Monitoring Stress-Induced Seismic Velocity Changes At SAFOD Using
Crosswell Continuous Active-Source Seismic Monitoring (CASSM)
Abstract
Monitoring of in-situ, stress-induced, seismic velocity change provides
an increasingly important contribution to the study of the earthquake
nucleation process. Continuous Active-Source Seismic Monitoring (CASSM)
with borehole sources and sensors has proven to be a very effective tool
to monitor seismic velocity and to identify its temporal variations at
depth. Since June 2017, we have been operating a crosswell CASSM field
experiment at the San Andreas Fault Observatory at Depth (SAFOD) where a
previous CASSM experiment identified the two seismic velocity reductions
approximately 10 and 2 hours before micro-earthquakes. The ultimate goal
of our experiment is to continuously monitor tectonic stress for the San
Andreas Fault near seismogenic depth. Our active-source experiment makes
use of two boreholes drilled at the SAFOD project site. A piezoelectric
source and a three-component accelerometer have been installed in the
SAFOD pilot and main holes, respectively, at about 1 km depth. A seismic
pulse is generated by the piezoelectric source four times per second,
and waveforms are recorded with a 48 kHz sample rate, with recordings
summed for 1 to 10 minutes to capture seismic velocity changes at a
high-temporal resolution. Since deployment in June 2017, and as of July,
2019, local seismicity has not been above our current threshold of
detection. However, we have identified a velocity reduction at the SAFOD
site (0.5 microsecond change in crosswell travel time, measured in a
coda window) possibly induced by dynamic stress changes from the distant
6 July 2019 M 7.1 Ridgecrest earthquake, California. We will
characterize and report the co-seismic change and post-seismic recovery
process for this remotely triggered velocity change. We will also report
on the overall status of this unique CASSM experiment.