Seismology and Earthquake Ground Motions of the August 24, 2014 M6 South Napa Earthquake
Tadahiro Kishida1, Shanshan Wang1, Silvia Mazzoni1, Christopher Markam1, Yuan Lu1, Yousef Bozorgnia1, Stephen Mahin1, Jonathan Bray1, Marios Panagiotou1, Jonathan P. Stewart2, Robert B. Darragh3, Norman A Abrahamson4, Justin Chow Hollenback1, Carlos Gutierrez5, Brian Chiou6, Sifat Muin1 and Douglas Scott Dreger1, (1)University of California Berkeley, Berkeley, CA, United States, (2)University of California Los Angeles, Civil and Environmental Engineering, Los Angeles, United States, (3)Pacific Engineering and Analysis, El Cerrito, CA, United States, (4)Pacific Gas and Electric Company, San Francisco, United States, (5)California Geological Survey Sacramento, Sacramento, CA, United States, (6)California Department of Transportation, Sacramento, United States
Abstract:
The
M6.0 South Napa earthquake produced strong ground motions in the northern San Francisco Bay area. A total of 214 three-component uncorrected digital accelerograms were downloaded from the CESMD website and processed following the PEER standard procedure (Ancheta et al. 2014). Intense ground motions were recorded in the heavily damaged area of Napa with peak acceleration greater than 0.3 g. Pulse-like waveforms were observed in several of the velocity time series at the near-fault stations. Near-fault velocity time series were rotated into fault normal and fault parallel directions and then characterized as pulse-like or non pulse-like according to previous studies by Hayden et al. (2014), Shahi (2013), and Lu and Panagiotou (2014). The near-fault velocity time series at five stations contained pulses with periods within the expected range of 0.7 s to 2.0 s for soil sites (Bray et al. 2009). However, they also contained longer period pulses than the expected range.
High-frequency spikes were recorded at Carquinez Bridge Geotechnical Array #1 (CBGA1) of approximately 1.0 g on the NS component. These spikes were in the S-wave portion and were consistently observed in the downhole arrays and several other sites along the same azimuth from the source. The spikes increase in amplitude both from the Hwy 37/Napa River East Geotechnical Array to CBGA1 and from a depth below 100 m to the surface. This suggests that the spikes could be a result of path effects and site amplification through the surficial soft soil deposits. However, these observations do not exclude the possibility of soil-structure interaction effects on the measured recordings.
The 5% damped pseudo-spectral accelerations (PSA) from the recorded ground motions compared well to those estimated from the recent NGA-West2 GMPEs. The exceptions are that PSA is under predicted from 1 to 3 seconds at several near fault records due to the velocity pulses and for short periods at Carquinez Bridge where the large high-frequency spikes in acceleration were recorded. The PSA of the recorded ground motions are compared to ASCE 7-10 design spectra and USGS uniform hazard spectra (UHS). The PSA exceeded the MCE spectra and the 2475-year UHS within the range of 1 to 2 s at two Napa stations, and short periods at CBGA1.
