THE MARCH 25, 1990 (M(W)=7.0, M(L)=6.8), EARTHQUAKE AT THE ENTRANCE OF THE NICOYA GULF, COSTA-RICA - ITS PRIOR ACTIVITY, FORESHOCKS, AFTERSHOCKS, AND TRIGGERED SEISMICITY

On March 25, 1990 a large earthquake (M(w)=7.0, M(L)=6.8) occurred at the entrance of the Nicoya Gulf, Costa Rica, at 1322:55.6 UTC, produci ng considerable damage in central Costa Rica and generating much inter est about whether or not the Nicoya seismic gap (Nishenko, 1989) had b roken. The local country-wide seismographic network recorded 6 years o f activity prior to this large earthquake, 16 hours of foreshocks, the mainshock, and its aftershocks. This network is operated jointly by t he Costa Rica Volcanological and Seismological Observatory at the Nati onal University (OVSICORI-UNA), and the Charles F. Richter Seismologic al Laboratory at the University of California, Santa Cruz (CFRSL-UCSC) . We obtained high resolution locations from this network and located the mainshock at 9 degrees 38.5'N, 84 degrees 55.6'W (depth is 20.0 km ) and the largest foreshock (M(w)=6.0, March 25, 1990, at 1316:05.8 UT C) at 9 degrees 36.4'N, 84(0)57.1'W (depth is 22.4 km). We find that t he aftershock zone abuts the southeast boundary of the Nicoya seismic gap, suggesting that the seismic gap did not rupture. Since the instal lation of the local network in April 1984 to March 24, 1990, nearly 19 00 earthquakes with magnitudes from 1.7 to 4.8 (318 with magnitude 3.0 or larger) have been located at the entrance of the Nicoya Gulf, one of the most active regions in Costa Rica. The March 25 earthquake occu rred at the northwest edge of this region, where a sequence of foresho cks began 16 hours prior to the mainshock. The spatial-temporal distri bution of aftershocks and directivity analysis of the mainshock ruptur e process using teleseismic records both indicate a southeast propagat ing rupture. The mainshock ruptured an asperity of approximately 600 k m(2) of area, with this area expanding to 4000 km(2) after 7 days. We present evidence that suggests that the ruptured asperity is produced by the subduction of a seamount. Inversion of teleseismic broadband an d long-period P and SH waves yields a thrust faulting mechanism with t he shallow plane striking 292 degrees, dipping 26 degrees, and with a rake of 88 degrees, in agreement with the subduction of the Cocos plat e under the Caribbean plate. Local first motions for the largest fores hock and the mainshock agree with this solution. We also present evide nce suggesting that the March 25, 1990, earthquake triggered and react ivated several seismic swarms in central Costa Rica and temporally dec reased the activity in the epicentral area of the July 3, 1983(M(s)=6. 2), Perez Zeledon earthquake.