Poster #209, Ground Motions

Detailed space-time variations of shallow velocity changes from dense array data

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Poster Presentation

2020 SCEC Annual Meeting, Poster #209, SCEC Contribution #10258
We compute high resolution space-time variations of subsurface seismic properties from autocorrelation functions (ACF) of noise and local earthquakes recorded at the Sage Brush Flat dense array, deployed around the Clark branch of the San Jacinto fault. The resolved temporal changes are referred to as apparent velocity changes because they reflect both nonlinear response and genuine changes of properties. Apparent velocity changes are estimated at four frequency bands (10-15, 10-20, 15-30, and 20-40 Hz) for two local earthquake data sets. In both cases the ACF from ambient seismic noise at each station is used as a local reference. In one analysis, stacked ACF’s from P and S-wave windows o...f 31 small events with magnitudes below 3.1 are used to compute the apparent velocity variations. In a second analysis, temporal evolution of properties is computed using moving time windows in continuous waveform over one-hour long data with noise and earthquake signals. The apparent velocity changes and recovery time are frequency dependent and present a strong spatial variability across the array. The resolved changes are larger and recovery time shorter with data associated with higher frequencies. Using 10-20 Hz, the apparent changes during the passage of the S- and P-waves from the small local events show average velocity changes over the whole network of 15% and 6%, respectively. The apparent velocity changes decrease to less than 1% for frequencies near 10 Hz. The relatively large changes in response to very low ground motion have important implications on nonlinear processes involving degradation and healing of the subsurface material during common earthquake shaking.