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

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

American Samoa tectonics

Depth sections

Moment tensors

South Pacific Zones for calculating b-value and Mmax

Seismicity analysis

Magnitude-frequency

4 5 6 7 8 91

10

100

1000

Comparison of rates of Samoa/Vanuatu catalogs (1900-2010)

rate all -declusteredrate all not declusteredrate 1964+ declusteredRate 1964+ not declustered

Magnitude

Annu

al ra

te o

f ear

thqu

akes

•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

Magnitude-frequency

5.25 5.75 6.25 6.75 7.25 7.75 8.251

10

100

1000

b-Values BackgroundBackground (elsewhere)0-50 (b=1.01)

Background50-100 (b=0.93)

Background100-200 (b=0.89)

Background200-300 (b=0.95)

Background300-400 (b=0.97)

Background400-500 (b=1.16)

Background500-600 (b=0.87)

Background600-723 (b=0.87)

Magnitude

N

Magnitude-frequency plot by source

5.25 5.75 6.25 6.75 7.25 7.75 8.251

10

100

1000

b-Values0-50 km

Tonga Outer Rise 0-50 (b=0.99)

New Hebrides Outer Rise 0-50 (b=1.11)

Tonga subduction 0-50 (b=0.87)

New Hebrides subduc-tion 0-50 (b=0.58)

Fiji zone0-50 (b=0.86)

Background (elsewhere)0-50 (b=1.01)

Magnitude

N

Magnitude-frequency plot Vanuatu

6.5 7.0 7.5 8.0 8.5 9.01.0

10.0

100.0

New Hebrides (Vanuatu) M>7 since 1900, M7+, 32 events

Observed - New Hebrides

predicted - New Hebrides, b=0.58

predicted - New Hebrides, b=0.87

predicted - New Hebrides, b=1

Magnitude

log

cum

ulati

ve n

umbe

r of e

arth

quak

es

Magnitude-frequency for Tonga-Kermedec trench

6.5 7 7.5 8 8.5 91

10

100

Tonga M>7 since 1900, b=0.87, M7+ 22 events

Observed-Tongapredicted-Tonga

Magnitude

log

num

ber o

f ear

thqu

akes

Model has two Mmax branches, M 8.5, 9, weighted equally

Ground motion analysis

Interface and Intraplate

Earthquakes

Interface and Intraplate, d=25 km

Interface and Intraplate Earthquakes

Intraplate and Interface Earthquakes

M 9 curves

Intraplate earthquakes by

depth (1 s and 0.2 s SA)

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.

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

Oceanic data by distance and depth

Red – interface, blue – intraplate

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

PGA station residuals

after subtracting station term and systematic offsetStation residuals

Residuals = data-prediction

PGA average single station residuals by magnitude and distance

Residuals = data-prediction

Comparison of Guam data and Zhao

Comparison of Guam data and Zhao

Comparison of Guam data and Zhao

Comparison of Guam data and Zhao

PGA 5HZ 1HZ

ZHAO -0.0542 0.0777 0.2018

Systematic offset of Pacific data with Zhao et al. (2006) model

Total Sigma, distance ≤ 300 km, M ≥ 6.0

Zhao and others (2006) GeomatrixAtkinson and Boore

(2003)

PGA 0.9297 0.9685 1.1587

5HZ .9709 1.0342 1.0194

1HZ .7024 .7434 .7554

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

since

1900

Type of

source

S=smoot

h

F=fault

Model

source

depth

(km)

Ground

motion

models

Tonga Outer Rise (0–50 km)

0.99 0.07 8.2 8.1 (8.0*)

8.2 S 10 Crustal Zhao and NGA

New Hebrides Outer Rise (0–50 km)

1.11 0.10 7.7 7.7 8.2 S 10 Crustal Zhao and NGA

Tonga Subduction Zone interface (0–50 km)

0.87 0.03 8.5 7.9 (8.0*)

7.0 (see interface zone below)

S 10 Crustal Zhao and NGA

New Hebrides Subduction Zone Interface (0–50 km)

0.58 0.04 7.9 7.8 7.0 (see interface zone below)

S 10 Crustal Zhao and NGA

Fiji zone (0–50 km) 0.86 0.03 7.8 7.7 8.0 S 10 Crustal Zhao and NGA

Background (elsewhere; 0–50 km)

1.01 0.07 7.1 7.1 7.3 S 10 Crustal Zhao and NGA

Background (50–100 km) 0.93 0.05 8.2 8.1 8.2 S 75 Intraslab

Background (100–200 km)

0.89 0.04 7.8 7.8 8.0 S 150 Intraslab

Background (200–300 km)

0.95 0.05 7.9 7.1 8.0 S 250 Intraslab

Background (300–400 km)

0.97 0.07 8.0 7.2 8.0 S 350 Intraslab

Background (400–500 km)

1.16 0.09 7.5 7.3 8.0 S 450 Intraslab

Background (500–600 km)

0.87 0.05 7.8 7.6 8.0 S 550 Intraslab

Background (600–723 km)

0.87 0.07 7.7 7.7 8.0 S 650 Intraslab

Tonga Subduction Zone Interface (0–50 km)M7–M9, 1900–2010

0.87 (fixed)**

8.5 7.9 (8.0(fixed)* )

8.5, 9.0 22 events / 110 years

F H=25, top of zone is 10 km

Inter-face

New Hebrides Subduction Zone Interface (0–50 km)M7–M9, 1900–2010

1.0 (fixed)** 

7.9 7.8 8.5, 9.0 32 events / 110 years

F H=25, top of zone is 10 km

Inter-face

Model and Results

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,

Hazard from subduction zones and seismicity ( 1 s SA)

Sensitivity

Maps and deaggregations

Seismic hazard map (10% PE of

exceedance in 50 years for 1 s SA)

Seismic hazard map (2% PE of

exceedance in 50 years for 1 s SA)

Seismic hazard map (10% PE of

exceedance in 50 years for 0.2 s SA)

Seismic hazard map (2% PE of exceedance

in 50 years for 0.2 s SA)

Deterministic analysis

Deaggregation for Pago Pago

Contribution to hazard is shown by the height from one color to the next color.

Geographic deaggregation ( 1 s top, 0.2 s bottom)

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