in
Stein, S., and Freymueller, J., eds., Plate Boundary Zones:
American Geophysical Union Geodynamics Series 30, p. 101–122,
https:// doi .org /10 .1029 /030GD06 .
Meade, B.J., and Hager, B.H., 2005, Block models of crustal motion
in southern California con- strained by GPS measurements: Journal
of Geophysical Research, v. 110, B03403, https:// doi
.org /10 .1029 /2004JB003209 .
Miyazaki, S., and Heki, K., 2001, Crustal velocity field of
southwest Japan: Subduction and arc- arc collision: Journal of
Geophysical Research, v. 106, p. 4305–4326, https:// doi
.org /10 .1029 /2000JB900312 .
Nakano, M., Nakamura, T., and Kaneda, Y., 2015, Hypocenters in the
Nankai Trough determined by using data from both ocean-bottom and
land seismic networks and a 3D velocity structure model:
Implications for seismotectonic activity: Bulletin of the
Seismological Society of Amer- ica, v. 105, p. 1594–1605,
https:// doi .org /10 .1785 /0120140309 .
551Nishimura et al. | Strain partitioning and interplate
coupling along the northern margin of the Philippine Sea
plateGEOSPHERE | Volume 14 | Number 2
Nishimura, S., and Hashimoto, M., 2006, A model with rigid
rotations and slip deficits for the GPS-derived velocity field in
southwest Japan: Tectonophysics, v. 421, p. 187–207,
https:// doi .org /10 .1016 /j .tecto .2006 .04 .017 .
Nishimura, T., 2011, Back-arc spreading of the northern
Izu-Ogasawara (Bonin) Islands arc clarified by GPS data:
Tectonophysics, v. 512, p. 60–67, https:// doi .org /10
.1016 /j .tecto .2011 .09 .022 .
Nishimura, T., 2017, Strain concentration zones in the Japanese
Islands clarified from GNSS data and its relation with active
faults and inland earthquakes: Active Fault Research, v. 46,
p. 33–39 (in Japanese with English abstract).
Nishimura, T., and Takada, Y., 2017, San-in shear zone in southwest
Japan, revealed by GNSS observations: Earth, Planets, and Space,
v. 69, p. 85, https:// doi .org /10 .1186 /s40623 -017
-0673 -8 .
Nishimura, T., Hirasawa, T., Miyazaki, S., Sagiya, T., Tada, T.,
Miura, S., and Tanaka, K., 2004, Tem- poral change of interplate
coupling in northeastern Japan during 1995–2002 estimated from
continuous GPS observations: Geophysical Journal International,
v. 157, p. 901–916, https:// doi .org /10 .1111 /j .1365
-246X .2004 .02159 .x .
Nishimura, T., Sagiya, T., and Stein, R.S., 2007, Crustal block
kinematics and seismic potential of the northernmost Philippine Sea
plate and Izu microplate, central Japan, inferred from GPS and
leveling data: Journal of Geophysical Research-Solid Earth,
v. 112, no. B5, https:// doi .org /10 .1029 /2005JB004102
.
Nishimura, T., Tobita, M., Murakami, M., Kanazawa, T., and
Shinohara, M., 2010, Fault model of 2007 M = 6.8 Chuetsu-oki
earthquake, central Japan, constructed using geodetic data: Ad-
vances in Geosciences, v. 20, p. 165–178, https:// doi
.org /10 .1142 /9789812838186_0009 .
Nishimura, T., Matsuzawa, T., and Obara, K., 2013, Detection of
short-term slow slip events along the Nankai Trough, southwest
Japan, using GNSS data: Journal of Geophysical Research: Solid
Earth, v. 118, no. 6, p. 3112-3125, https:// doi
.org /10 .1002 /jgrb .50222 .
Nishimura, T., Sato, M., and Sagiya, T., 2014, Global Positioning
System (GPS) and GPS-Acoustic observations: Insight into slip along
the subduction zones around Japan: Annual Review of Earth and
Planetary Sciences, v. 42, p. 653–674, https:// doi .org
/10 .1146 /annurev -earth -060313 -054614 .
Obara, K., 2010, Phenomenology of deep slow earthquake family in
southwest Japan: Spatio- temporal characteristics and segmentation:
Journal of Geophysical Research, v. 115, B00A25, https://
doi .org /10 .1029 /2008JB006048 .
Okada, Y., 1985, Surface deformation due to shear and tensile
faults in a half-space: Bulletin of the Seismological Society of
America, v. 75, p. 1135–1154.
