Shallow structure of the Landers fault zone from explosion-generated trapped waves

With fault-zone trapped waves generated by near-surface explosions within t he fault zone of the 1992 Landers, California, earthquake and recorded by l inear seismic arrays deployed along and across the fault, we have delineate d the shallow seismic structure near the 30-km-long southern rupture on the Johnson Valley fault. The explosion-excited trapped waves with relatively large amplitude and long-duration wave train after the S waves are similar to those generated by aftershocks [Li et al., 1994a, b] but have lower freq uencies and travel more slowly. Coda-normalized amplitude spectra of explos ion-excited trapped waves show a maximum at similar to 2 Hz, which decrease s rapidly with the station offset from the fault trace on the cross-fault p rofiles. Normalized amplitudes of trapped waves on the along-fault profile also decreased with distance between the explosion and station, giving an a pparent Q of similar to 18 at 1-2 Hz in the fault zone for the shot near th e profile. The dispersion of trapped waves from 0.6 to 2.5 Hz recorded on t his along-fault profile implies a shear velocity of similar to 1.0 km/s for the fault zone and similar to 1.8 km/s for the wall rock, while the data f rom the farther shot show an increase in velocity and Q with depth. Measure d group velocities and Q values were used as constraints in the numerical m odeling of trapped waves on cross-fault and along-fault profiles. Results r eveal that the shallow Johnson Valley fault is marked by a zone 250 m wide where the shear velocity is 1 km/s and Q is 20. Calculation of finite diffe rence synthetics for a depth-varying fault structure show that these model parameters apply to the depth of similar to 1 km, below which the fault zon e shear velocity increases to 1.9 km/s and Q increases to 30.