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

Last updated on 03-30-15. The revised website can be found at SNapa_Noise.

We analyze ambient seismic noise wavefield to monitor temporal variations in seismic velocity associated with the 24 August 2014 Mw 6.0 South Napa earthquake.

Temporal Evolution in Seismic Velocity

We estimate relative velocity changes (dv/v) with MSNoise [Lecocq et al., 2014]. To reduce the uncertainty of the dv/v measurement, we now use continuous waveforms collected at 10 seismic stations that locate near the epicenter of the 2014 South Napa earthquake.

The initial result with the eight stations can be found at SNapa_Noise.09-16-14 (last updated on 09-16-14). The new set of 10 stations also suppresses an effect of the reduced number of station pairs used in the time interval 2014.160-2014.235.

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 determine 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.

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.

Map View

Figure 2. Map view of stations and the South Napa earthquake. The 10 stations used is a combination of the Berkeley Digital Seismic Network & the Northern Hayward Fault Network operated by UC Berkeley, the Northern California Seismic Network operated by U.S. Geological Survey, and the EarthScope Plate Boundary Observatory operated by UNAVCO, Inc.


This work is partially support by the France-Berkeley Fund 2014-0051 and the National Science Foundation EAR-1053211. Waveform data, metadata, and earthquake catalog for this study were accessed through the Northern California Earthquake Data Center (NCEDC) [NCEDC, 2014].


Brenguier, F., M. Campillo, C. Hadziioannou, N. M. Shapiro, R. M. Nadeau, and E. Larose (2008), Postseismic relaxation along the San Andreas Fault at Parkfield from continuous seismological observations, Science, 321, doi:10.1126/science.1160943.

Lecocq T., C. Caudron, and F. Brenguier (2014), MSNoise, a Python Package for Monitoring Seismic Velocity Changes Using Ambient Seismic Noise, Seismological Research Letters, 85(3), 715‑726, doi:10.1785/0220130073.

NCEDC (2014), Northern California Earthquake Data Center, UC Berkeley. Dataset, doi:10.7932/NCEDC.