What Is the Potential for Large Deep Intraslab Earthquakes Beneath Northern California

Date: 4/25/2019

Time: 09:30 AM

Room: Cascade II

The subducting Gorda plate of the southern Cascadia subduction zone (CSZ) extends eastward to the western edge of the Sierra Nevada covering a large portion of northernmost California. If there exists an active Wadati-Benioff (WB) zone, there could be a significant seismic hazard for a large portion of northern California similar to western Washington. The USGS in the National Seismic Hazard Maps models the WB zone (depths > 40 km) uniformly north to south in terms of maximum magnitude (Mmax) but estimates the rates of large events for three regions: western Washington, western Oregon, and northwest California. There are obvious changes in the WB seismicity from north to south and in the physical, thermal, and stress conditions along the Juan de Fuca and Gorda plates (e.g., Wong, 2005; Wada et al., 2010). The USGS assigns Mmax of Mw 7.45 and 8.0 to the WB zone, weighted equally, based on global analogs although the largest known CSZ WB event is the 2001 Mw 6.8 Nisqually earthquake beneath Puget Sound. Despite the observation there are fewer than 20 earthquakes of Mw > 3.0 that have been observed in the Gorda WB zone deeper than 40 km since 1980, the USGS calculates that Mw 7.2 to 7.5 earthquakes have a recurrence interval of 4761 yrs compared to 1563 yrs beneath Puget Sound. The long recurrence intervals for the Gorda WB zone are obviously a significant extrapolation of the 150-year historical record where no WB earthquake larger than Mw 5 has ever been observed at depths > 40 km beneath northern California. In fact, all large earthquakes (Mw > 6) in the Gorda plate have been shallow and occurred much farther to the west generally off the coastline. Three potential issues regarding the USGS modeling of the Gorda WB zone will be addressed: 1) why are there so few deep earthquakes and is there an active internally deforming WB zone; 2) is the Mmax range of Mw 7.45 to 8.0 appropriate; and 3) how best to model the WB zone and address epistemic uncertainties based on the available including a small historical catalog.

Presenting Author: Ivan G. Wong

Authors