Shallow intraplate seismicity in the Buenos Aires province (Argentina) and surrounding areas: is it related to the Quilmes Trough?

Abstract

In the light of the November 30th, 2018 (N30) earthquake activity, some neighborhoods of the city of Buenos Aires were shaken by a 3.8 mb earthquake (4.53 km estimated depth). We examined the historical and recent seismic records in order to analyze possible mechanisms related to the distribution of tectonic stresses as responsible for such unusual earthquakes in a region where only very little seismic activity is reported. According to this, at list one historical event occurred on June 5th, 1888 and other small magnitude earthquakes are mentioned since 1848 interpreted as being associated with the Rio de la Plata faulting. But there is, still no consensus about the role of this structure compared to other structures with orientation SW-NE. The lack of evidence to support one over the other structures makes it difficult to analyze these earthquakes. The presence of the Quilmes Trough connecting the Santa Lucía Basin in Uruguay and the Salado Basin in Argentina was recently proposed to play a tectonic role by a system of ENE-WSW trending controlled by extensional faulting related to the beginning of the Gondwana breakup. This depocenter with a thickness of almost 2,000 m of Mesozoic and Tertiary sequences could be acting as a zone of weakness in the crust and therefore responsible for the mentioned earthquake activity. The orientation of this structure correlates well with the present convergence vector between the Nazca and the South American plates and could therefore be propitious for strain release triggering shallow intraplate seismicity. We propose that most of the epicenters from historical and recent earthquakes might be aligned sub-parallel to the principal axis of the Quilmes Trough. Nevertheless, more data is needed to produce a reliable earthquake monitoring system in order to elucidate the tectonic stress regime and the existence of such structures at depth

Transtensive Origin of the Encadenadas-Vallimanca Corridor (Buenos Aires, Argentina): A Revision and a New Proposal From Satellite Images

“…From the tectonic point of view, the Pacific margin of South America is subjected to compression since the Mesozoic when the Nazca-South America convergence established a sub-latitudinal deformation regime (Brooks et al, 2003;Sperner et al, 2003;Cobbold et al, 2005;Costa et al, 2006;Guzmán et al, 2007). The revised and updated compilation of focal mechanisms in intraplate South America shows that horizontal compressional stresses predominate, not only in the Andean foreland belt, but also in mid-plate areas of the continent (Assumpção et al, 2016;Rossello et al, 2020). The cited works allow assuming the current maximum horizontal stress orientation for the Pampean region with an azimuth of 80º, which is slightly oblique with respect to the EVC, putting in evidence the right-lateral component of the wrench faulting (Figure 5).…”

“…The right lateral EVC involves basement levels and suggests the flow of crustal material farther east, possibly facilitating the widening of Cenozoic depocenters located in the Atlantic margin such as those documented by Rossello et al (2017). Richardson et al (2013) argued that the increased coupling between the subducting flat slab and the overriding plate beneath the Andean Cordillera at 32ºS resulted in the propagation of Neogene and Quaternary crustal deformation eastward to the Rio de la Plata craton (Rossello et al, 2020). In this sense, the EVC located far East could constitute the prolongation of a major slab discontinuity zone.…”

“…Although this author did not make emphasis on structural features deductible from the geophysics, it is possible to infer a not properly displayed pattern of interruption for the anomalies matching the EVC position (Figure 3). The Pampean plain, covering the Buenos Aires and La Pampa provinces, has been traditionally considered to be seismically inactive in contrast with the active Andean system associated with the subduction of the Nazca plate under the South American plate (Assumpção et al, 2016;Rossello et al, 2020). However, two moderate earthquakes with magnitudes close to 4.0 magnitude (Richter scale) and epicenters at the SW of the Buenos Aires province (Figure 1) were registered in 2016 by the INPRES (2019):…”

“…These data would be most relevant for validating the activity and type of movement on the controlling faults of the EVC, and for elucidating whether we deal with a pure strikeslip phenomenon or a rift system associated with a regional-scale extensional regime. These earthquakes could be related to the EVC for their proximity, but other not yet defined basement discontinuities are capable of reactivation despite being located away from typically seismic areas (Nivière et al, 2013;Reuber & Mann, 2019;Rossello et al, 2020). The referred magnitude and depth have no direct strong incidence in inducing surficial deformation, but the softness of the sedimentary cover could be easily accommodated on the underlying fractured basement.…”

Possible hydrogeological and thermal conditions of the Quilmes Tectonic Trough (Buenos Aires Province, Argentina): a working hypothesis

“…Rossello et al (2011) identified first-order structural features with subparallel highs and lows and matching general ENE-WSW directions in the basement, similar to those in the Santa Lucía basin, and with extensional tectonics related to the Gondwana breakup and the early opening of the Atlantic Ocean (Figure 3). This structuring is considered to be one of the factors responsible for the 3.8-degree earthquake recorded approximately 20 km south of Buenos Aires on November 30, 2018, with an estimated epicenter of approximately 4.5 km (Rossello et al, 2020).…”