Was the 23 November 1873 California-Oregon Border Earthquake an Inslab Earthquake?

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

The 23 November 1873 earthquake along the coast near the California-Oregon border is the largest historic earthquake in that area. Oregon and California newspaper reports from 1873 suggest that this M~6.75 earthquake occurred within the subducting oceanic Juan de Fuca slab. Historically, few felt aftershocks have followed inslab earthquakes beneath the Puget Lowland, whereas crustal or megathrust earthquakes near the coast are expected to have many more. Thus, an inslab origin for the 1873 earthquake is more consistent with the only three felt aftershocks reported following it. Newspapers reported that the region experiencing the highest seismic intensities (MMI=VII and VIII) was bounded by Crescent City, California, to the south, Port Orford, Oregon, to the north, and Jacksonville, Oregon, to the east. Subject to the caveat that we lack felt reports from offshore, these highest intensities are centered at 42.3°N and 123.9°W, about 35 to 40 km northeast of proposed intensity centers of Toppozada et al. (1981) and Bakun (2000). Newspaper reports that did not indicate that the earthquake was felt locally imply that the earthquake was not generally felt south of the southern edge of the subducting slab in northern California. Felt reports at San Francisco and Sacramento are the exception, perhaps related to local site amplification at those cities, and appear to represent outliers in that each city was surrounded by communities whose newspapers did not report that the earthquake had been felt locally. In addition, the earthquake was not generally reported felt east of locations where the top of the subducting slab is more than 80 km deep. The abrupt truncation of felt reports along the southern edge of the subducting slab and to the east would appear to be most consistent with an inslab earthquake. Depth contours to the top of the subducted slab at the location of the highest intensities indicate an earthquake depth of at least 20 km.

Presenting Author: Tom Brocher

Authors