JOINT INVERSION OF FAULT ZONE HEAD WAVES AND DIRECT-P ARRIVALS FOR CRUSTAL STRUCTURE NEAR MAJOR FAULTS

A newly recognized class of seismic phases, namely fault zone head waves arising from refraction at a transfault velocity contrast, provides additional constraints to direct P waves in inversions of near-fault arrival time data for earthquake location and velocity structure. Incorporation of fault zone head waves in the inversion process increases the available data and broadens the spatial coverage of ray paths between sources and receivers. Equally important, the explicit recognition of fault zone head waves eliminates a source of error resulting from misidentifying head wave first arrivals as direct P waves. A simple algorithm for the joint inversion of fault zone head waves and direct P phases is illustrated with observed and synthetic data. Application to a small data set recorded by the borehole seismic network at Parkfield, California, provides separate seismic velocity depth profiles for the crustal blocks on the two sides of the San Andreas fault. For depths less than 3 km the obtained velocity contrast is 10-20%. For greater depths it decreases to 3-7%. Numerical tests with synthetic data show that for sources with known locations and for well located local earthquakes (randomized mislocation errors of up to 600 m) proper use of fault zone head waves can significantly improve the accuracy of the fitted velocity structure. The results of this work motivate the inclusion of fault zone head waves in state-of-the-art joint structure-hypocenter inversions using high quality near-fault data.