EL TEMBLOR DE QUEPOS DEL 20 DE AGOSTO DE 1999 (6,9 Mw): FUENTE SÍSMICA Y EVOLUCIÓN DE LA SISMICIDAD

Abstract

The seismic sequence began on august 20, 1999 at 04:02 with an event of 6.9 Mw (seismic moment magnitude), located 50 km south of Quepos and a maximum intensity of VII (MM). Immediately the mainshock was followed by 500 aftershocks of magnitude up to 2.5 ML within the first 24 hours. During the secondday after the main shock the activity quickly decayed to an average of 5 events by day. On august 21, 1999 at04:49 LT, took place an event of 5,6 Mw with an intensity of V (MM) in Quepos and IV in San José, which isconsidered the major aftershock of the sequence. Seismic wave analysis by SEISAN computer program and macro seismic data indicated that the rupture area hadan extension of 1000 km2 and a displacement of 43 cm. The seismicity was related to two different seismic sources, a pure subduction process and an asperity by a submarine mountain on the Cocos plate. The main shock wasa characteristic subduction event and the second was an upper crustal event, related with a strike slip fault.