Aftershock pattern of the July 9, 1997 Mw=6.9 Cariaco earthquake in Northeastern Venezuela

Abstract

The Mw=6.9 Cariaco earthquake of July 9, 1997 was the strongest earthquake that occurred on the El Pilar fault between Cumaná and the Gulf of Paria in the 20th century. The El Pilar fault is the most prominent strike-slip fault in the transition zone between the Caribbean plate and the South American plate in this region. We installed 18 seismic and 10 strong motion stations for recording aftershocks. We calculated the hypocenters of 1306 aftershocks with magnitudes between −0.5 and 4.1. Most aftershocks concentrated near the documented trace of the El Pilar fault. The spatial distribution of the aftershocks allowed us to separate the aftershock region on the El Pilar fault into six segments with different dip angles. We computed and classified fault-plane solutions of 222 aftershocks. The majority of fault-plane solutions are of strike-slip type. Some segments show differences in the dominating type of focal mechanisms. From the aftershock distribution and spatial variation of b-values we inferred a rupture length of the Cariaco earthquake of 60 km, and thus an average displacement of ∼1.3 m during the main shock is estimated. Two out of four stepovers, which were found on this length, were seismically activated during the monitoring time interval. The b-value distribution on the fault plane appears to define two areas of major stress relaxation: beneath Cariaco and half way between Casanay and El Pilar close to the surface.