Shallow low-velocity zone of the San Jacinto fault from local earthquake waveform modelling

P>We developed a method to determine the depth extent of low-velocity zone (LVZ) associated with a fault zone (FZ) using S-wave precursors from local earthquakes. The precursors are diffracted S waves around the edges of LVZ and their relative amplitudes to the direct S waves are sensitive to the LVZ depth. We applied the method to data recorded by three temporary arrays across three branches of the San Jacinto FZ. The FZ dip was constrained by differential traveltimes of P waves between stations at two side of the FZ. Other FZ parameters (width and velocity contrast) were determined by modelling waveforms of direct and FZ-reflected P and S waves. We found that the LVZ of the Buck Ridge fault branch has a width of similar to 150 m with a 30-40 per cent reduction in V(p) and a 50-60 per cent reduction in V(s). The fault dips 70 +/- 5 degrees to southwest and its LVZ extends only to 2 +/- 1 km in depth. The LVZ of the Clark Valley fault branch has a width of similar to 200 m with 40 per cent reduction in V(p) and 50 per cent reduction in V(s). The Coyote Creek branch is nearly vertical and has a LVZ of similar to 150 m in width and of 25 per cent reduction in V(p) and 50 per cent reduction in V(s). The LVZs of these three branches are not centred at the surface fault trace but are located to their northeast, indicating asymmetric damage during earthquakes.