Actice fault recognition in northwestern Venezuela and its seismogenic characterization: Neotectonic and paleoseismic approach

Abstract

Northweslern Venezuela lies within the interaction zone of the Cari,bbean and South America plates and is being subjected to a stress field characterized by a NNW-SSE maximum horizontal stress and a ENE-WSW minimum horizontal stress (strike-slip regime). This stress tensor, calculated from microtectonic data collected at various sites in Plio-Pleistocene formations of northern Falcon state, is responsible for present kinematics and activity of five sets of faults: east-west right-lateral faults, NW-SE right-lateral fault~ (synthetic to the east-west faults), NNW-SSE normal faults, north-south to NNE-SSW left-lateral faults and ENE-WSW reverse faults (sub-parallel to fold axes). The most conspicuous tectonic feature of northwestern Venezuela is the east-west, right-la!eral Oca-Ancón fault system which extends for 650 km from Santa Marta (Colombia) to Boca de Aroa (eastern Falcon stare). It is also the grealest seismogenic source of this region, as paleoseismic trenching carried out on the Oca and the Ancón faults revealed the occurrences of earthquakes of magnitudes Ms 7.4 to 7.5 on both faults . The recurrence time of such events is 1752 ± 133 yr. on the Ancon fault and 4300 ± 1000 yr. on the Oca fault. Magnitude assessment of maximum credible earthquakes for those fault segments from trench data coincides fairly well with magnitude estimates by means of segmentation criteria. Minor faults of northern Falcon state can generate maximum credible earthquakes ranging between Ms 6.2 and 7.0 and their recurrence time is longer than 1500 yr. due to their rather slow (generally ~ 0.5 mm/yr) slip rate.