Unusual Downhole and Surface Free‐Field Records Near the Carquinez Strait Bridges during the 24 August 2014Mw 6.0 South Napa, California, Earthquake

Abstract

This paper reports the results of Part A of a study of the recorded strong‐motion accelerations at the well‐instrumented network of the two side‐by‐side parallel bridges over the Carquinez Strait during the 24 August 2014 ( M w 6.0) South Napa, California, earthquake that occurred at 03:20:44 Pacific Daylight Time (PDT) with epicentral coordinates 38.22° N, 122.31° W (http://earthquake.usgs.gov/earthquakes/eqarchives/poster/2014/20140824.php; last accessed October 2014). Both bridges and two boreholes were instrumented by the California Strong motion Instrumentation Program (CSMIP) of the California Geological Survey (CGS; Shakal et al. , 2014). A comprehensive comparison of several ground‐motion prediction equations as they relate to recorded ground motions of the earthquake is provided by Baltay and Boatright (2015). The moniker “odd couple” is appropriate for the pair of bridges across the Carquinez Strait, because each bridge is of a different design and a different age. The westbound suspension bridge (CGS station 68185), also known as the Alfred Zampa Memorial bridge, opened for service in 2003, when it replaced an older 1927‐era‐construction cantilever truss bridge that was deemed seismically unsafe. The eastbound cantilever truss bridge (CGS station 68184) opened for service in 1958. Figure 1 shows the layout of the two bridges and the locations of the associated geotechnical arrays. The inset in the figure indicates where the bridges are with respect to San Francisco, California. A photograph of both bridges is provided in Figure 2. The azimuths of the west‐ and east‐bound bridges are listed as 358° and 357°, respectively (Center of Engineering Stron Motion Data [CESMD] at http://www.strongmotioncenter.org/NCESMD/photos/CGS/lllayouts/ll68185.pdf and http://www.strongmotioncenter.org/NCESMD/photos/CGS/lllayouts/ll68184.pdf; last accessed April 2015). These parallel bridges are important lifelines on Interstate 80, connecting the California cities of Crockett and Vallejo, the San Francisco Bay area and the Sacramento regions, and the well‐known Napa Valley wine‐country and much of the remainder of northern …