Monitoring seismic velocity changes associated with the 2014 Mw 6.0 South Napa earthquake

We analyze ambient seismic noise wavefield to explore temporal variations in seismic velocity associated with the 24 August 2014 Mw 6.0 South Napa earthquake. We estimate relative velocity changes (dv/v) with MSNoise [Lecocq et al., 2014, SRL] by analyzing continuous waveforms collected at 10 seismic stations that locate near the epicenter of the 2014 South Napa earthquake. Following Brenguier et al. [2008, Science], our preliminary analysis focuses on the vertical component waveforms in a frequency range of 0.1-0.9 Hz. We determine the reference Green’s function (GF) for each station pair as the average of 1-day stacks of GFs obtained in the time interval, January through July 2014. We estimate the time history of dv/v by measuring delay times between 10-day stacks of GF and reference GF. We find about 0.07% velocity reduction immediately after the 2014 South Napa earthquake by measuring the delay times between stacked and reference GFs. Our preliminary result also reveals a post-seismic relaxation process. The velocity reduction is down to 0.04% about 20 days after the 2014 South Napa earthquake. We have implemented an automated system to monitor the time history of dv/v (http://earthquakes.berkeley.edu/~taira/SNapa/SNapa_Noise.html) by using waveforms archived at the Northern California Earthquake Data Center. We will characterize the detailed temporal evolution of velocity change associated with the 2014 South Napa earthquake.