Regional Design Ground Motion Criteria for the Southern Bering Sea

This paper discusses the development of earthquake design criteria for the Southern Bering Sea—Aleutian Shelf region. The study reported herein was primarily motivated by a high level of seismic activity along the Aleutian Shelf and its potential impact on the design of future offshore and onshore facilities in the region. The criteria were developed for the Strength Level Earthquake (SLE) and Ductility Level Earthquake (DLE). The SLE has an average return period of 200 yr, while the DLE has a return period on the order of 1000’s of years. The criteria were based on the results of probabilistic and deterministic seismic hazard analyses of the region. The probabilistic seismic hazard analyses (SHA) were performed to establish 5 percent damped pseudovelocities (PSV) associated with an average return period of 200 yr for various sites in the region. Although seven seismotectonic provinces were considered, the larger moment magnitude events (M_w > 8.5) occurring in the shallow subduction province contribute most to the 200-yr PSV. A DLE event of M_w 9.2 was selected deterministically to be the maximum earthquake likely to occur in the region based on the tectonics and the historic seismicity. Both SLE and DLE events are associated with major ruptures along the postulated Shumagin-Unalaska seismic gap. Because no earthquake records exist for such severe earthquakes, the design ground motion criteria were partly established using a computer program to simulate ground motions for giant subduction zone earthquakes. Because the SLE and DLE events are similar and because ground motions do not increase significantly beyond M_w = 8.5, the DLE pseudovelocity spectra are about only 10 to 20 percent greater than the SLE spectra. The SLE spectra were compared with the API RP 2A [1] design spectra for this region and significant differences were noted for the region north of the Aleutian Islands. The revised zoning map recommended in this study is believed to provide more realistic standards for the seismic design of future facilities in the region.