MECHANISM OF THE 1992 NICARAGUA TSUNAMI EARTHQUAKE

The 1992 Nicaragua earthquake generated larger tsunamis than expected from its surface wave magnitude (M(s) 7.2) and is known as a 'tsunami earthquake'. Seismological studies showed that the duration was very long for its size, about 100 s. Other studies have shown that the seismic moment estimated from tsunamis is an order of magnitude larger than that from seismic waves, even after the long duration is accounted for. Numerical computations of tsunamis from various fault models are made to reconcile this discrepancy. Comparison of calculated waveforms with tide gauge records shows that the fault width is 40 km, much narrower than the aftershock area, and extends only into the upper 10 km of the ocean bottom. Slip amount on the fault is estimated to be 3 m from amplitude comparisons. The fault length is estimated to be 250 km, slightly longer than the aftershock area, from comparison of the tsunami height distribution. The rigidity around the shallow fault may be smaller than that of a standard underthrust fault, and the seismic moment is estimated as 3 x 10(20) Nm, consistent with the seismic observations. A slow rupture on the shallow fault, presumably in the subducted sediments, is responsible to the unusually large tsunami excitation.