Coseismic displacements and Holocene slip rates for two active thrust faults at the mountain front of the Andean Precordillera (∼33°S)

During the last few hundred years several destructive earthquakes occurred along the eastern margin of the Andean Precordillera, where GPS data reveal a shortening rate of ∼4.5 mm/a. We use fault scarp profiles and age determinations of deformed terraces (T 1 –T 4 ) to infer coseismic displacements and quantify slip rates for the Peñas and Cal thrust faults near Mendoza city. Scarps on the lowest terrace level T 1 reveal vertical offsets of 0.8–1.0 m for both faults, which are interpreted as coseismic displacements during the last earthquake. Together with the fault dip these offsets indicate that both faults are capable of producing magnitude M W ∼6.9 earthquakes, which is corroborated by a magnitude M S = 7.0 event on the Cal fault that destroyed Mendoza in 1861. At the Peñas thrust fault, terrace T 2 has an age of ∼3.3 ka and is offset by ∼1.9 m, whereas the ∼12‐ka‐old terrace T 3 is displaced by ∼11 m. Combined with the fault dip of ∼25°, the age and offset of terrace T 3 define a shortening rate of ∼2.0 mm/a on the Peñas fault, i.e., about half of the present‐day shortening at the eastern margin of the Precordillera. At the Cal fault, terraces T 2 to T 4 have ages of ∼0.8 ka (OSL), ∼3.9 ka ( 14 C), and ≤12 ka ( 10 Be) and are vertically offset by ∼2.6, ∼3.6, and ∼7.0 m, respectively, which implies that slip on the fault has recently accelerated. Hence, the Cal fault poses a serious seismic hazard to the one million inhabitants of Mendoza.