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ABSTRACT: Observations of Damage in the Marina District, San Francisco, California, during the Northern California Loma Prieta Earthquake of 1989

James A. Nikas, III, James A. Jacobs

At 5:04 p.m. on October 17, 1989, movement along the San Andreas fault zone was recorded as a 7.1 magnitude earthquake with an epicenter at Loma Prieta, about 100 km south of the Marina District in San Francisco, California. Strong shaking lasted about 15 sec. Based on historic maps, the heaviest damage in the Marina District during the 1989 Loma Prieta earthquake occurred in an environmental transition zone between Strawberry Island, a tidal inlet, a marsh, and the San Francisco Bay. This lowland area originally was the site of a landfill for debris from the 1906 earthquake. In 1915, fine-grained sand was subsequently hydraulically pumped on top of the poorly sorted earthquake debris for the Panama-Pacific Exposition. On this uncompacted, water-saturated, fine-grained sa d, residential construction was conducted in the Marina District from the 1920s to the 1940s, prior to modern earthquake building codes.

Evidence of liquefication of the fill observed after the Loma Prieta earthquake included fine-grained sand boils and extensional cracks formed by lateral spreading. Of the 11 buildings completely destroyed, and up to 60 buildings that may need to be demolished in the Marina District, most were corner-lot, three- and four-story, multifamily residences having full garages on the ground floor without any supporting shear walls. Without sufficient lateral resistance to withstand the loss of vertical support during liquefication, these buildings lacked the strength and stiffness that could have been provided by adjacent buildings. In contrast with the heavy structural damage sustained by buildings underlain by liquefied fill, buildings in the Marina District underlain by a competent sandst ne member of the Franciscan Formation sustained only minor structural damage.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990