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