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

摘要

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 Penas 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 Penas 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 Penas 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.