Alaska (ALS)

Authors:

R.L. Wesson, O.S. Boyd, C.S. Mueller, C.G. Bufe, A.D. Frankel, M.D. Petersen

Summary

The 2007 seismic hazard model for Alaska (USA) was developed by the United States Geological Survey (Wesson et al., 2007). The model covers the state of Alaska including the Aleutian Islands. The model has been translated from its original format into the OpenQuake (OQ) engine by GEM.

Information about the OQ model versions and input files can be found on the Results and Dissemination page.

Interactive Viewer

The viewer below depicts the seismic sources and hazard results in terms of PGA for a return period of 475 years. Click on the menu in the upper right corner to select the layer.

viewer_legend

Tectonic overview

Alaska sits at the northwestern edge of the North American continent. Active faulting in the region is related to the interaction of the neighboring Pacific and Eurasian plates with North America. The Pacific plate subducts underneath central and western Alaska at the Aleutian subduction zone; plate convergence is about 60 mm/yr at the eastern end of the subduction zone and 80 mm/yr in the west. The 1964 Gulf of Alaska earthquake has been estimated at \(M_w\) 9.2, one of the largest earthquakes in history; this event occurred on the Aleutian megathrust. Relative motion between the Pacific and North American plates is oblique, and the substantial strike-slip component is taken up on faults in the interior of Alaska; this slip is mostly localized on the right-lateral Denali fault system, which hosted the 2001 \(M_w\) 7.9 Denali earthquake. The Denali fault slips about 10 mm/yr. Additional faults in the upper plate of the subduction zone, such as the Fairweather Fault and the Chugach-St. Elias thrust belt, may be important sources of earthquakes in the relatively populated south of Alaska. However, active faults relating to the relative motions of Eurasia and the hypothesized 'Bering microplate' in western Alaska are distributed throughout the state and pose some hazard.

Source: Yeats, 2012. Active Faults of the World. Cambridge University Press.

Basic Datasets

See Wesson et al. (2007) for a description of the datasets used for developing the hazard model.

Hazard Model

Seismic Source Characterisation

The seismic source characterisation (SSC) consists of various seismic source typologies to describe earthquake occurrence in different tectonic settings. Distributed seismicity is used to model both active shallow and deep intraslab seismicity, while fault sources are used to model seismicity occurring on shallow crustal faults and large subduction interface events.

The OQ implementation uses three OQ source typologies. The background (gridded) seismicity models are implemented as collections of Point Sources. Crustal faults are modelled using Simple Fault Sources, and and subduction faults are modeled using Complex Fault Sources. The OQ sources are depicted in the interactive viewer.

Epistemic Uncertainties

The SSM considers epistemic uncertainty in the characterisation of active shallow crustal faults. Each is modelled with both characteristic and Guteberg-Richter (GR) magnitude frequency distributions (MFDs), with the magnitude depending on the MFD type; the two occurrence models are weighted equally in the logic tree. In the case of the Fairweather fault, the characteristic MFD considers the on- and offshore fault lengths separately, while the GR MFD treats it as a single fault.

Ground Motion Characterisation

The table below shows the ground motion characterisation (GMC), which is comprised of a set of ground motion prediction equations (GMPEs). The GMM for the 2007 Alaska model distinguishes between three main tectonic regions: Active Shallow Crust, Subduction Interface, and Subduction IntraSlab.

Epistemic Uncertainties

For every tectonic region, epistemic uncertainty is considered by using multiple GMPEs, each with an associated logic tree weight.

Active Shallow Crust Weight
BooreEtAl1997GeometricMean 0.25
SadighEtAl1997 0.25
AbrahamsonSilva1997 0.25
CampbellBozorgnia2003NSHMP2007 0.25
Subduction Interface Weight
YoungsEtAl1997SInterNSHMP2008 0.5
SadighEtAl1997 0.5
Subduction IntraSlab Weight
YoungsEtAl1997SSlab 0.5
AtkinsonBoore2003SSlabNSHMP2008 0.5

Results

Hazard curves were computed with the OQ engine for peak ground acceleration (PGA) and spectral acceleration (SA) at 0.2s, 0.5s, 1.0s, and 2s. The computation was performed on a grid of 29904 sites (spaced at approximately 10 km) with reference soil conditions corresponding to a shear wave velocity in the upper 30 meters (Vs30) of 760-800 m/s.

The hazard map for PGA corresponding to a 10% probability of exceedance in 50 years (475 year return period), can be seen using the interactive viewer. For a more comprehensive set of hazard and risk results, please see the GEM Visualization Tools.

References

Wesson, Robert L., Boyd, Oliver S., Mueller, Charles S., Bufe, Charles G., Frankel, Arthur D., Petersen, Mark D., 2007, Revision of time-Independent probabilistic seismic hazard maps for Alaska: U.S. Geological Survey Open-File Report 2007-1043