Geological, geomorphological, and geophysical evidence associated with the Cerritos fault, and implications for seismic hazard in Morelia, Michoacán, México

Abstract

The Cerritos fault is located SW of Morelia city, in the state of Michoacán, Mexico. The fault belongs to the western part of the Morelia-Acambay Fault System, an area with several active segmented faults that form various grabens and half-grabens. In this area, faulting affects Miocene to Holocene lithologies; some of these faults even control the distribution of regional monogenetic volcanoes. This work is an interdisciplinary approach to characterize the Cerritos fault, an important structure with a high seismic hazard potential. The Cerritos fault is a 12 km-long oblique fault (normal-left lateral) with a 130±5 m-high topographic relief (100 m of surface displacement + 30 m of subsurface displacement estimated from the inverted resistivity study). This fault is ENE-WSW oriented (255°), with a 76° NNW dip. In a well-exposed cross section of the Cerritos fault, a few meters from the master fault trace, a colluvial wedge with organic material was identified, yielding a radiocarbon age of 3.37–3.21 cal ka BP. Our geologic and geomorphological analyses indicate that the Cerritos fault is a young, tectonically-active fault, especially in its eastern part. The vertical and horizontal linearity of the fault scarp and the accumulation of lake deposits in the down-thrown block suggest active vertical motion (uplift and subsidence, respectively) along this fault. Geophysical surveys, including seismic refraction, terrestrial magnetometry, and electrical resistivity tomography, show the subsurface geometry of the fault to be characterized by a main listric fault plane and a damage zone in the footwall block, extending as far as 75 m from the main scarp. The damage zone is characterized by secondary, synthetic, and antithetic faults, forming roll-over anticlines and two crestal collapse grabens that accumulate colluvial material. Paleoseismic estimates of activity and seismic hazard potential indicate that the Cerritos fault has a slip-rate of 0.03±0.01 mm/yr, with mean vertical displacements of 0.5 m per event and a mean recurrence interval of 16700 years. The Cerritos fault can generate single-segment ruptures with magnitudes of M_w 6.2 to 6.6. Still, in a worst-case scenario, it could also rupture with the subparallel and adjacent Morelia and Cointzio faults, generating earthquake magnitudes up to M_w 6.9.