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Revisiting an Enigma on California's North Coast: The Seismotectonics of the M6.5 Fickle Hill Earthquake of December 1954

Description: 

Many earthquakes occur along the North Coast of California around the Mendocino Triple Junction (MTJ). There the Pacific, Gorda and North American (NA) plates meet, forming the Mendocino and San Andreas faults and the Cascadia subduction zone (CSZ). Historically, the majority of large earthquakes around the MTJ have been intraplate – within the Gorda plate offshore and in its subducted portion beneath the NA plate. On land north of the MTJ, many active faults mapped in the NA plate are interpreted as part of the CSZ fold and thrust belt. While some events have been detected in the NA plate, no large historic events have been clearly identified as associated with mapped surface faults with one possible exception, the M6.5 earthquake of 21 Dec 1954 near Fickle Hill, CA, in Humboldt County. The location and seismotectonics of this event remain enigmatic, as it occurred well before the present era of broadly available, digital seismic data. It was well recorded for the time, by two nearby accelerometers and other stations of the US Coast and Geodetic Survey (USCGS), on seismographs operated by UC Berkeley and the California Seismological Lab in Pasadena, CA, and at other regional and teleseismic sites. The 14 epicenters given in the literature cluster between 40.78N and 124.17W in the SW and 40.94N and 124.0W in the NE. Using  published catalog data, unpublished data from Berkeley’s archives, and digitized accelerometer records, we redetermine the earthquake’s hypocenter. Its most likely location is just east of Arcata at 40.87N, 124.03W, at ~11 km depth. P-wave polarities read from Berkeley station records and the digitized waveforms from the two accelerometer stations are consistent with a reverse mechanism. With the depth uncertainties, this implies that the source is more likely to be the subduction interface or the NW-SE trending crustal Mad River Fault zone. We update the ShakeMap using reinterpreted detailed felt/damage reports from the USCGS, newspaper archives and eyewitness accounts. The intensity distribution also supports a relatively shallow location beneath Fickle Hill, to the east of Arcata.

Session: Unusual Earthquakes and Their Implications - I

Type: Oral

Date: 4/16/2025

Presentation Time: 03:00 PM (local time)

Presenting Author: Margaret

Student Presenter: No

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