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 broadband seismic stations from the SIEMBRA and ESP, two temporary experiments deployed in the Pampean flat slab region, 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, we relocated 100 earthquakes using the JHD technique and a local velocity model. The focal depths of our relocated events vary between 6 and 50 km. We estimated local magnitudes between 0.4 ≤ ML ≤ 5.3 and moment magnitudes between 1.3 ≤ Mw ≤ 5.3. Focal mechanisms were determined from new hypocenter relocations and first motion P-wave polarities. Our solutions exhibit a majority of the earthquakes with reverse faulting mechanism. 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 structures beneath the Iglesia-Calingasta Basin in the west and the Eastern Precordillera in the east. The nucleation of deep earthquakes beneath the Iglesia Basin could be related to the presence of a major ramp accommodating the crustal shortening between the Frontal Cordillera and the Precordillera. The crustal seismicity beneath the Precordillera seems to correlate with west-dipping structures rooting deep into the Cuyania basement suggesting a thick-skinned basement deformation system beneath the Precordillera and its shallow thin-skinned fold and thrust belt.