Crustal seismicity in the Andean Precordillera of Argentina using seismic broadband data

Abstract

In this study, we analyze 100 crustal Precordilleran earthquakes recorded in 2008 and 2009 by 52 broadbandseismic stations from the SIEMBRA and ESP, two temporary experiments deployed in the Pampeanflat slabregion, between the Andean Cordillera and the Sierras Pampeanas in the Argentine Andean backarc region.In order to determine more accurate hypocenters, focal mechanisms and regional stress orientations, werelocated 100 earthquakes using the JHD technique and a local velocity model. The focal depths of our relocatedevents vary between 6 and 50 km. We estimated local magnitudes between 0.4≤ML≤5.3 and momentmagnitudes between 1.3≤Mw≤5.3. Focal mechanisms were determined from new hypocenter relocations andfirst motion P-wave polarities. Our solutions exhibit a majority of the earthquakes with reverse faulting me-chanism. Regional stress tensor from the inversion of P- and T-axis orientations, shows a maximum stress axis(σ1) almost horizontal with a strike of 85° and a minimum stress axis (σ3) almost vertical.We correlate this small-to-moderate magnitude seismicity with the presence of large basement structuresbeneath the Iglesia-Calingasta Basin in the west and the Eastern Precordillera in the east. The nucleation of deepearthquakes beneath the Iglesia Basin could be related to the presence of a major ramp accommodating thecrustal shortening between the Frontal Cordillera and the Precordillera. The crustal seismicity beneath thePrecordillera seems to correlate with west-dipping structures rooting deep into the Cuyania basement suggestinga thick-skinned basement deformation system beneath the Precordillera and its shallow thin-skinned fold andthrust belt.