Surface deformation associated with the November 23, 1977, Caucete, Argentina, earthquake sequence

Abstract

Leveling data are combined with seismic observations from the November 23, 1977, Ms = 7.4 Caucete earthquake in western Argentina, in order to constrain models of crustal deformation in the Sierras Pampeanas tectonic province. The spatial pattern and amount of uplift ( ∼1 m) that accompanied this earthquake suggest 4 m of slip on a north‐south trending, west dipping fault with a dip of 35°, a downdip width of 24 km, and a length of 80 km. The top of the fault is 17 km below the surface. These fault parameters are generally consistent with the seismic focal mechanism and moment of the main shock, which occurred near the top of the fault. The fault length is derived from the distribution of local and relocated teleseismic aftershocks and from analysis of the main shock (a double event in which the two shocks occurred 21s apart and were separated by 65 km along the proposed fault plane strike). The fault width is determined from the locations of the larger hypocenters along the preferred fault plane. This fault is buried beneath the north‐south striking Sierra Pie de Palo, a Laramide style crystalline basement uplift in the western part of the Sierras Pampeanas province. The buried fault model is consistent with the lack of observed coseismic surface faulting and the broad, east‐west symmetric character of the overlying Sierra Pie de Palo. By combining the 1977 earthquake observations with topographic relief and with estimates of the age of deformation for the Sierras Pampeanas basement uplifts, we calculate a shortening rate of the order of 10 mm/yr for the past 5 m. y. for this Andean back arc province and estimate an average recurrence time of the order of 1000 years for earthquakes of this magnitude beneath the Sierra Pie de Palo.