Comparisons of Site-Specific Ground Motion Estimates and Observations in the 2018 Anchorage M 7.0 Earthquake

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

The lack of significant structural damage in the Anchorage area in the 30 November 2018 Mw 7.0 earthquake was credited to the robust seismic design regulations and construction that occurred after the 1964 Great Alaska Mw 9.2 earthquake notwithstanding the event was at a depth of 47 km. Reported PGA values in the 2018 earthquake ranged from 0.12 to 0.47 g within hypocentral distances of 56 km. Wong et al. (2010; 2014) performed site-specific probabilistic seismic hazard analyses for the Anchorage area and more recently, for four representative sites that ranged from firm soil to soft rock (Vs30 236 to 570 m/sec). The site-specific PGA estimates for a return period of 2475 years were higher than 0.9 g. In contrast, the 2007 National Seismic Hazard Maps for Alaska (firm rock Vs30 of 760 m/sec) indicate PGA values at the same return period of about 0.7 g. The differences between our site-specific estimates and the National Seismic Hazard Maps stem not only from site conditions but also from differences in the ground motion models (GMMs) and modeling of the Wadati-Benioff (WB) zone. The 2007 USGS maps used pre-2003 GMMs and modeled the WB zone using their gridded seismicity approach. In the latter, they modeled uniform average rates on planes at constant depths between 50-80 km and 80-120 km beneath Anchorage. In our site-specific analyses, the WB zone, which controlled the PGA hazard at most return periods, was modeled based on the geometry of the dipping subducting slab. Acceleration response spectra from our site-specific studies, the National Seismic Hazard Maps, IBC, and the 2018 strong motion recordings are compared in order to assess the adequacy of the IBC design ground motions in Anchorage as compared to site-specific analyses.

Presenting Author: Ivan G. Wong

Authors