Abstract

Humboldt County on California’s north coast was the furthest region from the 1906 epicenter to suffer significant damage in the 1906 earthquake. This study reassesses the impacts described in the state earthquake investigation commission report (Lawson, 1908) and subsequent intensity studies by examining reports from local newspapers, weather service logs, and the large collection of photographs, journals, and letters collected by local historical societies and museums. The highest intensity (modified Mercalli [MMI] IX) occurred in the Petrolia and upper Mattole Valley. The entire coastal region from Shelter Cove to the Eel River Valley experienced intensities of at least VIII. The most detailed damage descriptions come from Ferndale, where newspaper reports detail the damage to over 40 structures in the downtown area. Fewer than 2% of chimneys survived, both brick buildings were severely damaged, and about 25% of the commercial wood buildings were twisted by the ground shaking. Liquefaction was observed in the Mattole Valley throughout the Eel River Valley and as far north as Fields Landing on Humboldt Bay, which exceeded the aerial extent and scale of any other historic event, including the 1992 Cape Mendocino earthquake (Mw 7.2). In contrast to the isoseismal map published in the Lawson Report, damage in Eureka and the Humboldt Bay region was less than in Ferndale. Based on the severity of damage and scale of liquefaction, the 1906 earthquake was Humboldt County’s strongest historic event. Peak intensities likely exceeded the Cape Mendocino earthquake, the intensity VII or larger isoseismal was about twice as large, and peak ground velocity values likely exceeded 50 cm/sec in much of southern Humboldt County. The strong shaking in Humboldt County supports models showing significant slip along the northern segment of the 1906 rupture zone.

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