FINE-STRUCTURE OF THE LANDERS FAULT ZONE - SEGMENTATION AND THE RUPTURE PROCESS

Observations and modeling of 3- to 6-hertz seismic shear waves trapped within the fault zone of the 1992 Landers earthquake series allow the fine structure and continuity of the zone to be evaluated. The fault, to a depth of at least 12 kilometers, is marked by a zone 100 to 200 meters wide where shear velocity is reduced by 30 to 50 percent. This zone forms a seismic waveguide that extends along the southern 30 kilo meters of the Landers rupture surface and ends at the fault bend about 18 kilometers north of the main shock epicenter. Another fault plane waveguide, disconnected from the first, exists along the northern rupt ure surface. These observations, in conjunction with surface slip, det ailed seismicity patterns, and the progression of rupture along the fa ult, suggest that several simple rupture planes were involved in the L anders earthquake and that the inferred rupture front hesitated or slo wed at the location where the rupture jumped from one to the next plan e. Reduction in rupture velocity can tentatively be attributed to faul t plane complexity, and variations in moment release can be attributed to variations in available energy.