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

Schmidt, S., Hetzel, R., Mingorance, F., Ramos V.A.

Zusammenfassung

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 ~4.5mm/a. We use fault scarp profiles and age determinations of deformed terraces (T1–T4) to infer coseismic displacements and quantify slip rates for the Peñas and Cal thrust faults near Mendoza city. Scarps on the lowest terrace level T1 reveal vertical offsets of 0.8–1.0m 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 MW ~6.9 earthquakes, which is corroborated by a magnitude MS=7.0 event on the Cal fault that destroyed Mendoza in 1861. At the Peñas thrust fault, terrace T2 has an age of ~3.3ka and is offset by ~1.9m, whereas the ~12-ka-old terrace T3 is displaced by ~11m. Combined with the fault dip of ~25°, the age and offset of terrace T3 define a shortening rate of ~2.0mm/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 T2 to T4 have ages of ~0.8ka (OSL), ~3.9ka (14C), and ≤12ka (10Be) and are vertically offset by ~2.6, ~3.6, and ~7.0m, 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.