The 1900 Mw 7.6 earthquake offshore north–central Venezuela: Is La Tortuga or San Sebastián the source fault?

Abstract

A new high resolution shallow marine seismic survey provides further insights on the fault source of the October 29th, 1900 earthquake, which affected a large region of North–central Venezuela, both offshore islands and mainland. This Mw 7.6 to 7.7 seismic event has been indistinctly ascribed to either the La Tortuga or San Sebastián fault, which has installed a live debate among different authors. These two faults allegedly run roughly east–west offshore, along the Coastal range. The San Sebastián fault (SSF) bounds this range to the north, being responsible for its linearity and the steep slope of its northern flank, whereas the La Tortuga fault (LTF) would do about the same, but some 20–30 km farther north, based on acoustic surveys carried out in the late 70's and early 80's. We herein bring strong evidence to support that the SSF is the source fault of this major offshore earthquake, mainly based on the freshness of its submarine scarp and recent fault throw, preserved at the Chuspa bay (close to Cabo Codera, eastern end of the Coastal range in central Venezuela), as depicted by one of the profiles. In addition, it is shown that most of the previous extent ascribed to LTF is not so. The LTF, besides exhibiting mainly normal slip, is much shorter than originally proposed. This precludes that LTF may have the seismic potential for an earthquake as big as the 1900 event. Finally, available reflection seismic data reveal the existence of a major fault system bounding the La Guaira shelf on the north, which exhibits tectonic inversion of pre existing extensional faults, shortening and strong syn-tectonic deformation during the Plio-Quaternary deposition. Sedimentation in the Bonaire basin during the Plio-Quaternary time is strongly controlled by this system activity.