Three-dimensional shear wave velocity structure revealed with ambient noise tomography in the Parkfield, California region

Abstract

Knowledge of the shear wave velocity structure in the Parkfield, California region is a key factor for obtaining accurate earthquake locations and improving wave propagation simulations, but it has been poorly resolved in previous studies. We have conducted an ambient noise tomography study using continuous waveform data from available seismic stations, and higher resolution was achieved. The shear wave velocity structure in the crust was inverted from Rayleigh wave dispersion curves between 1 and 16 s. The new shear wave velocity structure correlates well with an existing compressional wave velocity model obtained from body-wave arrival time tomography. A low velocity zone (LVZ) in the lower crust was revealed on the southwest side of the San Andreas Fault, which is opposite to the pattern in the shallow crust. Two LVZs in the mid- and lower crust were revealed whose locations correlate with a high conductivity body, which supports the existence of fluid channels from depth into the SAF. Velocity variations at depth near the SAF also suggest a correlation with changes of the amplitude of low frequency earthquakes and with the resistivity structure.