HR: 0830h
AN: S51E-0101    [PDF]
TI: Site Specific Analysis of Recent CSMIP Downhole Array Data
AU: * Haddadi, H
EM: hhaddadi@consrv.ca.gov
AF: California Geological Survey, Strong Motion Instrumentation Program, 801 K Street, MS13-35, Sacramento, CA 95814 United States
AU: Graizer, V
EM: vgraizer@consrv.ca.gov
AF: California Geological Survey, Strong Motion Instrumentation Program, 801 K Street, MS13-35, Sacramento, CA 95814 United States
AU: Shakal, A
EM: tshakal@consrv.ca.gov
AF: California Geological Survey, Strong Motion Instrumentation Program, 801 K Street, MS13-35, Sacramento, CA 95814 United States
AB: California Strong Motion Instrumentation Program (CSMIP) started operating downhole arrays in 1989 to provide essential data for studying the effects of local soil conditions. As of September 2003, 19 downhole arrays were instrumented, 13 of those with cooperation of California Department of Transportation (Caltrans). More than 90 low amplitude recordings from earthquakes with 2.0$<$M$<$7.1 were recorded at the arrays. The strongest acceleration of 0.5g was recorded at the La Cienega array in Los Angeles area during the M4.2 earthquake of September 9, 2001. This is a relatively small event with the epicentral distance of about 3 km and depth of 8 km (almost vertical wave incidence). A new downhole array was instrumented in May 2003 at Rohnert Park in Sonoma. The accelerometers are installed at the surface, depth of 11 m, and 47 m. The M4.3 Santa Rosa earthquake of May 25, 2003 is recorded at the array with the maximum acceleration of 0.9%g. The low amplitude recordings are used to evaluate P- and S- wave velocities at the site. The velocities obtained by correlation analysis agree with those of suspension logging. The M4.8 earthquake of September 3, 2002 is recorded at Los Angeles Obregon Park downhole array. The array was installed in 2000 and its local site geology consists of deep alluvium over sandstone. The accelerometers are installed at the surface and depth of 69 m. The recordings of the earthquake are used to correlate seismic wave motion at the surface and depth in horizontal and vertical directions. The S- and P- wave velocities obtained by correlation analysis are compared with those of suspension logging method. The new geotechnical array at Aptos in Santa Cruz is designed to study the effects of the cliff on seismic ground motion. The accelerometers are located at the surface and planned to install at depth of 30 m. Also, a three components accelerometer recorder is installed about 15 m south of the borehole toward the seacliff near bluff. The low amplitude recordings (max 1.5%g) of M4.3 earthquake of January 7, 2003 are recorded at the surface of downhole and at the bluff. The spectral ratios of motions at the surface of downhole and near the bluff are significantly different in two horizontal directions at periods shorter than about 0.5 seconds. Recent low amplitude recordings were obtained last year at the La Cienega, El Centro, Eureka and Tarzana downhole arrays and the data from Tarzana are presented in a separate paper. Directional topographic site effect is observed at Tarzana.
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2003 Fall Meeting