S062-0017
Inverting temporal velocity changes at SAFOD from cross-well active source seismic data using a Markov Chain Monte Carlo approach
Wednesday, 16 December 2020
Poster
Fenglin Niu, Rice University, Earth, Environmental and Planetary Sciences, Houston, TX, United States, Hongrui Qiu, University of Southern California, Los Angeles, CA, United States, Taka'aki Taira, Univ California Berkeley, Berkeley, CA, United States, Thomas M Daley, Lawrence Berkeley Lab, Berkeley, CA, United States, Michelle Robertson, Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA, United States, Pierpaolo Marchesini, Lawrence Berkeley National Laboratory, Earth & Environmental Sciences, Berkeley, CA, United States and Todd Wood, Lawrence Berkeley National Laboratory, Berkeley, CA, United States
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.
To obtain robust temporal changes in delay time from the repeated active source recordings, we employed a Markov Chain Monte Carlo approach to avoid uncertainties introduced in the selection of reference trace. We have identified a velocity reduction at the SAFOD site right after the 6 July 2019 M 7.1 Ridgecrest earthquake and other changes, which we will investigate and report in our presentation.
