Large Holocene earthquakes in the Sierras Pampeanas and sorrounding plains: more likely than once thought
by
Carlos H. Costa
Departamento de Geología, Universidad Nacional de San Luis, Argentina
Coauthors: Guillermo L. Sagripanti (Departamento de Geología, Universidad Nacional de Río Cuarto, Argentina)

The Sierras Pampeanas (Pampean Ranges) have traditionally been envisaged as a pericratonic area, with both the recurrence and seismic potential for large earthquakes considerably lower than the nearby Andean orogen. These characteristics have also been suggested by instrumental and historical seismicity, since historically recorded seismic crisis have been much sparse and less destructives than the episodes located along the precordilleran piedmont. However, intraplate faults are characterized by low deformation rates and a seismic cycle commonly beyond 104 years, but the capability for storing and releasing seismic energy is similar or even larger than many high slip-rate faults of interplate regions. A first order consequence of these facts is that both instrumental and historical seismicity does not adequately image the seismogenic capability of these long-term recurrence faults, basically because the time span needed for a suitable analysis of their seismogenic parameters (recurrence, maximum probable earthquake, elapsed time since last surface rupture), is far beyond the temporal penetration provided by seismic catalogs (< 5.102 years in the American continent).

Many earthquake-related evidences have been found along several neotectonic structures in the Sierras Pampeanas, such as primary surficial ruptures (Comechingones, Sierra Chica, Sierra de San Marcos and Velazco faults), large rock-avalanches (San Luis fault) and paleoliquefaction features (Las Lagunas fault). These phenomena have no historical analogs in the region, but they testify for the occurrence of large earthquakes (M > 7.5) since Late Pleistocene.

Most of the bounding faults of the pampean blocks show evidences of quaternary movements and information suggesting a considerable seismogenic potential for most of them are added year by year. Despite the long-term recurrence interval characterizing any individual neostructure, the evidences so far recognized highlight that many of these faults or fault segments gave rise to large earthquakes with surface signature and hence with a considerable damaging potential. Conversely, it is understood that the Sierras Pampeanas and surrounding regions have witnessed several significant seismic crisis during the Holocene.

Date received: December 2, 2004