The Need for Alaska-Specific Ground Motion Models for Updating U.S. Geological Survey Alaska Seismic Hazard Maps

Session: Forthcoming Updates of the USGS NSHMs: Hawaii, Conterminous U.S. and Alaska
Type: Oral
Date: 4/28/2020
Time: 11:45 AM
Room: 230 + 235
Description: 

Appropriate ground motion models (GMMs) are needed to properly estimate seismic hazard in a region. The transportable array of the USArray project has provided a wealth of Alaska crustal and subduction attenuation information. Recent M5-7 inslab subduction earthquakes have provided ground motions that reveal under or over estimates from current GMMs in different portions of Alaska. Ground motions in the fore-arc region of south-central Alaska tend to be significantly higher than predicted by current GMMs. Back-arc ground motions north of the Alaskan Peninsula and west of Cook Inlet tend to be significantly lower than current GMMs for inslab earthquakes with focal depths below 80 km. The 2016 M7.1 Iniskin deep inslab earthquake and other nearby deep inslab earthquakes revealed higher than expected ground motions in the Cook Inlet and Kenai Peninsula fore-arc region due to lower attenuation raypaths through the subducting slab and into the overlying crust. They also revealed order-of-magnitude lower back-arc ground motions to the north and west of the M7.1 epicenter. The 2018 M7.1 Anchorage shallow inslab earthquake revealed higher than expected ground motions in the Cook Inlet and Kenai Peninsula fore-arc region for the mainshock but expected levels of ground motion for aftershocks. The back-arc region of lower ground motions is not seen at stations north and west of the Iniskin epicenter for the shallow inslab earthquakes like they were seen for the deep inslab earthquakes. Additionally, crustal attenuation based on Lg-waves from crustal earthquakes indicate a lower crustal attenuation throughout Alaska compared with California and the Pacific Northwest (1 Hz Q of 220 for Alaska vs. 100 to 150). Specifically in the Cook Inlet and Kenai Peninsula fore-arc region, crustal Q below 2 Hz exceeds 300, possibly due to the effect of low-angle subduction in that region. All these differences from expected ground motions affect seismic hazard estimates and will be important to incorporate into the USGS update of the 2007 Alaska seismic hazard maps.

Presenting Author: Chris H. Cramer

Authors