SOURCE CHARACTERISTICS OF THE NICARAGUAN TSUNAMI EARTHQUAKE OF SEPTEMBER 2, 1992

A large, Ms = 7.2, shallow earthquake took place off the Pacific coast of Nicaragua on September 2, 1992, and set off large tsunamis. The ground motion of the main shock was very weak along the whole Nicaraguan coast. A tsunami as high as 10 m was observed at El Transito, whereas a height of about 2 m is empirically expected for an earthquake of Ms 7.2. The tsunami magnitude Mt is estimated to be 7.9 from tide gage data. The Nicaraguan event is a tsunami earthquake which generates unusually large tsunamis for its earthquake magnitude. The source mechanism is studied in detail by using waveforms of body waves and surface waves recorded on global broadband seismographs. The possibility of single force source is ruled out from radiation patterns and the amplitude ratio of Rayleigh and Love waves. The main shock is interpreted as low-angle thrust fault with strike = 302-degrees, dip = 16-degrees and slip 87-degrees, the Cocos plate underthrusting beneath the Caribbean plate. The seismic moment from surface wave analysis is 3.0 x 10(20) Nm (Mw = 7.6). The source dimension is estimated to be 200 km x 100 km from the aftershock area. The inversion results of body waves suggest bilateral rupture with rupture velocity as low as 1.5 km/s and duration time of about 100 s. The source process time is unusually long, indicating that the associated crustal deformation has a long time constant.