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Structure-Related and Stress-Induced Shear-Wave Velocity Anisotropy: Observations from Microearthquakes near the Calaveras Fault in Central California

Stanford University
Department of Geophysics, Mitchell Building B55
Stanford, California 94305
Zinke{at}pangea.stanford.edu

We attempt to distinguish between structure-related and stress-induced shear-wave velocity anisotropy in the upper crust using microearthquakes (M 2.6) that occurred before and after the Tres Piños, California earthquake (M 5.5) of 26 January 1986. The Tres Piños earthquake occurred on the Quien Sabe fault, a northwest–southeast trending right-lateral strike-slip fault, approximately 10 km east of the Calaveras fault in central California. Digital, three-component seismograms are available from 1984 to 1992 from the Quien Sabe Ranch station (HQR), located only 7.3 km north-northeast of the epicenter of the main event.

We studied polarization directions and delay times using a method that combines cross-correlation techniques and particle-motion plots. To enhance the significance of our interpretations we reduced uncertainties related to the hypocenter-location of the earthquakes by source relocation based on joint-hypocenter determination and cross correlation.

In the study area we found two zones with different but stable polarization directions. Around the Quien Sabe fault system, shear-wave polarizations of over 30 microearthquakes clearly indicate that the slow shear-wave direction is normal to the northwest-southeast trend of this vertical right-lateral strike-slip fault. Thus, velocity anisotropy appears to be caused by the fault system itself. However, anisotropy associated with a cluster of microearthquakes located about 4 km to the east is quite different. In this zone the fast and slow shear-polarization directions are correlative with the maximum and minimum horizontal-stress directions, respectively. Thus in this case, it appears the crustal velocity anisotropy is controlled by the state of stress in the crust. The observation of these two polarization directions at one station suggests that the shallow crust below the station HQR does not influence shear-wave polarization directions.

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