Revised seismic history of the El Pilar fault, Northeastern Venezuela, from the Cariaco 1997 earthquake and recent preliminary paleoseismic results

Abstract

In light of the July 9, 1997, Cariaco earthquake, it is clearly understood now that damage in the city of Cumana – located in northeastern Venezuela and frequently destroyed by the largest earthquakes since the first recorded event in 1530 – is strongly enhanced by poor soil conditions that, in turn, are responsible for site amplification and widespread earthquake-induced effects. Therefore, most previous macroseismic studies of historical earthquakes must be revaluated because those localized high-intensity values at Cumana surely led to the misestimation of past epicenters. Preliminary paleoseismic results, gathered at three exploratory trenches dug across the surface break of the Cariaco 1997 earthquake in 1998, allow us to associate the 1684 earthquake with this recently ruptured fault segment that extends between the towns of San Antonio del Golfo and Rio Casanay (roughly between the two gulfs of Cariaco and Paria, state of Sucre). Other major results from the reassessment of the seismic history of this fault are: (a) the 1766 event seems to have generated in a different source to the El Pilar fault because the size of the felt area suggests that it is an intermediate-depth earthquake; (b) damage to Cumana produced by the 1797 event suggests that this was a local earthquake, perhaps equivalent to the 1929 earthquake, which ruptured for some 30 km just east of Cumana into the Gulf of Cariaco; and (c) seismogenic association of the 1530 and 1853 earthquakes still remains unclear but it is very likely that these ruptures occurred offshore, as suggested by the rather large tsunami waves that both events have generated, placing their hypocenters west of Cumana in the Cariaco Trough. This reassessment also sheds light into the El Pilar fault segmentation and the behavior of its seismogenic barriers through time.