Seismic structures of the Calico fault zone inferred from local earthquake travel time modelling

Abstract

We analysed high-frequency body waves of local earthquakes to image the damage zone of the Calico fault in the eastern California shear zone. We used generalized ray theory and finite difference methods to compute synthetic seismograms for a low-velocity fault zone (FZ) to model the direct and FZ-reflected P and S traveltimes of local earthquakes recorded by a temporary array across the fault. The low velocity zone boundaries were determined by apparent traveltime delays across the fault. The velocity contrast between the fault zone and host rock was constrained by the traveltime delays of P and S waves and differential traveltimes between the direct and FZ-reflected waves. The dip and depth extent of the low velocity zone were constrained by a systematic analysis of direct P traveltimes of events on both sides of the fault. We found that the Calico fault has a ∼1.3-km-wide low velocity zone in which the P- and S-wave velocity decreased 40 and 50 per cent, respectively, with respect to the host rock. The low velocity zone dips 70° northeast and extends 3 km in depth.