VERY BROAD-BAND SEISMIC ANALYSIS OF THE 1992 FLORES, INDONESIA, EARTHQUAKE (M(W)=7.9)

The December 12, 1992, Fiores Island earthquake (M(W) = 7.9), which caused a destructive tsunami, is located in the back are of the transition zone between the Sunda and Banda arcs, a region in active collision with the Australian continental margin. We study the earthquake using four seismological techniques spanning a period range of 1-1000 s and infer that the Flores event may be characterized as a shallow back are thrust with fault plane strike 70 degrees +/- 30 degrees, dip 28 degrees +/- 10 degrees and rake 80 degrees to 140 degrees (linear trade-off with strike), a rupture consisting of three discrete pulses with a total duration of 70 s (centroid time 26.3 s) and an estimated moment release between 7.5x10(20) and 8.0x10(20) N m. From the slip vector we infer that compressional forces play an important role in the deformation mechanism of the back are north of Flores Island. Body waves constrain the hypocentral depth to be 16 km with rupture mainly propagating updip to shallow depth and bilaterally along strike with most subevents located toward the northeast. Evidence for rupture directivity from surface wave deconvolutions is inconclusive, but the overall source function is consistent with body wave results. Our body wave finite fault model is in rough agreement with two models proposed to explain tsunami runup observations. Normal mode modeling of W phase observations at periods longer than 70 s shows that slow slip is negligible. From this and the slip distribution determined from the body wave analysis we infer that the seismic source is an unlikely candidate to explain anomalously high tsunami run-ups measured on northeastern Flores Island.