Ozawa, S., 2014, Shortening of recurrence interval of Boso slow
slip events in Japan: Geophysical Research Letters, v. 41,
p. 2762–2768, https:// doi .org /10 .1002 /2014GL060072
.
Ozawa, S., Yarai, H., Tobita, M., Une, H., and Nishimura, T., 2008,
Crustal deformation associated with the Noto Hanto Earthquake in
2007 in Japan: Earth, Planets, and Space, v. 60,
p. 95–98, https:// doi .org /10 .1186 /BF03352767 .
Ozawa, S., Yarai, H., Imakiire, T., and Tobita, M., 2013, Spatial
and temporal evolution of the long- term slow slip in the Bungo
Channel, Japan: Earth, Planets, and Space, v. 65,
p. 67–73, https:// doi .org /10 .5047 /eps .2012 .06 .009
.
Sagiya, T., 1999, Interplate coupling in the Tokai district,
central Japan, deduced from continu- ous GPS data: Geophysical
Research Letters, v. 26, p. 2315–2318, https:// doi
.org /10 .1029 /1999GL900511 .
Sagiya, T., 2004, A decade of GEONET: 1994–2003—The continuous GPS
observations in Japan and its impact on earthquake studies: Earth,
Planets, and Space, v. 56, p. xxix–xli, https:// doi .org
/10 .1186 /BF03353077 .
Sagiya, T., and Thatcher, W., 1999, Coseismic slip resolution along
a plate boundary megathrust: The Nankai Trough, southwest Japan:
Journal of Geophysical Research, v. 104, p. 1111–1129,
https:// doi .org /10 .1029 /98JB02644 .
Sagiya, T., Miyazaki, S., and Tada, T., 2000, Continuous GPS array
and present-day crustal defor- mation of Japan: Pure and Applied
Geophysics, v. 157, p. 2302–2322, https:// doi .org /10
.1007 /PL00022507 .
Savage, J.C., 1983, A dislocation model of strain accumulation and
release at a subduction zone: Journal of Geophysical Research,
v. 88, p. 4984–4996, https:// doi .org /10 .1029
/JB088iB06p04984 .
Shen, Z., Jackson, D.D., and Ge, B.X., 1996, Crustal deformation
across and beyond the Los Angeles basin from geodetic measurements:
Journal of Geophysical Research, v. 101,
p. 27,957–27,980, https:// doi .org /10 .1029 /96JB02544
.
Shen-Tu, B., Holt, W.E., and Haines, A.J., 1995, Intraplate
deformation in the Japanese Islands: A kinematic study of
intraplate deformation at a convergent plate margin: Journal of
Geophysi- cal Research, v. 100, p. 24,275–24,293,
https:// doi .org /10 .1029 /95JB02842 .
Suito, H., and Ozawa, S., 2009, Transient crustal deformation in
the Tokai district—The Tokai slow slip event and postseismic
deformation caused by the 2004 off southeast Kii Peninsula earth-
quake: Seismological Society of Japan Journal, v. 61,
p. 113–135, https:// doi .org /10 .4294 /zisin .61 .113 (in
Japanese) .
Tabei, T., Hashimoto, M., Miyazaki, S., and Ohta, Y., 2003,
Present-day deformation across the south- west Japan arc: Oblique
subduction of the Philippine Sea plate and lateral slip of the
Nankai forearc: Earth, Planets, and Space, v. 55,
p. 643–647, https:// doi .org /10 .1186 /BF03352471 .
Tadokoro, K., Ikuta, R., Watanabe, T., Ando, M., Okuda, T., Nagai,
S., Yasuda, K., and Sakata, T., 2012, Interseismic seafloor crustal
deformation immediately above the source region of anticipated
megathrust earthquake along the Nankai Trough, Japan: Geophysical
Research Letters, v. 39, L10306, https:// doi .org /10 .1029
/2012GL051696 .
Takagi, R., Obara, K., and Maeda, T., 2016, Slow slip event within
a gap between tremor and locked zones in the Nankai subduction
zone: Geophysical Research Letters, v. 43, p.
1066–1074, https:// doi .org /10 .1002 /2015GL066987 .
Takayama, H., and Yoshida, A., 2007, Crustal deformation in Kyushu
derived from GEONET data: Journal of Geophysical Research,
v. 112, B06413, https:// doi .org /10 .1029 /2006JB004690
.
