Seismic Hazard Analysis for Guam & the Northern Mariana Islands

1. Seismic Hazard Analysisfor Guam & the Northern Mariana Islands Chuck Mueller U.S. Geological Survey Golden, Colorado, USA

2. Tectonic Setting&Seismic History

3. Westward subduction of Pacific plate at Mariana Trench • Back-arc spreading at Mariana Trough • Complex oblique deformation in south

5. Northern & central subduction zone: • Deep seismicity (down to 700km) • Steeply dipping Benioff zone

6. Southern subduction zone: • Less deep; less steep

7. 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!

8. 08Aug1993, MW7.8 Harada & Ishibashi (2008): faulting on sub-horizontal plane ~70km deep within the subducting slab

9. Historical earthquakes shallow Benioff

10. Historical earthquakes shallow Benioff

11. 1) Gridded Historical Seismicity

12. Source catalogs: EVC PDE ISC Decluster with G&K

13. 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

16. 2-D Gaussian Smoothing•50-km for shallow• 30-km for deeper

19. Historical earthquakes and seismicity hazard models * Mmax from Am Samoa

20. 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)

21. 2) Megathrust Interface

22. Megathrust modeling issues Limited seismic history complicates estimates of maximum magnitude. Use MW8 based on local history (80%) and MW9 from other subduction zones (20%). 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).

23. 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

24. Ruff & Kanamori (1980)

25. Uyeda & Kanamori (1979)

26. 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 ...

27. 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)

28. Mw 8+ ~ 450 years

29. Ground Motions for Megathrust Interface• Zhao interface + epistemic

30. 3) Two Crustal Faults on Guam

31. Based primarily on Tracy et al. (1964)

32. Ground Motions for Crustal Faults•NGA B&A (0.333)• NGA C&B (0.333)• NGA C&Y (0.333)

33. Results

37. Probabilistic ground motions (g)

43. Extra slides