SOURCE CHARACTERISTICS OF THE 1992 NICARAGUA TSUNAMI EARTHQUAKE INFERRED FROM TELESEISMIC BODY WAVES

We analyzed the broadband body waves of the 1992 Nicaragua earthquake to determine the nature of rupture. The rupture propagation was represented by the distribution of point sources with moment-rate functions at 9 grid points with uniform spacing of 20 km along the fault strike. The moment-rate functions were then parameterized, and the parameters were determined with the least squares method with some constraints. The centroid times of the individual moment-rate functions indicate slow and smooth rupture propagation al a velocity of 1.5 km/s toward NW and 1.0 km/s toward SE. Including a small initial break which precedes the main rupture by about 10 s, we obtained a total source duration of 110 s. The total seismic moment is M(o) = 3.4 x 10(20) Nm, which is consistent with the value determined from long period surface waves, M(o) = 3.7 x 10(20) Nm. The average rise time of dislocation is determined to be tau approximate to 10 s. The major moment release occurred along a fault length of 160 km. With the assumption of a fault width W = 50 km, we obtained the dislocation D = 1.3 m. From tau and D the dislocation velocity is D = D/tau approximate to 0.1 m/s, significantly smaller than the typical value for ordinary earthquakes. The stress drop Delta sigma = 1.1 MPa is also less than the typical value for subduction zone earthquakes by a factor of 2-3. On the other hand, the apparent stress defined by 2 mu E(s)/M(o), where mu and E(s) are respectively the rigidity and the seismic wave energy, is 0.037 MPa, more than an order of magnitude smaller than Delta sigma. The Nicaragua tsunami earthquake is characterized by the following three properties: I)slow rupture propagation; 2) smooth rupture: 3) slow dislocation motion.