Seismic Hazard Analysis for Guam & the Northern Mariana Islands Chuck Mueller U.S. Geological Survey Golden, 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, M W Aug2002, M W 6.5 No very large eqk has ever been associated with the Mariana interface!

08Aug1993, M W 7.8 Harada & Ishibashi (2008): faulting on sub-horizontal plane ~70km deep within the subducting slab

Largest Observed (M W ) Depth (km) mt7.3 (1940)6.5 (2002) 0-40 or7.4 (1990) 0-40 other7.5 (1902)6.9 (2007) Historical earthquakes shallow Benioff

Largest Observed (M W ) Depth (km) mt 0-40 or 0-40 other (1993) (1914)7.6 (2000) (1957)6.6 (2005) (1931)6.9 (1953) (2007) (1905)6.5 (2001) (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”)

2-D Gaussian Smoothing50-km for shallow30-km for deeper

Largest Observed (M W )Model Parameters Depth (km) bMminMmaxModel depth 0-40 mt7.3 (1940)6.5 (2002) or7.4 (1990) 7.5 (80%) 8.2* (20%) 0-40 other7.5 (1902)6.9 (2007) (1993) (1914)7.6 (2000) (1957)6.6 (2005) (1931)6.9 (1953) (2007) (1905)6.5 (2001) (1995)600 * Mmax from Am Samoa Historical earthquakes and seismicity hazard models

Ground Motions for Background Seismicity Shallow (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 adjusted)

2) Megathrust Interface

Megathrust modeling issues 1)Limited seismic history complicates estimates of maximum magnitude. Use M W 8 based on local history (80%) and M W 9 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 => M W 8+ 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 M W 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 M W > 8.5 events … may reflect the short earthquake history sampled.” (BSSA, Jan2007)

M w 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 sSA sSA Probabilistic ground motions (g)

Extra slides