INVESTIGATION OF THE RUPTURE PROCESS OF THE 28 JUNE 1992 LANDERS EARTHQUAKE UTILIZING TERRASCOPE

Displacement seismograms recorded by TERRAscope for the 28 June 1992 L anders earthquake (M(w) 7.3) are deterministically modeled using a for ward, point-source summation technique. Although the data set is spars e, it was possible to robustly determine important rupture parameters such as gross slip distribution, rupture velocity, rise time, and tota l source duration. The relatively simple approach lends itself to rapi d application following large earthquakes, provided that a catalog of Green's functions appropriate for the region is available. The fault u sed in the modeling of the Landers mainshock has a length of 70 km alo ng strike and a width of 15 km along dip. A model was found in which t he distribution and amplitude of slip at the surface matches the obser ved surface slip and provides a very good level of fit to the seismic data. Seismically, the Landers earthquake is characterized as two sube vents. The peak slip of the first subevent is 10 km north of the epice nter and the second is 40 km northwest along strike from the epicenter . The seismic moment is distributed as 2 x 10(26) dyne-cm to the first and 6 X 10(26) to the second subevent, respectively. It was assumed i n our modeling that the distribution of seismic moment along strike wa s the same at all depths. This assumption implies that slip at depth i s 69% of that at the surface as a result of differences in the materia l properties in the velocity model. The sensitivities of the source mo del to rupture velocity and dislocation rise time were examined. A rup ture velocity of 2.9 km/sec (80% of the shear-wave velocity) and a ris e time of 1 to 3 sec were found to satisfy the data. The rise time is only a fraction of the total source process time of 24 sec, and implie s that slip on the fault occurred within a narrow band (3 to 10 km), a t any instant during the rupture.