EART 118 SeismotectonicsLecture 17

CONCEPTS:

Variable frictional properties seem ubiquitous

Subduction Zone Geometry…

Earthquakes!

No seismicsignal

Episodic SlipSteady

SlipTremor

36-40 mm/yr loading rate Coast moves ~2 cm/yr today (SNARF)Margin-wide recurrence: 550 years (var ~200 yrs); 313 years into the eq cycle

~10 meters of post-1700 accumulated slip deficitLast eq appears to have been margin-wide (M9)

LVZLOC

Sealed plate boundary Permeable plate

boundary

Forearc

Arc volcano

Serpentinized mantle wedge

Overpressuredoceanic crust

Layer 3

Layer 1Layer 2 Oceanic crust Oceanic mantle

Spreading ridge

Hydrothermal circulation

EcoglitizationLFE hypocentersW-B hypocenters

MODEL

1. upper oceanic crust intensely hydrated through hydrothermal circulation at ridge;2. sediments carpet seabottom, plate cools, free water incorporated in hydrous minerals;3. upon subduction, increased P/T cause prograde metamorphic reactions, freeing fluids at lithostatic pore pressures;4. pressure maintained by impermeable plate boundary above and massive gabbro below5. metamorphic reactions culminate near 45 km depth with eclogitization and ruptureof plate boundary seal;6. fluids escape into overlying mantle wedge causing serpentinization and pore pressure reduction

Base of Seismogenic Zone

• CATACLASITE (P-sensitive)

~350°C - ~450°C(quartz vs. feldspar)

• MYLONITE (T-sensitive)

Cascadiavelocity

field(SNARF)

2010

vertical

2010

+

Szeliga et al., 2008

Chapman and Melbourne., 2009

Many magnitude 6s

Subtract from convergence rate of 34 mm/yr

•~Half of convergence is accommodated by large ETS events

•All imaged slip occurs below 25 km depth, above 40 km

• But max slip ~ 1/smoothing

6 Gomberg and Peng, 2010

Tremor (and slow slip) vs LFE

vs earthquake

TimevsDistancealongstrike

•  Along-strikepropagationvelocityvaries–  7to12km/day

•  Generaltendencytomoveupdip– WechandCreager

•  Originatein3places

Houston,Delbridge,Wech,CreagerNatureGeoscience,inrevision

Non-volcanic tremor always correlates with GPS-detectable ETS events

• Space and time correlation between slow slip and tremor detection along nearly the entire margin

Brudzinski etc 2009

GPS-inferred slip: Mw= 6.75 PNSN tremor

August 2010 event

Slip Tremor?

Melbourne

extendsupdipfrom

SimilarresultsfromSchmidtandKrogstad

ETS may delineate a 25 km lower limit to interseismic strain accumulation

After Hyndman, Dragert, Wang, etc, 1992-2003

A rough forecast of future slip after full recurrence interval

Tremor, VLFs, SSEs - All Slow, Interplate Earthquakes

TREMOR EPICENTRAL DISTRIBUTION NANKAI vs CASCADIA

Nonvolcanic Deep TremorAssociated with Subduction in

Southwest JapanKazushige Obara

Deep long-period tremors were recognized and located in a nonvolcanic regionin southwest Japan. Epicenters of the tremors were distributed along the strikeof the subducting Philippine Sea plate over a length of 600 kilometers. The depthof the tremors averaged about 30 kilometers, near the Mohorovic discontinuity.Each tremor lasted for at most a few weeks. The location of the tremors withinthe subduction zone indicates that the tremors may have been caused by fluidgenerated by dehydration processes from the slab.

Long-period events and tremors with typi-cal periods in the range of 0.2 to 2 s areoften observed at active volcanoes and re-flect the internal dynamics of the volcanicsystem (1). A possible tremor-generatingmechanism is flow-induced oscillation inchannels transporting magmatic fluid (2).We have identified and studied anomalouslong-period tremors from a nonvolcanic

area in southwest Japan by using the Na-tional Research Institute for Earth Scienceand Disaster Prevention’s (NIED) high-sensitivity seismograph network (Hi-net),which is composed of about 600 stationsinstalled throughout Japan to detect mi-croearthquakes (3). The densely distributedhigh-sensitivity seismic stations provide ahigh-level detection capability for micro-earthquakes and offer us an opportunity tofind and investigate very small amplitudetremors. Because the amplitudes of thesetremors are very small, it is difficult to

identify them with a single station or asparse network.

