BASIN GENERATED SURFACE WAVES IN SAN FRANCISCO BAY

摘要

Site response is often complicated by the regional conditions beyond the local geology directly beneath the site. A prime example is the Treasure Island (TI) and Yerba Buena Island (YBI) sites in the San Francisco Bay, where surface waves propagate in the shallow sediments at TI but are absent at the nearby rock site, YBI. Ground motions recorded during three earthquakes at the Treasure Island Geotechnical Array (TIGA) are evaluated to assess the consistency of basin-generated surface waves in the San Francisco Bay. The TIGA recorded ground motions at up to seven levels in the soil (surface, 7 m, 16 m, 31 m, 44 m) and in the underlying bedrock (104 m and 122 m). The soil conditions at the site consist of 12 m of fill overlying 15 m of soft marine sediments (young bay mud) over dense sand and overconsolidated stiff clays (old bay clays) to bedrock at 91 m depth. The three earthquakes are the 4 December 1998 Richmond event (M_w=4.0), the 18 August 1999 Bolinas event (M_w=4.6) and the 3 September 2000 Yountville event (M_w=5.0). Each earthquake represents a different azimuth to the TIGA encompassing approximately 90°. Basin generated surface waves exist at TIGA for all three earthquakes and show similar character in terms of frequency (0.6-2 Hz) and duration (>20 seconds). Finite-difference waveform modeling, based on a simplified two-dimensional velocity structure of the northern San Francisco Bay, was used to predict and analyze the wavefield for the 18 August 1999 Bolinas earthquake ground motions. Linear site response at TI was found to be controlled by horizontally propagating surface waves trapped in the Bay sediments. The best-fit model included a single 100 m sedimentary basin (constant shear wave velocity of 400 m/s) over a 1.5 km/s layer of Franciscan bedrock, which trapped energy in the basin and produced ringing ground motions similar to that observed in the data.