Imaging and monitoring temporal changes of shallow seismic velocities at the Garner Valley near Anza, California, following the M7.2 2010 El Mayor-Cucapah earthquake

Lei Qin, Yehuda Ben-Zion, Luis F. Bonilla, & Jamison H. Steidl

Published August 1, 2019, SCEC Contribution #9318, 2019 SCEC Annual Meeting Poster #041

Recordings from ~700 events in 2010 at Garner Valley Downhole Array (GVDA) stations in southern California at various depths (0 m, 6 m, 15 m, 22 m, 50 m, and 150 m) are used to analyze the shallow velocity structure and temporal changes of seismic velocities after the 2010 M7.2 El Mayor-Cucapah (EMC) earthquake. Travel time differences of direct P- and S- wave travel times between surface and borehole stations reveal very low shear wave velocity (178-241 m/s) and high Vp/Vs ratio (6.3) in the top 15 m. Seasonal variations of S-wave velocities in the top 6 m are observed. Temporal changes of seismic velocities after the EMC earthquake are estimated using autocorrelations of free surface reflections at two borehole stations (22 m and 50 m) and seismic interferometry between multiple GVDA station pairs. The S-wave velocity in the top 6 m drops abruptly by 14.3±3.3% after the passage of surface waves from the EMC event with a peak ground acceleration of 39 Gal, and recovers in ~100-200 sec. The average velocity reductions decrease with depth and are 10.9±3.1%, 9.6±2.1%, 6.3±2.1%, and 4.5±2.0% in the top 15 m, 22 m, 50 m, and 150 m, respectively. Comparisons of the seismic interferometry results between the 0-22 m and 22-150 m sensors suggest no velocity changes below 22 m. Pore pressures recordings are in phase with the surface displacement and reach maxima when the highest velocity drop occurs, implying fluid interactions with the soft surface material likely contribute to the observed velocity reductions.

Citation
Qin, L., Ben-Zion, Y., Bonilla, L. F., & Steidl, J. H. (2019, 08). Imaging and monitoring temporal changes of shallow seismic velocities at the Garner Valley near Anza, California, following the M7.2 2010 El Mayor-Cucapah earthquake. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology