Search

In this work we propose a detailed analysis of the crustal seismicity characterizing the Sierras de Córdoba (Eastern Sierras Pampeanas, central Argentina). We considered a large number of first P- and S-wave arrivals with good azimuthal coverage by the SIEMBRA and ESP temporary seismic networks along with a regionally calibrated velocity model. We obtained accurate hypocentral solutions and related them with possible faulting basement structures. Our results indicate that the crustal seismicity of the Sierras de Córdoba is mainly concentrated to the north, west and south-west of the Sierra Grande. In addition, the seismicity at depth seems to be preferentially distributed at middle crustal levels (depths ranging from 10 to 26 km). Focal mechanisms solutions obtained from P-wave first motion polarities and P–S amplitude ratios are predominantly reverse and could be associated with the extension at depth of the Sierra de Pocho, Sierra Grande and Ciénaga del Coro faults. The magnitudes are comprised between 1.6 ≤ M L ≤ 3.3 and 1.7 ≤ M W ≤ 3.4. They represent the first estimates for small-to-moderate sized earthquakes occurring in the Sierras de Córdoba and recorded by local, temporary seismological networks. • In plan, the earthquakes in the Sierras de Córdoba are heterogeneously distributed. • The middle crustal levels concentrate the highest number of earthquakes. • Focal mechanisms show the extension of the Sierra de Pocho fault at depth. • Sierra de Grande fault is recognized at depth from determined focal mechanisms. • Ciénaga del Coro fault is identified from focal mechanisms in the middle-crust. [ABSTRACT FROM AUTHOR]

Elastic properties in twelve representative rock samples from Central and Western Precordillera in the Andean backarc region of Argentina between 30 and 31°S were estimated from detailed petrological analysis. Thus, P and S seismic-wave velocities (Vp, Vs) as well as Poisson's ratio (σ) among other parameters were derived for gabbros, leuco-gabbros and wehrlites, in greenschist and amphibolite metamorphic conditions using a framework of a wide variety of empirical observations from active continental margins. In addition, Vs lithosphere models along two west–east cross sections were obtained using a joint inversion of teleseismic Rayleigh waves and receiver functions. These models clearly delineate the upper-plate crustal structure and the flat-slab subduction of the Nazca plate at about 100 km depth in this region. The suggested seismic velocity structure shows a relatively low (<3.3 km/s) Vs layer located in the first 15–18 km depth, then an increase of it from 3.3 to 4 km/s between 20 and 55 km depth with a mayor change at 40 km depth beneath the Precordillera showing an increase in Vs from 3.3 to 3.8 km/s. The Moho discontinuity was identified at around 65 km depth beneath the Precordillera (Vs = 4.3 km/s) and shows a low shear-wave velocity contrast in comparison with the upper continental mantle's parameters. Using this seismological model, Vs estimations derived from the petrological analyses for the 12 collected samples can be projected at depths greater than 30 km. These geophysical and petrological results agree with the hypothesis of a mafic thickened and partially eclogitized lower crust beneath the Precordillera, which has been predicted previously on a base of seismological studies only. Our petrological and seismological results collectively support a thick crustal model of a mafic–ultramafic composition extending to middle-to-lower crustal levels beneath Central and Western Precordillera; this region correlates with a suture zone between the eastern Cuyania terrane and the western Chilenia terrane. [ABSTRACT FROM AUTHOR]

