Abstract
Earthquakes with magnitudes M ≤ 3.5 were registered in the Andes backarc between ~32.5°S 33.75°S within a temporary experiment from November 2002 to March 2003. Data were collected from 15 seismological broad band stations, deployed above flat subduction section and also above the transitional to normal section of the Nazca plate. Seismic events were located mostly in part of the Southern Precordillera and Cerrilladas Pedemontanas of Mendoza Province. Focal mechanism solutions were obtained, for selected data between15 kmand35 kmdepths, from P wave first motion using FOCMEC software. A band trending NW-NNW of normal focal mechanism earthquakes is located just by the thrust front towards West, and covers the Southern tip of the Southern Precordillera and the Western side of the Cerrilladas Pedemontanas, Mendoza Province, Argentina. Thrust focal mechanism solutions obtained in the present work for events with magnitude less than or equal to 3.5 also show that the thrust front South of ~33.1°S is located in the same place as Mw ≥ 3.6 earthquakes. The most significant findings in this work are these signals of extensional regimen which appear in a compressional subduction tectonic regimen. Nevertheless the short temporal experiment precludes firm interpretations about this particular phenomenon. Probably it is related to a temporary stress relaxation. The normal earthquakes are likely associated to normal faults, and/or to their subsidiaries, that were inverted by contraction and now re-inverted at least transitorily as normal. These faults, which are near and to the East of the suture between Chilenia and Cuyania Paleozoic terranes, were originally involved in the Cuyo Triassic basin formation.
Highlights
Seismological data from CHARSME (CHile ARgentina Seismological Measurement Experiment) Experiment (Figure 1), allowed us characterize the crustal seismicity with magnitude less than or equal to 3.5
A quality classification was applied to the focal mechanism solutions considering the family solutions behavior, i.e. strike and dip variation within the family solutions for each event
The seismicity showed in the present work, between 32.5 ̊S and 33.75 ̊S in the backarc of the Andes, is mainly related to two morphotectonic units (Figures 3 and 4), part of the Southern Precordillera [22] and Cerrilladas Pedemontanas of Mendoza Province, Argentina
Summary
Seismological data from CHARSME (CHile ARgentina Seismological Measurement Experiment) Experiment (Figure 1), allowed us characterize the crustal seismicity with magnitude less than or equal to 3.5. The major percent of focal mechanisms show normal solutions. Focal mechanisms from earthquakes with magnitude greater than 3.6 were obtained from Harvard CMT Catalog, [1,2,3,4,5]. Subduction process supplied compressive stress to the crust, principally in the East-West direction [8]. This explains the almost total dominance of observed thrust focal mechanisms (Mw ≥ 3.6). The notable distinction in this work is the presence of small events with normal focal mechanism solutions.
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