APPLICATION OF MORPHOLOGIC DATING OF COMPOSITE FAULT SCARPS AT THE EL CARRIZAL FAULT, BAJA CALIFORNIA SUR: TOWARDS AN ESTIMATION OF POTENTIAL SEISMIC HAZARDS

Seismic hazard and its derived societal impact have been systematically neglected in the southern portion of the Baja California Peninsula, nearby La Paz metropolitan area. This is mainly a result of a very short and incomplete record of past seismic activity, mostly restricted to the last half century. Since the late 90’s microseismicity surveys of nearby fault zones have been undertaken, showing substantial activity in some of them. Thus paleoseismic reconstruction on the main fault zones near the urban area of La Paz city is a powerful tool for the estimation of potential hazards. The El Carrizal Fault is a NNW-SSE trending normal fault dissecting two alluvial fans of Pliocene age. The whole El Carrizal Fault system is located less than 40 km from La Paz. The studied area is characterized by alluvial fans dissected by fluvial currents and faults. The main fault scarp is clearly visible in some sites across the system. Topographic construction of cross sections of the El Carrizal fault confirms the existence of a series of stepped scarps to the east of the main fault trace. These composite scarps are located on transport-limited hillslopes, displaying characteristic soft-rounded topography. Displacement of such scarps suggests that El Carrizal fault has been active at least during the Pleistocene. Simple diffusion and discriminant analysis models of scarp degradation devised for semi-arid settings were applied to the resulting cross sections. Results confirm a Pleistocene age to the scarps. This implies considerable seismogenic activity for at least Pleistocene time. A field survey of outcropping units in the area of study shows that palustrine deposits (late Pleistocene) are affected by small faults associated with the El Carrizal fault system. These palustrine deposits along the faulted area show clear promise for dating purposes due to their organic matter contents.