Two parallel strike-slip fault zones east of Cape Mendocino are the apparent northward continuation of a northwest-trending line (San Andreas fault) that bifurcates into two great en echelon, recently active right-slip fault zones. North from Hollister, the eastern fault zone–the Hayward–Lake Mountain fault system–extends past Arcata onto the continental shelf southwest of Crescent City. The Hayward–Lake Mountain fault system defines the Humboldt plate, a small northwest-elongate sliver of the North American continent bounded on the west by the San Andreas fault zone and on the north by the Gorda plate. This newly recognized plate is converging northeastward against the Gorda plate, which is being thrust beneath it.

Steps between en echelon member fault zones in the Hayward–Lake Mountain fault system suggest that the line of faults is so recent that there has been insufficient time to integrate the fault zones into a single surficial break. The fault system may have resulted from a recent and still continuing realignment of the North American plate boundary near the Mendocino triple junction.

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