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High-resolution analysis of 1906 earthquake intensities in the city of San José, California

N. SHOSTAK, San Jose State University, nshostak@aol.com

The goal of this study is to determine key factors controlling distribution of seismic intensities in San José, California during the April 18, 1906 earthquake. The study was conducted using a high-resolution map of shaking in the city developed specifically for this purpose. The city of San José, located 20 km east of the San Andreas fault on a sedimentary basin south of San Francisco Bay, lies on alluvium approximately 200 to 300 m deep. Over 450 official damage inspection reports completed by a group of architects and building contractors by April 26, 1906, together with photographs, sites from the 1908 Lawson report, and contemporary media and personal accounts, form the data set from which nearly 600 structures – commercial, residential, municipal and church – have been geolocated and assigned Modified Mercalli intensities (MMI). Contemporary Sanborn fire insurance maps of San José at a scale of 1" to 50’ provide construction details and precise locations of buildings. Knowledge of building detail and comments in the inspection reports eliminate most construction-related variability. The high density of data enables assignment of MMI values to areas several blocks in extent; in the center of the city, intensities can be assigned to single city blocks. With this high resolution, it is possible to correlate variations in intensity with mapped Quaternary geologic units. The resulting patterns of damage in San José indicate that shaking was not uniform throughout the city, that damage appears to be more intense in a linear band running from south to north in the central part of the city, and that areas of higher seismic intensity appear to correlate with mapped areas of Quaternary alluvial levee deposits.

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Last Modified: 2011 Aug 10

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