Abstract: The subduction of the Nazca plate under the South American plate around 31°S is characterized by flat slab geometry. The (Chilean) Pampean flat slab of Argentina associated with the subduction of the Juan Fernandez ridge lies in a region of a series of foreland uplifts corresponding to the thin-skinned Precordillera and basement cored Sierras Pampeanas ranges. The SIEMBRA project deployed 40 broadband stations in 2008–2009 in both the Precordillera and the Sierras Pampeanas with the aim to foster the understanding of the entire central Andean flat slab region. One of the SIEMBRA station (DOCA) located on the western flank of Sierra de la Invernada in the Central Precordillera appears particularly appropriate to study the crustal structure and eventually detect discontinuities related to terranes establishment. We thus performed a receiver function analysis using teleseismic data recorded at the DOCA station during the SIEMBRA project and from October 2011 to June 2012 using a broadband UNSJ (National University of San Juan) seismic station with the purpose to obtain crustal images with details of the intracrustal structure consistent with a mechanism that could explains both the observed earthquake depths and the uplift pattern in the Central Precordillera. Our results show that the Moho beneath the Precordillera lies at a depth of about 66 km. The Moho signal appears diminished and behaves irregularly as a function of azimuthal orientations. Although this observation could be the result of an irregular geometry it also correlates with the hypothesis of partial eclogitisation in the lower crust. Two mid-crustal discontinuities have also been revealed. The shallower one could correspond to a décollement level between the Precordilleran strata and the Cuyania basement at 21 km depth. The deeper one which the presence has been matched with a sharp decrease of the crustal seismic activity drove us to the hypothesis of a major change in crustal composition at 36 km. Finally the flat portion of the subducted slab has been imaged lying at about 100 km depth. [Copyright &y& Elsevier]

The Magallanes-Fagnano fault (at ~51-56°S) is a continental NW-SE transform fault zone that marks the boundary between the South American Plate (north) and the Scotia Plate (South). In 1949, two crustal seismic events of estimated magnitude M > 7 were located in this area using teleseismic records. Although, this region is scarcely populated, such shallow seismicity could represent a considerable threat to the remote urban centers in this sector. The characterization of the local seismicity always lacked good quality records and permanent instrumentation. In the recent past years, permanent seismic stations (CSN and INPRES-CTBTO) were deployed in the Tierra del Fuego Island and Southern Patagonia, providing with continuous time, broadband records of waveform data. In order to obtain better insights about the local seismicity associated with the Magallanes-Fagnano fault, we jointly inverted teleseismic receiver functions with Rayleigh-wave dispersion data to build a calibrated velocity model for the study region. In parallel, we analyzed the continuous waveforms to automatically detect local earthquakes and locate them using our regional velocity model. Our results show: 1) Shallow seismic events distributed along the Magallanes-Fagnano fault. 2) Diffuse seismicity characterizing the South American crust, north of 53°S. Our observations suggest two mechanisms of crustal deformation. In the Tierra del Fuego Island the seismicity is mostly associated with the Magallanes-Fagnano fault system. In the Southern Patagonia, along the Patagonian Andes the seismicity would be mostly due to isostatic rebound following the retreat of the Patagonian Ice sheet. • A new regional velocity model for the Southern Patagonia and Tierra del Fuego. • New hypocenter locations from local seismicity records. • The Magallanes-Fagnano fault is the main mechanism of crustal deformation in Tierra del Fuego. • Isostatic rebound may be the main mechanism of crustal deformation in the Southern Andes. [ABSTRACT FROM AUTHOR]

This paper aims to better comprehend the crustal structure of the Precordillera and the Iglesia Valley. Located in the northern sector of the Pampean flat-slab (San Juan, Argentina), this region was the epicenter of the historical 1894, M > 7 earthquake (intensity IX on the modified Mercalli scale) and thus considered as a region of particularly high seismic hazard. The structure and deformation style associated with such events remain poorly understood. In this study, we analyze 4 months of regional seismic records from national and international stations in order to characterize a sequence of 74 crustal events that followed the occurrence of an intermediate magnitude (M W = 4.8 and M L = 5.2) earthquake on October 28th, 2019 for which we constrain the source parameters using regional moment tensor inversion. The analysis also includes the determination of 11 focal mechanisms from P-wave polarities and P–S waves amplitude ratios. Most of the focal mechanisms indicate reverse motion solutions. Seismological results are then integrated with other geophysics (gravity and seismic reflection profiles) as well as geological data, to build a 3D structural model that allows a better understanding of the crustal deformation mechanisms of the study area. Our results suggest that the seismic deformation is mostly concentrated in the basement of the Western Precordillera, at the transition with the Frontal Cordillera. This deformation seems respond to compressional stresses. The proposed model provides evidence of ongoing seismic activity related to the interaction between the thick-skinned deformation of the Frontal Cordillera with the thin-skinned fold and thrust belt of the Western Precordillera. • We analyzed the crustal seismicity in the Western Precordillera and Iglesia Basin. • We modeled the source parameters for the October 28, 2019 earthquake. • We located 74 crustal earthquakes mainly distributed between 10