Detailed space-time variations of shallow seismic velocities at the San Jacinto fault zone based on autocorrelation analysis of dense array data

Luis F. Bonilla, & Yehuda Ben-Zion

Published August 12, 2019, SCEC Contribution #9470, 2019 SCEC Annual Meeting Poster #179 (PDF)

The Sage Brush Flat (SGB) site in the trifurcation area of the San Jacinto fault zone (SJFZ) southeast of Anza, California, had a dense array of 1,108 vertical ZLand nodes recording continuous waveforms for ~30 days in spring of 2014. The dense array covered ~ 600 m x 600 m region around the Clark branch of the SJFZ with a core grid consisting of 20 rows perpendicular to and centered on the fault trace. Each row had 50 sensors at a nominal 10 m interstation spacing and the nominal separation between rows was 30 m. The array recorded earthquake and noise data continuously at 500 samples/sec. We use the dense array data to analyze changes of seismic velocities with autocorrelation functions of moving time windows within the continuous waveforms. The recordings include signals generated by small earthquakes and various other sources of ground motion (e.g. air/train traffic, wind shaking obstacles above the ground). We examine velocity changes obtained at different frequency bands (10-150 Hz) related to different depth sampling. The response of the network is highly variable and is likely related to the local structure underneath the array. The spatial distribution of velocity changes is similar for all examined earthquakes. However, the time of the recovery process following velocity drops strongly depends on the used frequency band. Non-earthquake sources of ground motion produce similar changes as seismic events. Separating earthquakes from the other sources of motion is important for improving the knowledge on velocity changes at shallow depths.

Key Words
velocity changes, autocorrelation functions, dense array, San Jacinto fault

Citation
Bonilla, L. F., & Ben-Zion, Y. (2019, 08). Detailed space-time variations of shallow seismic velocities at the San Jacinto fault zone based on autocorrelation analysis of dense array data. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)