American Samoa Seismic Hazard Maps

1. American Samoa Seismic Hazard Maps Mark D. Petersen, Stephen C. Harmsen, Kenneth S. Rukstales, Charles S. Mueller, Daniel E. McNamara, Nicolas Luco, and Melanie Walling

2. American Samoa • Formed by migration of tectonic plate over hot spot 2 to 28 Ma • Pacific Plate motion GPS vector • Tonga trench is one of most active subduction zones ( 15-24 cm/yr), 22 M 7 earthquakes in last 110 yrs • September, 2009 M 8.1 event on outer rise and interface

3. American Samoa tectonics

4. Depth sections

5. Moment tensors

6. South Pacific Zones for calculating b-value and Mmax

7. Seismicity analysis

8. Magnitude-frequency • For this analysis we use 1964+ catalog • The rate of M 5’s seems to be higher • The rate of M 7+ is similar • Completeness near M 5

9. Magnitude-frequency

10. Magnitude-frequency plot by source

11. Magnitude-frequency plot Vanuatu

12. Magnitude-frequency for Tonga-Kermedec trench Model has two Mmax branches, M 8.5, 9, weighted equally

13. Ground motion analysis

14. Interface and Intraplate Earthquakes Interface and Intraplate, d=25 km

15. Interface and Intraplate Earthquakes

16. Intraplate and Interface Earthquakes M 9 curves

18. Intraplate earthquakes by depth (1 s and 0.2 s SA)

19. Ground motions for Shallow crustal earthquakes(1 s and 0.2 s SA) • For M 8 crustal earthquakes Zhao is highest (less gm saturation with M) • For M 6 crustal earthquakes Zhao is lowest • In 2011 paper Zhao et al. suggests more magnitude saturation for M 7+ earthquakes.

20. Zhao et al. (2006) crustal, inslab, and interface earthquake ground motions • Inslab earthquakes cause high gm • Guam ground motions are dominated by • Intraslab ground motions • American Samoa ground motions are • dominated by crustal sources

21. Oceanic data by distance and depth Red – interface, blue – intraplate

22. Zhao et al. (2006) model with Pacific strong ground motion data ( 1 s and PGA)

23. PGA station residuals Station residuals after subtracting station term and systematic offset Residuals = data-prediction

24. PGA average single station residuals by magnitude and distance Residuals = data-prediction

25. Comparison of Guam data and Zhao

26. Comparison of Guam data and Zhao

27. Comparison of Guam data and Zhao

28. Comparison of Guam data and Zhao

33. Systematic offset of Pacific data with Zhao et al. (2006) model Total Sigma, distance ≤ 300 km, M ≥ 6.0

35. Model and Results

36. Logic tree In zones that include subduction zones, we use 30 percent strike-slip and 70 percent reverse in the ground-motion models. In all other zones, we use half strike-slip and half normal faulting mechanisms in the ground-motion models,

37. Hazard from subduction zones and seismicity ( 1 s SA)

38. Sensitivity

43. Maps and deaggregations

44. Seismic hazard map (10% PE of exceedance in 50 years for 1 s SA)

45. Seismic hazard map (2% PE of exceedance in 50 years for 1 s SA)

46. Seismic hazard map (10% PE of exceedance in 50 years for 0.2 s SA)

47. Seismic hazard map (2% PE of exceedance in 50 years for 0.2 s SA)

48. Deterministic analysis

49. Deaggregation for Pago Pago Contribution to hazard is shown by the height from one color to the next color.