Takeda, T., Kasahara, K., and Kimura, H., 2007, Geometry of the
Philippine Sea plate in and around the Sagami Trough—Digital
restoration from paper records of multichannel seismic data: Chikyu
Monthly, v. S57, p. 115–123 (in Japanese).
Toda, S., and Stein, R., 2003, Toggling of seismicity by the 1997
Kagoshima earthquake couplet: A demonstration of time-dependent
stress transfer: Journal of Geophysical Research, v. 108,
2567, https:// doi .org /10 .1029 /2003JB002527 .
Tong, X., Smith-Konter, B., and Sandwell, D.T., 2014, Is there a
discrepancy between geological and geodetic slip rates along the
San Andreas fault system?: Journal of Geophysical Research,
v. 119, p. 2518–2538, https:// doi .org /10 .1002
/2013JB010765 .
Tsutsumi, H., Okada, A., Nakata, T., Ando, M., and Tsukada, T.,
1991, Timing and displacement of Holocene faulting on the Median
Tectonic Line in central Shikoku, southwest Japan: Journal
of Structural Geology, v. 13, p. 227–233, https://
doi .org /10 .1016 /0191 -8141 (91)90069 -U .
Wald, D.J., and Somerville, P.G., 1995, Variable-slip rupture model
of the great 1923 Kanto, Japan, earthquake: Geodetic and
body-waveform analysis: Bulletin of the Seismological Society of
America, v. 85, no. 1, p. 159–177.
Wallace, L., Ellis, S., Miyao, K., Miura, S., Beavan, J., and Goto,
J., 2009, Enigmatic, highly active left-lateral shear zone in
southwest Japan explained by aseismic ridge collision: Geology,
v. 37, p. 143–146, https:// doi .org /10 .1130 /G25221A
.1 .
Wallace, L.M., et al., 2016, Near-field observations of an
offshore Mw 6.0 earthquake from an inte- grated seafloor and
subseafloor monitoring network at the Nankai Trough, southwest
Japan: Journal of Geophysical Research, v. 121,
p. 8338–8351, https:// doi .org /10 .1002 /2016JB013417
.
Wang, K., and Bilek, S.L., 2011, Do subducting seamounts generate
or stop large earthquakes?: Geology, v. 39, p. 819–822,
https:// doi .org /10 .1130 /G31856 .1 .
Wang, K., and Bilek, S.L., 2014, Fault creep caused by subduction
of rough seafloor relief: Tectono- physics, v. 610,
p. 1–24, https:// doi .org /10 .1016 /j .tecto .2013 .11 .024
.
Watanabe, S., Ishikawa, T., and Yokota, Y., 2015, Non-volcanic
crustal movements of the northern- most Philippine Sea plate
detected by the GPS-acoustic seafloor positioning: Earth, Planets,
and Space, v. 67, p. 184, https:// doi .org /10 .1186
/s40623 -015 -0352 -6 .
Yarai, H., and Ozawa, S., 2013, Quasi-periodic slow slip events in
the afterslip area of the 1996 Hyuga- nada earthquakes, Japan:
Journal of Geophysical Research, v. 118, p. 2512–2527,
https:// doi .org /10 .1002 /jgrb .50161 .
Yasuda, K., Tadokoro, K., Ikuta, R., Watanabe, T., Nagai, S.,
Okuda, T., Fujii, C., and Sayanagi, K., 2014, Interplate locking
condition derived from seafloor geodetic data at the northernmost
part of the Suruga Trough, Japan: Geophysical Research Letters,
v. 41, p. 5806–5812, https:// doi .org /10 .1002
/2014GL060945 .
Yokota, Y., et al., 2015, Heterogeneous interplate coupling
along the Nankai Trough, Japan, de- tected by GPS-acoustic seafloor
geodetic observation: Progress in Earth and Planetary Sci- ence,
v. 2, p. 10, https:// doi .org /10 .1186 /s40645 -015
-0040 -y .
Yokota, Y., Ishikawa, T., Watanabe, S., Tashiro, T., and Asada, A.,
2016, Seafloor geodetic constraints on interplate coupling of the
Nankai Trough megathrust zone: Nature, v. 534, p.
374–377, https:// doi .org /10 .1038 /nature17632 .
Yoshioka, S., and Matsuoka, Y., 2013, Interplate coupling along the
Nankai Trough, southwest Japan, inferred from inversion analyses of
GPS data: Effects of subducting plate g