Imaging and Monitoring Temporal Changes of Shallow Seismic Velocities at the Garner Valley Near Anza, California, in Relation to the M7.2 2010 El Mayor – Cucapah Earthquake

Date: 4/26/2019

Time: 01:45 PM

Room: Pine

We investigate temporal changes of seismic velocities after the M7.2 2010 El Mayor – Cucapah (EMC) earthquake, as well as the shallow velocity structure at the Garner Valley site northeast of ANZA, CA, with data from the Garner Valley Downhole Array (GVDA). The analyzed data include all events recorded by GVDA stations at various depths (0 m, i.e. surface, 6 m, 15 m, 22 m, 50 m and 150 m) in 2010. The local shallow velocity structure is first analyzed using travel times of direct arrivals between the surface and borehole stations. We observe extremely low shear wave velocity (~160 m/s) and high VP/VS ratio (~8) in the top 15 m and large variations of seismic velocities, especially the shear wave velocities, in the top 150 m (VS from ~160 m/s to ~1000 m/s and VP from 2000 m/s to 2800 m/s). The temporal changes of velocities in relation to the EMC earthquake are estimated from evolving autocorrelation functions at the two shallow stations (surface and 6 m borehole) and seismic interferometry between the surface station and borehole stations at 15 m, 22 m, 50 m and 150 m. The obtained average SH wave velocity reductions decrease with depth (~4.5% in the top 150 m, ~6.3 % in top 50 m, ~9.6% in top 22 m, ~10.9% in top 15 m, and ~13.2% in top 6-8 m). Based on the travel time and average velocity drops between different stations, the results imply that the velocity changes are concentrated in the top 22 m. The velocity reductions occur at the passage of large amplitude surface waves (~39 Gal) and the highest pore pressure variation of ~7 kPa, and are followed by rapid recovery in about 100-300 seconds. More than 10% of S wave velocity reductions in the top 15 m are produced by peak ground accelerations (PGA) smaller than 40 Gal, indicating that nonlinear site behavior may be more common and prominent than previously thought.

Presenting Author: Lei Qin

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