The Alaska Seismic Hazard Model

C. Mueller & The NSHM Project

U.S. Geological Survey

Golden, Colorado

USGS Seismic Hazard Maps

• Seismic hazard maps & engineering-design maps for

building codes and other applications

• 48 States + Alaska + Hawaii + US Territories

• Probabilistic methodology

Models for seismic sources & ground motions

Uncertainties, logic trees with weights

Multidisciplinary

• Time-independent probabilistic ground motions with a 2%

and 10% probability of exceedance in 50 years

Basic Methodology

• Faults: slip-rate/recurrence => eqk rate model (MFD)

• Catalog: completeness, recurrence, zones => eqk rate

model (MFD)

• Ground-motion models (GMMs)

• Uncertainties

• Hazard computed for sites on a grid

• At each site:

Loop over source MFDs, source-to-site distances, GMMs

Sum up ground-motion exceedances (hazard curve)

• Choose a risk level & map the probabilistic ground motions

The 2007 hazard model (Wesson and others, OFR 2007-1043) extends &

refines the 1999 model (OFR 99-36)

✓ Implications of the 2002 MW 7.9 Denali earthquake

Better characterization of Susitna Glacier – Denali – Totschunda system

Improved paleo-earthquake record

✓ New geologic information for Quaternary faults on Kodiak Island

✓ Better characterization of megathrust segmentation, recurrence, geometry

✓ Update fault slip rates: Fairweather – Queen Charlotte, Transition

✓ Update characterization of Castle Mountain fault

✓ Extend and improve seismicity catalog

✓ Update ground-motion models

✓ Update hazard codes

Physiographic features and crustal faults used in the hazard calculations (red).

(Wesson and others, USGS OFR 2007-1043, Fig.1)

Earthquake Catalog (MW 5.5+, 1990-2004)

(Wesson and others, USGS OFR 2007-1043, Fig.2)

Catalog mix:

Centennial (Engdahl & Villasenor) +

U.S. Earthquakes (Stover & Coffman) +

PDE + ISC + AEIC

Large Historical Earthquakes

(Wesson and others, USGS OFR 2007-1043, Fig.3)

(Wesson and others, USGS OFR 2007-1043, Fig.4)

Earthquake Mechanisms & Pacific Plate Motion

2007 Models for Megathrust Sources

The segmentation model was based on:

• Rupture areas of past large eqks

• Tectonic features that block through-going rupture

- e.g., decoupling at Shumagin

- e.g., transition from continental to island-arc subduction

• Variations in subduction geometry or coupling

- e.g., highly oblique subduction in the western Aleutians

• Also:

- spatial variations in recurrence mode (characteristic/G-R)

- spatial variations in maximum magnitude

- our judgment about persistence

Plate convergence rates & directions across the megathrust

are known, but a significant fraction of the deformation is

aseismic – up to 90% in some places.

Modeled rates of large earthquakes based solely on plate

convergence rates are too large, so parts of the megathrust

recurrence model are based on the seismicity catalog.

Recurrence Data

Recurrence of 1964-type eqk from geologic studies

• Copper River Delta & Cook Inlet => ~750 yrs

• Girdwood => six great eqks in 3300 yrs = ~650 yrs

Kodiak Island

• Participated in 1964 and previous 1964-type eqks

• Also ruptures more frequently in somewhat smaller eqks

Western Aleutians: recurrence behavior is not characteristic

MW 5-7

1. Make catalog 0-50 km for each sub-zone

2. Compute recurrence for sub-zone

3. Distribute rate uniformly in sub-zone

4. Compute hazard with G-R

MW 7-8 (excluding Yakataga)

1. Make catalog 0-50 km for whole megathrust

2. Compute recurrence for megathrust

3. Distribute rate uniformly in megathrust

4. Compute hazard with G-R

MW 8+ Komandorski

None

MW 8-9.2 Western Aleutians

1. Make catalog 0-50 km for zone

2. Compute recurrence for zone

3. Distribute rate uniformly in zone

4. Compute hazard with G-R

MW 8+ Shumagin

None

MW 8-8.5 Semidi

1. Make catalog 0-50 km for zone

2. Compute recurrence for zone

3. Distribute rate uniformly in zone

4. Compute hazard with G-R

MW 8+ PWS & Kodiak

PWS + Kodiak: characteristic MW 9.2, 650 yrs

Kodiak alone: characteristic MW 8.8, 650 yrs

PWS+Kodiak: b=0.738

Whole Megathrust (x-Yakataga): b=0.689

Estimating recurrence from a catalog

Transition Fault

• Thrust/oblique fault that accommodates the tectonic transition

from strike-slip to thrust deformation

• Under Gulf of Alaska; poorly known

• Tectonics & geodetics => slip ~ 12 mm/yr

• But this leads to very high hazard

• So we (somewhat arbitrarily) assume 50% coupling; 6 mm/yr

released as characteristic MW 8.2 every 325 yrs

2007 Source Models for Crustal Faults

Mix of characteristic & G-R models

2007 Source Models for

Crustal Eqks & Benioff Zone Eqks

Seismicity model for shallow eqks 0-50km: incremental rate of MW=6 eqks (/100km2/yr)

Seismicity model for Benioff eqks 50-80km: incremental rate of MW=6 eqks (/100km2/yr)

Seismicity model for Benioff eqks 80-120km: incremental rate of MW=6 eqks (/100km2/yr)

2007 Ground Motion Models

10% in 50 years 2% in 50 years

Model Update

• Catalogs

USGS PDE/ComCat + GSCanada + USHIS (other?)

Improve partitioning into crustal, interface, in-slab (use Slab 2.0?)

Best for SHA: mag 4+ since 1970?

• Crustal faults

Update fault inventory

Better model for Transition fault

Role for lake-sediment paleoseismology? GPS?

• Megathrust: geometry, segmentation, recurrence

Paleoseismology research

Better model for Yakutat block

Role for GPS? Slab 2.0? Time dependence? UCERF-type model?

• Ground-motion models

NGA-West, NGA-Subduction (other?)

PSHA technical lead: Dr. Hong Kie Thio

Figure from K. Rukstales, USGS

Segmentation proposed at the October 2018 Powell Center workshop

with megathrust 10 km & 50 km depth contours from Slab 2.0