ROCK-AVALANCHES AS EVIDENCES OF PALEOSEISMIC ACTIVITY IN THE SIERRAS PAMPEANAS, ARGENTINA


             E.F. Gonzalez Diaz (1), C.H. Costa(2), A.D. Giaccardi(2) and L.E. Fauque(1)
      (1) Universidad de Buenos Aires, Argentina (2) Universidad Nacional de San Luis, Argentina

      The Sierras Pampeanas are basement uplifts located in Central Western Argentina where considerable neotectonic activity took place during Pliocene-Pleistocene times. Quaternary faulting along these ranges uplift front as well as some historical earthquakes indicate that paleoseismic records should be studied for a better understanding of the regional seismic hazard. Two rock-avalanches have been recognized at the northern piedmont of the southernmost Sierra de San Luis. Differential preservation of their geomorphological attributes suggests different ages. Avalanche accumulations are characterized by large blocks (200 m3). Deposits and morphologies belonging to the older one are not well preserved, hampering the reconstruction of their morphometric parameters. However, deposits signature as well as the direct relationship with a hillslope scar clearly indicate a rock avalanche nature. The younger one still shows its original morphological features, allowing the calculation of its volume (600 . 106 m3), velocity (190 km/hr), area covered (8 km2), thickness (65-90 m) and maximum run-off (4,6 km). Avalanche volumes, morphotectonic location, morphometric parameters and isolated occurrence, favour the association of these geomorphological features with a seismic origin. This fact open some interesting points as for the regional seismic hazard characterization such as:

      1. Earthquakes with capabilities for movilizing important volumes of rocks took place during the Quaternary;
      2. No rock avalanches were induced as a consequence of the nearby 1936 San Francisco earthquake M=6.0. It means that rock avalanches may have been triggered by a larger prehistorical earthquake and/or by another climatical conditions;
      3. Dating avalanches ages would provide preliminary insights on paleoseismological recurrence.

      These data are critical for a proper seismic risk assessment, since as in many intraplate settings historical and instrumental seismicity are not representative enough of the long-recurrence rate of Quaternary faulting and hence their seismic potential could be significatively undervaluated.