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 after subtracting station term and systematic offsetStation residuals Residuals = data-prediction

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

25. Comparison of Guam data and Zhao

33. PGA5HZ1HZ ZHAO -0.05420.07770.2018 Systematic offset of Pacific data with Zhao et al. (2006) model Total Sigma, distance ≤ 300 km, M ≥ 6.0 Zhao and others (2006)Geomatrix Atkinson and Boore (2003) PGA 0.92970.96851.1587 5HZ.97091.03421.0194 1HZ.7024.7434.7554

34. Source Model (depth) Calculate d b-value b-value standar d deviatio n Maximum recorded magnitude since 1900 Maximum recorded magnitud e since 1964 Maximum magnitude for calculations Rate of M≥7 s inc e 1900 Type of source S=smoot h F=fault Model source depth (km) Ground motion models Tonga Outer Rise (0–50 km) 0.990.078.28.1 (8.0*) 8.2S10Crustal Zhao and NGA New Hebrides Outer Rise (0–50 km) 1.110.107.7 8.2S10Crustal Zhao and NGA Tonga Subduction Zone interface (0–50 km) 0.870.038.57.9 (8.0*) 7.0 (see interface zone below) S10Crustal Zhao and NGA New Hebrides Subduction Zone Interface (0–50 km) 0.580.047.97.87.0 (see interface zone below) S10Crustal Zhao and NGA Fiji zone (0–50 km)0.860.037.87.78.0S10Crustal Zhao and NGA Background (elsewhere; 0–50 km) 1.010.077.1 7.3S10Crustal Zhao and NGA Background (50–100 km)0.930.058.28.18.2S75Intraslab Background (100–200 km) 0.890.047.8 8.0S150Intraslab Background (200–300 km) 0.950.057.97.18.0S250Intraslab Background (300–400 km) 0.970.078.07.28.0S350Intraslab Background (400–500 km) 1.160.097.57.38.0S450Intraslab Background (500–600 km) 0.870.057.87.68.0S550Intraslab Background (600–723 km) 0.870.077.7 8.0S650Intraslab Tonga Subduction Zone Interface (0–50 km) M7–M9, 1900–2010 0.87 (fixed)* * 8.57.9 (8.0(fix ed)* ) 8.5, 9.022 events / 110 years FH=25, top of zone is 10 km Inter-face New Hebrides Subduction Zone Interface (0–50 km) M7–M9, 1900–2010 1.0 (fixed)* * 7.97.88.5, 9.032 events / 110 years FH=25, top of zone is 10 km Inter-face

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.

52. Geographic deaggregation ( 1 s top, 0.2 s bottom)

53. Conclusions Model seismicity rates defined by historic seismicity We don’t know of any crustal faults on or near American Samoa For Pago Pago outer rise earthquakes and subduction interface earthquakes dominate hazard For Pago Pago crustal GMPE’s are the most important For Pago Pago we apply equally weighted Zhao et al. (2006, class 1-rock of about 600 m/s) and 3 NGA equations (NEHRP B/C, Vs30=760) Even though the ground motions are half the current IBC, we feel that the model for Pago Pago includes plausible sources and ground motion models and represents a reasonable estimate of the hazard