AFTERSHOCK OBSERVATION AND S-WAVE VELOCITY SURVEYS AFTER. THE 2014 SOUTH NAPA EARTHQUAKE

摘要

A M6.0 earthquake occurred in South Napa County on August 24, 2014. We recorded aftershocks and conducted S-wave velocity (VS) surveys using active and passive surface wave methods at four sites in Napa. Portable accelerometers were deployed at three locations for aftershock observation on the day after the mainshock. One of the three sites is located directly on the line of the surface rupture. The accelerometers recorded continuous data for two weeks. At least 550 aftershocks with magnitudes between -0.6 and 3.9 were recorded during a one-week period starting the day after the mainshock. Surface wave surveys were conducted at each of the three aftershock observation sites and also at Napa Valley College. Dispersion curves from active and passive methods were combined and phase velocities were obtained to a minimum frequency of 0.3 Hz. VS profiles were determined to a depth of 100 to 2000 m by joint inversion of dispersion curves and horizontal to vertical spectral ratio (HVSR) curves. Depth to bedrock with VS higher than 3000 m/s appears to be at least 2000 m at three aftershock observation sites. The VS profiles obtained by the surface wave surveys are generally consistent with site-specific differences in amplification and S-P time observed in aftershock observations.