Documentation

Abstract

Probabilistic seismic hazard maps have been prepared for Alaska portraying ground motion values (peak ground acceleration and spectral amplitude at periods of 0.2, 0.3 and 1.0 seconds) at probabilities of exceedance of 2% and 10% in 50 years. Preparation of these maps followed the same general strategy as that followed for the USGS seismic hazard maps of the contiguous United States, combining hazard derived from spatially-smoothed historic seismicity with hazard from fault-specific sources.

Preparation of the Alaska maps presented particular challenges in characterizing the hazard from the Alaska-Aleutian megathrust. In the maps of the contiguous United States the rate of seismicity for recognized active faults was determined from slip rates estimated from geologic data. This approach is not appropriate for the megathrust because it has been demonstrated that a significant fraction of the subduction occurs aseismically. The characteristic earthquake hypothesis, based on recurrence rates determined from geologic data, is appealing for the portion of the megathrust that ruptured in the 1964 Alaskan earthquake, but is shown to be inappropriate for the western portion of the megathrust by the recent large earthquakes in the region which did not follow the characteristic model.

Consequently the hazard from the western portion was estimated based on a truncated Gutenberg and Richter model derived from historic seismicity, and the hazard for the 1964 zone was estimated from a combination of a Gutenberg and Richter model derived from historic seismicity and the characteristic earthquake hypothesis with recurrence rates estimated from geologic data. Owing to geologic complexity and limited data, hazard models of the easternmost portion of the megathrust in the vicinity of Yakataga are the least satisfactorily constrained. Hazard is estimated for the recognized crustal faults of the Denali, Fairweather-Queen Charlotte and Castle Mountain fault systems based on available geologic slip rates. Hazard from other sources is estimated from spatially smoothed historic seismicity. Disaggregations of the hazard for Anchorage, Fairbanks and Juneau reveal the dominant sources of the hazard at each location.

Introduction

Alaska is the most seismically active state in the United States, and in 1964 the site of one of the largest earthquakes since the beginning of instrumental recording. Although the current population of the region is small by comparison with, say California, the consequences of a large earthquake in the region could be much greater now than at the time of the 1964 Alaskan earthquake. The probabilistic seismic maps we have prepared are intended to extend those prepared by Frankel et al. (1996) for the 48 contiguous states, and with soon-to-be-published maps for Hawaii. Our methodology follows the basic approach of Cornell (1968). These maps are intended to summarize the available quantitative information about seismic hazard from geologic and geophysical sources. Full color maps at a scale of 1:7,500,000 are available as U.S. Geological Survey Miscellaneous Investigations Series I-2679 (Wesson et al., 1999).

The process of preparing these maps included a workshop held in Anchorage in the fall of 1996 attended by many scientists and engineers involved in aspects of earthquake research and structural engineering practice in Alaska. Preliminary calculations of hazard were presented for discussion and a number of recommendations were made that affected the subsequent preparation of the map. Draft maps were posted on the World Wide Web and circulated for comment in the fall of 1997. The maps presented here have benefited greatly from both the original workshop in 1996 and from the review comments received.

The strategy for preparing these maps is similar to that for recently prepared seismic hazard maps of the contiguous United States (Frankel et al., 1996). The maps presented here include maps for peak ground acceleration and 1.0, 3.3 and 5.0 Hz spectral acceleration at probabilities of exceedance of 10% in 50 years (annual probability of 0.0021) and 2% in 50 years (annual probability of 0.000404).

The historical instrumental seismicity of Alaska and the Aleutians for earthquakes greater than or equal to magnitude 6 is shown in Figure 1. The preparation of the earthquake catalogs used for the analyses in this report (and shown in Figure 1) is discussed in a companion report, Mueller et al., 1998). Clearly the majority of the seismicity in the region is associated with the Alaska-Aleutian megathrust fault extending eastward along the Aleutian arc into south central Alaska. The northwestward-moving Pacific plate is subducted along this megathrust beneath the North American Plate giving rise to the Aleutian trench and islands. Additional significant seismicity occurs along the northwestward-striking system of right-lateral strike-slip faults extending southeastward through and offshore of the panhandle of southeast Alaska. This system of faults forms the northeast boundary of the Pacific plate. Additional seismicity occurs in central Alaska.

The estimated rupture zones of the largest earthquakes in this century are shown in Figure 2 (Plafker et al., 1993). During this century virtually the entire plate boundary from the westernmost Aleutian Islands to the Queen Charlotte Islands off British Columbia has ruptured in large to great earthquakes. The only exceptions are areas near the Komandorsky Islands, near the Shumagin Islands, and near Cape Yakataga (Sykes, 1971; Davies et al., 1981). Near the Komandorsky Islands, historical records of large earthquakes in 1849 and 1858 at the extreme western end of the arc have been judged as insufficient to conclude that plate-margin-rupturing earthquakes have occurred there (Sykes et al., 1981; Taber et al., 1991). At this location subduction is occurring at a highly oblique angle, and it has been argued that the recurrence properties of large earthquakes here may differ significantly from those else along the arc. Indeed, Cormier (1975) has argued that the region may be incapable of supporting a great earthquake. In the vicinity of the Shumagin Islands, that is, in the region between the 1957 and 1938 earthquakes, it has been argued that no great earthquake has occurred in this century. Similarly, the vicinity of Cape Yakataga has experienced no great earthquakes in this century. These two regions have been identified as "seismic gaps," that is, the potential sites of future large earthquakes (Davies et al., 1981; Sykes, 1971, Lahr et al., 1980).

Disclaimer

This report is preliminary and has not be reviewed for conformity with the U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Although these maps have been used by the U.S. Geological Survey, no warranty, expressed or implied is made by the USGS as to the accuracy of the maps and related material nor shall the fact of distribution constitute any such warranty, and no responsibility is assumed by the USGS in connection therewith.