Revisiting an enigma on California’s north coast: The Mw6.5 Fickle Hill earthquake of 21 December 1954

Many earthquakes happen alongside the North Coast of California within the neighborhood of the Mendocino Triple Junction (MTJ), the place the Pacific, Gorda, and North American (NA) plates meet, and on the adjoining plate boundaries. The MTJ marks the nexus of the Mendocino and San Andreas faults with the Cascadia subduction zone (CSZ). Historically, most giant earthquakes across the MTJ have been inside the offshore Gorda plate and its subducted portion beneath the NA plate. North of the MTJ, energetic faults mapped within the NA plate are a part of the CSZ fold‐and‐thrust belt. Although some occasions have been detected within the NA plate, no giant historic occasions have been related to mapped floor faults. The 21 December 1954 M_w 6.5 earthquake in Humboldt County is one potential exception. Using printed information from catalogs and articles, unpublished information from Berkeley’s archives, and S‐P occasions interpreted from two U.S. Coast and Geodetic Survey (USCGS) accelerometers, we decide a likelihood cloud for the earthquake’s hypocenter utilizing NonLinLoc. The highest likelihood location lies beneath Fickle Hill simply east of the town of Arcata, California, at 40.87° N, 124.03° W, and ∼11 km depth. Using P‐wave polarities from Berkeley stations and the digitized waveforms from the accelerometers, we discover that the focal mechanism most per the info signifies thrust motion with strike, dip, and rake of 350°, 10°, and 90°, respectively, at a depth of 14 km. Given the depth uncertainties of each this occasion and the megathrust, this means that the earthquake more than likely occurred on the subduction interface fairly than on the mapped faults within the Mad River fault zone that development 322° and dip to the northeast. The revisited depth within the epicentral area additionally helps a location beneath Fickle Hill to the east of the town of Arcata, California.