Revisiting an Enigma on California’s North Coast

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.