THE APRIL 22, 1991, VALLE-DE-LA-ESTRELLA, COSTA-RICA (MW = 7.7) EARTHQUAKE AND ITS TECTONIC IMPLICATIONS - A BROAD-BAND SEISMIC STUDY

The rapture process of a large back arc thrusting earthquake, the Apri l 22, 1991, Valle de la Estrella, Costa Rica (M(W) = 7.7), earthquake, is investigated using broadband body waves and long-period surface wa ves. We find the source process to be relatively simple, with the sour ce models separately obtained from body and surface waves being very c onsistent. The event occurred on a shallow, southwest dipping rupture plane on which most energy is released updip of the hypocentral locati on (10-20 km deep). High-frequency radiation appears to have been rele ased over a relatively small source area. Our preferred model has a fo cal mechanism with strike 102 +/- 10, dip 17 +/- 140 and rake 63 +/- 1 7-degrees, a seismic moment of 3.8 +/- 1.5 x 10(20) Nm. and a total ru pture duration of 40 +/- 6 s. The earthquake appears to be associated with the North Panama Deformed Belt (NPDB), a thrust and fold complex that has accommodated the oroclinal deformation of Panama. This event, along with previous large events north of Panama in 1882 and 1916, in dicates that there is substantial convergence along the NPDB, marking the NPDB as a probable emerging plate boundary. It remains difficult t o gauge the earthquake hazard in the region because of the tectonic co mplexity.