Depth-dependent structure of the Landers fault zone from trapped waves generated by aftershocks

We delineate the internal structure of the Johnson Valley and Kickapoo faul ts (Landers southern rupture) at seismogenic depth using fault zone trapped waves generated by aftershocks. Trapped waves recorded at the dense linear seismic arrays deployed across and along the surface breaks of the 1992 M7 .5 Landers earthquake show large amplitudes and dispersive wave trains foll owing the S waves. Group velocities of trapped waves measured from multiple band-pass-filtered seismograms for aftershocks occurring at different dept hs between 1.8 km and 8.2 km show an increase in velocity with depth. Veloc ities range from 1.9 km/s at 4 Hz to 2.6 km/s at 1 Hz for shallow events, w hile for deep events, velocities range from 2.3 km/s at 4 Hz to 3.1 km/s at 1 Hz. Coda-normalized amplitude spectra of trapped waves peak in amplitude s at 3-4 Hz for stations located close to the fault trace. The amplitude de cays rapidly with the station offset from the fault zone. Normalized amplit udes also decrease with distance along the fault, giving an apparent Q of 3 0 for shallow events and 50 for deep events. We evaluated depth-dependent f ault zone structure and its uncertainty from these measurements plus our pr evious results from near-surface explosion-excited trapped waves [Li et al. , 1999] in a systematic model parameter-searching procedure using a three-d imensional (3-D) finite difference computer code [Graves, 1996]. Our best m odel of the Landers fault zone is 250 m wide at the surface, tapering to 10 0-150 m at 8.2 km depth. The shear velocity within the fault zone increases from 1.0 to 2.5 km/s and Q increases from 20 to 60 in this depth range. Fa ult zone shear velocities are reduced by 35 to 45% from those of the surrou nding rock and also vary along the fault zone with an increase of similar t o 10% near ends of the southern rupture zone.