This research involves the continuation of the monitoring, by means of fault zone guided waves, the Landers fault zone 3-D structure and strengthening process after the M = 7.5 earthquake of 1992. Earlier studies of fault zone trapped waves from Landers aftershocks allowed the delineation of the Johnson Valley fault (the Landers southern rupture plane) to a depth of at least 10km, marked by a low velocity waveguide approximately 200 m wide where shear velocity is reduced by 30 to 50 percent and Q is about 30. Repeat seismic surveys in 1994 and 1996 using explosive sources along the Johnson Valley revealed an increase in seismic velocities with time. These observations indicate a strengthening of the fault zone with time that may be due to a closure of cracks. This grant will support a repeat of this experiment in 1998 and 2000. The goal is to further understand the earthquake rupture process and develop methods for predicting the segmentation of known faults in future earthquakes. This research is a component of the National Earthquake Hazard Reduction Program. ***
