Modern crustal seismicity in the northern Andean Precordillera, Argentina

The Precordillera in the Andean backarc region of Argentina has a long record of large and damaging earthquakes in the last century. Its northern part is not well known seismically neither its style of deformation. In this work we study modern crustal seismicity occurring in the same epicentral area of the historical 1894 earthquake. That earthquake caused deaths and severe damage even at large distances of about 500 km. Because of the unavailability of instrumental records to study this large earthquake, not many details of its seismic source are known. Our study involves broadband and short period seismic data from modern seismicity to characterise the same epicentral area. We used seismic records from the permanent Argentine network (INPRES) and from two temporary broadband experiments (CHARGE and SIEMBRA). We determine 44 seismic locations and their focal depths which are in the range between 10 and 30 km and magnitudes 1.2 ≤ M ≤ 4. In addition, we obtain 33 focal mechanism solutions for some of these earthquakes using P-wave first motions and amplitude ratios between P and S waves; for the same region, five more moderate earthquakes have been ever observed globally and reported by the Global Centroid Moment Tensor catalogue in the last 55 years. We note that epicentres are mainly distributed in the southern sector of the study region, with a relatively greater concentration in the foothills of the Frontal Andes Cordillera. Focal mechanisms show mainly reverse, and in less proportion strike-slip solutions. Overall these results agree with compression and shortening in an east-west orientation of the Andean northern Precordilleran backarc region.