We observed small-amplitude tremors thatlasted from a few minutes to a few days, asshown in fig. S1A. The tremors were observedsimultaneously at several Hi-net stations, whichindicates that they are not related to artificialnoise. The predominant frequency of the trem-ors ranged from 1 to 10 Hz and was lower thanthat of ordinary earthquakes of similar size (10to 20 Hz). We transferred the raw seismogramto root-mean-square (rms) amplitude for thefiltered output (fig. S1B), and tremors wereclearly seen for time windows of 35 to 50 min.The envelope shapes of the tremors were verysimilar at different stations. The envelopes hadgradual rise times and differed from those of anormal earthquake, which has a spike-like en-velope shape. The similar envelope amplitudepattern seemed to have been propagated with avelocity of 4 km/s, which we roughly estimatedfrom paste-up traces plotted with the increasingepicentral distance. This means that the sourceof the tremors was located at a deep portion andthe envelopes were propagated not by P-wave,but by S-wave velocity. Because it was verydifficult to identify the initial P- and S-waveonset for the hypocentral determination, weapplied a cross-correlation technique to get thedistribution of the relative arrival time of theenvelope (4). The spatial distribution of the

National Research Institute for Earth Science andDisaster Prevention, Tenno-dai 3-1, Tsukuba, Ibaraki,305-0006, Japan. E-mail: obara@bosai.go.jp

Fig. 1. Epicentral distribution of the deep long-period tremors in the yearof 2001. The circle indicates the center of epicenters of tremors calcu-lated every hour. The crosses represent the Hi-net stations. The depth

contour line indicates the maximum frequent depth-distribution ofearthquakes inside the subducting Philippine Sea plate, and the gray linerepresents the leading edge of the subducting Philippine Sea plate (11).

R E P O R T S

www.sciencemag.org SCIENCE VOL 296 31 MAY 2002 1679

on N

ovem

ber

26, 2010

ww

w.s

cie

ncem

ag.o

rgD

ow

nlo

aded fro

m

Wech & Creager (2012) Obara (2002)

• tremor locations parallel 30-40 km slab contours• tremor width narrower in Japan (~30 km) than Cascadia (~80 km)

20, 30, 40 km

Tremor Mechanism

Slow shear slip, not fluid flow.

Does tremor in Japan, Cascadia, California, Costa Rica, Mexico, Alaska,… share the same mechanism?

Responds fast to stress(talk by Chastity Aiken)

Rubinstein, Vidale, et al.

…Turned on its side

Tremor under the San Andreas

Earthquakes

Tremorsand episodic slow slip?

Tremor under the San Andreas

Nadeau, 2005

Parkfield

Much of what we know about fault behavior at

depth comes from seismicity

Source Properties from SeismogramsEarthquake

???

P and S arrivals, polarity, waveform modeling, etc...

Source: unr.edu1 minute

Tremor

40 minutes

Matched filter technique:Template Waveforms

10 minutes 25 seconds

Shelly et al., Nature, 2007

Scan template through continuous data,sum correlations

13

Stacked LFE Templates

Single LFE template 100 LFE stack template

stacking

P-wave S-wave

17

Parkfield Tremor

Locations

• 88 stacked LFE templates• Located by P and S arrivals

on stacked waveforms, using a 3D velocity model.

• Sources extend 75 km both NE and SW of Parkfield

Shelly and Hardebeck, GRL, 2010

18

• 12+ years of continuous data

• ~50 trillion cross-correlation measurements

• ~850,000 events detected since mid-2001 (3000-30,000 per family)

• Detectible tremor activity in some area every day

Parkfield Tremor Catalog

Shelly and Hardebeck, GRL, 2010

Conclusion 1.1: The San Andreas fault does not end at the base of the “seismogenic zone.” Tremor sources are located on the deep extension of the fault, in the lower crust.

Conclusion 1.2: At least some portions of the deep fault deform brittlely. Tremor contains seismic waves of 30+ Hz even with temperatures ~500-600ºC

Conclusion 1.3: Tremor amplitude varies coherently along strike, possibly reflecting control from variations in geology and/or fluids.

What does it mean? (1)

Tremor Migration: characteristic velocity?

29

Recurrence: Shallower sources have larger, less frequent bursts

Cum

ulat

ive

even

ts

1 year

Cum

ulat

ive

even

ts

1 year

Cum

ulat

ive

even

ts

1 year

Cum

ulat

ive

even

ts

1 year

Shelly and Johnson, GRL, 2011