Monitoring of Relative Velocity Change Following the 2014 Mw 6.0 South Napa Earthquake (Archived)

Temporal Evolution in Seismic Velocity¶

We estimate relative velocity changes (dv/v) with MSNoise [Lecocq et al., 2014]. We use continuous waveforms collected at eight seismic stations that locate near the epicenter of the 2014 South Napa earthquake. Following Brenguier et al. [2008], our preliminary analysis focuses on the vertical component waveforms in a frequency range of 0.1-0.9 Hz.

We estimate the reference Green’s function (GF) for each station pair as the average of 1-day GFs obtained in the time interval, January through July 2014. The time history of dv/v is determined by measuring delay times between 30-day stacks of GFs and the reference GF.

We obtain about 0.05% velocity reduction following the South Napa earthquake (Figure 1). Note that dv/v is plotted at the end of the 30-day-length time window that is used for computing 30-day stacks of GFs. Note also that the temporal variability of dv/v in the time interval 2014.160-2014.235 is likely due to the reduced number of station pairs. Three stations (NC.NAPC, NC.NSP, and NC.NMI) were temporally down in this time interval. We therefore examine the dv/v without these three stations. Although the uncertainty of the dv/v measurement is increased, we observe the change in dv/v following the South Napa earthquake (Figure 2).

To monitor the relative velocity change, we implement an automated system that updates the time history of dv/v shown in Figure 1. This automated process makes use of waveforms archived at the Northern California Earthquake Data Center [NCEDC, 2014]. Figure 1 is currently updated every day.

Figure 1. Top panel shows the number of stations pairs used to infer the relative velocity change (dv/v). The bottom panel shows the time history of dv/v. The error bars represent the two-sigma standard deviation. The dashed red line indicates the origin time of the South Napa earthquake.

Figure 2. Comparison of dv/v measurements between the two different sets of stations. Red circles are dv/v inferred with all 8 stations. Gray squares are dv/v obtained with 5 stations in which we exclude data from the 3 stations that were temporally down during 2014.160-2014.235.

Map View¶

Figure 3. Map view of stations and the South Napa earthquake.