Source characteristics of the 1989 Boca del Tocuyo earthquakes in northwestern Venezuela

Abstract

The Boca del Tocuyo earthquake of April 30, 1989, has a special significance in assessing the seismic hazard of northern Venezuela and in understanding the complex tectonic deformation in this area. Although it was an event of moderate magnitude (M_w=6.2), the mainshock and its largest aftershock (M_w=5.6) produced considerable damage in low–rise structures, mainly due to soil failure, and induced intense liquefaction in most of the coastal towns near the epicentral area. Both earthquakes show body–wave trains which are much longer than those expected for events of such magnitude. The reason for this anomalous duration was analyzed through the formal inversion of the P, SH, and SV waves recorded teleseismically. The results show that the Boca del Tocuyo main event was generated by multiple rupture processes composed of at least two and perhaps three subevents, whereas the aftershock was formed by two subevents. Although the direction of rupture propagation could not be gleaned directly from the teleseismic body wave data, the epicentral relocation of the largest aftershock, the hypocentral distribution of about 60 aftershocks (m_b<4.5), and the linearity of the coast, which is parallel to the geologic structures in this area, suggest right–lateral, strike–slip faulting in a NW–SE direction. The Boca del Tocuyo earthquakes were apparently generated on a system of conjugate faults that are oblique to the east–west–trending, major fault systems that define the plate boundaries in northern Venezuela.