SEGMENTACIÓN SISMOGENÉTICA DE LA FALLA DE BOCONÓ A PARTIR DE INVESTIGACIONES PALEOSÍSMICAS POR TRINCHERAS, VENEZUELA OCCIDENTAL: ¿MIGRACIÓN DE LA RUPTURA HACIA EL NORESTE EN TIEMPOS HISTÓRICOS?

Resumen

Seismogenic segmentation of the Bocono fault from paleoseismic trench investigations, western Venezuela: NE-directed rupture Migration in historical times? The Bocono fault is a seismogenically-active right-lateral strike-slip fault that extends for over 500 km, in a SW-NE direction, mainly along the backbone of the Merida Andes, but also along the northern margin of the Yaracuy depression. So far, this fault had been previously subdivided into 5 sections (Bocono-a through -e from SW to NE) based strictly on geometric criteria. In order to assess the reliability of this preliminary “segmentation”, 7 paleoseismic trenches have been excavated between 2004 and 2006 to complement 4 previous assessments. The integration of all trench results allows to determining that: 1) The Bocono Fault is actually segmented, as initially suggested by the few large, known, destructive historical earthquakes; 2) the geometric segment boundaries appear to be actual seismogenic barriers to rupture propagation, thus supporting the existing preliminary segmentation, although some may be “leaky” boundaries (between Bocono-b and -c in 1674 AD); 3) all segments appear to have ruptured in historical times. Furthermore, it appears that there is a northeastward time and space migration of the before-the-last event sequence (similar to the North Anatolian fault in Turkey), starting in 1610 AD on Bocono-a and closing the cycle on Bocono-e in 1812 AD. Segments Bocono-b and -c broke during the 1673-1674 earthquakes sequence (similar to the Izmit and Duzce 1999 events) and Bocono-d would seem to have ruptured during the 1736 AD earthquake, which is a poorly documented event although significant damage to Barquisimeto (at NE tip of Bocono-d) was reported. Should this cy cle repeat in the future, the 1894 event on Bocono-a has started a new cycle and the repeat of the 1674 event should be the next large shock on the Bocono fault. Based on the recurrence determined from trenching, the next ≈Ms 7.0 event on Bocono-c should occur in a time window between year 2070 and 2130 AD. Mitigation, particularly as to the social component, should be developed in preparation for a future event of such characteristics in the central Merida Andes.