Seismic Hazard Analysisfor Guam & the Northern Mariana
Islands
Chuck MuellerU.S. Geological SurveyGolden, Colorado, USA
Tectonic Setting&
Seismic History
Westward subduction of Pacific plate at Mariana Trench
Back-arc spreading at Mariana Trough
Complex oblique deformation in south
Northern & central subduction zone:
Deep seismicity (down to 700km)
Steeply dipping Benioff zone
Southern subduction zone:
Less deep; less steep
Largest modern eqks with likely shallow thrust mechanisms:
06Jun1993, MW6.4
14Aug2002, MW6.5
No very large eqk has ever been associated with the Mariana interface!
08Aug1993, MW7.8
Harada & Ishibashi (2008): faulting on sub-horizontal plane ~70km deep within the subducting slab
Largest Observed (MW)
Depth (km) 1900-2011 1964-2011
0-40 mt 7.3 (1940) 6.5 (2002)
0-40 or 7.4 (1990)
0-40 other 7.5 (1902) 6.9 (2007)
41-80
81-120
121-160
161-200
201-300
301-500
501-700
Historical earthquakes
shallow
Benioff
Largest Observed (MW)
Depth (km) 1900-2011 1964-2011
0-40 mt
0-40 or
0-40 other
41-80 7.8 (1993)
81-120 8.2 (1914) 7.6 (2000)
121-160 6.8 (1957) 6.6 (2005)
161-200 7.0 (1931) 6.9 (1953)
201-300 7.5 (2007)
301-500 7.1 (1905) 6.5 (2001)
501-700 7.1 (1995)
Historical earthquakes
shallow
Benioff
1) Gridded Historical Seismicity
Source catalogs:
1) EVC
2) PDE
3) ISC
Decluster with G&K
Divide declustered catalog into eight sub-catalogs:• 0-40 km (megathrust, outer-rise, “other”)• 41-80• 81-120• 121-160• 161-200• 201-300• 301-500• 501-700
2-D Gaussian Smoothing• 50-km for shallow• 30-km for deeper
Largest Observed (MW) Model Parameters
Depth (km)
1900-2011 1964-2011 b Mmin MmaxModel depth
0-40 mt 7.3 (1940) 6.5 (2002)
1.06
5.0
7.0
200-40 or 7.4 (1990)
7.5 (80%)
8.2* (20%)
0-40 other
7.5 (1902) 6.9 (2007) 7.5
41-80 7.8 (1993)
8.2
60
81-120 8.2 (1914) 7.6 (2000) 100
121-160 6.8 (1957) 6.6 (2005) 140
161-200 7.0 (1931) 6.9 (1953) 180
201-300 7.5 (2007)
0.80 8.0
250
301-500 7.1 (1905) 6.5 (2001) 400
501-700 7.1 (1995) 600* Mmax from Am Samoa
Historical earthquakes and seismicity hazard models
Ground Motions for Background SeismicityShallow (0-40):• NGA B&A (0.167)• NGA C&B (0.167)• NGA C&Y (0.167)• Zhao crustal (0.5)Deep (41-700):• Zhao in-slab + epistemic (0.70 as-published + 0.30 adjusted)
2) Megathrust Interface
Megathrust modeling issues
1) Limited seismic history complicates estimates of maximum magnitude. Use MW8 based on local history (80%) and MW9 from other subduction zones (20%).
2) Evidence for weak coupling precludes estimating rates of large earthquakes from plate-motion data. Instead, extrapolate rates of historical earthquakes associated with the megathrust => MW8+ eqk every 450 yrs.
2) 3) Define downdip edge of megathrust surface as 40-km depth contour on west-dipping seismicity. This closely matches Hayes etal Slab1.0. Support for choice of 40 km from co-seismic slip patterns in recent great eqks and depths of thrust-mechanism eqks along Izu-Bonin (Hayes).
Conventional wisdom…
Weak plate coupling & weak seismicity on the interface correlate with…
Extension in the upper plate
Active back-arc spreading
Weak/no accretion
Deep trench
Old subducting plate
Slow subduction
Steep Benioff zone
Ruff & Kanamori (1980)
Uyeda & Kanamori (1979)
Mariana megathrust Mmax?
Based on its weak seismic history and the
traditional classifications, it would have been
difficult to justify an upper magnitude greater
than about MW 8 for the interface model prior
to 2004 Sumatra and 2011 Tohoku ...
Stein & Okal on the 2004 Sumatra-Andaman earthquake:
“The December earthquake was much larger than expected from a previously proposed relation, based on the idea of seismic coupling, in which such earthquakes occur only when young lithosphere subducts rapidly. Moreover, a global reanalysis finds little support for this correlation. Hence, we suspect that much of the apparent differences between subduction zones, such as some trench segments but not others being prone to MW > 8.5 events … may reflect the short earthquake history sampled.”(BSSA, Jan2007)
Mw 8+ ~ 450 years
Ground Motions for Megathrust Interface
• Zhao interface + epistemic
3) Two Crustal Faults on Guam
Based primarily on Tracy et al. (1964)
Ground Motions for Crustal
Faults
• NGA B&A (0.333)
• NGA C&B (0.333)
• NGA C&Y (0.333)
Results
Guam (Agana) Saipan
2% in 50y
10% in 50y
2% in 50y
10% in 50y
PGA 0.94 0.49 0.57 0.29
0.2sSA 2.86 1.43 1.75 0.83
1.0sSA 0.61 0.30 0.37 0.18
Probabilistic ground motions (g)
Extra slides