Velocity structure (1D) and earthquakes relocation in the flat-slab to normal plate transition zone in the Argentinean transarc

Abstract

The Argentinean Andean foreland region between latitudes 31°S to 33.5°S presents an intense crustal and upper mantle seismic activity that was recorded by a local seismological network called CHARSME (Chile Argentina SeisMic Experiment), and was used to obtain new velocity determinations. In a first moment, from 4 P-wave velocity (VP) models proposed by other authors, the detail in the layers was increased, resulting in better models used as initial model in the investment. Finally, by carefully selecting a set of earthquakes, a simultaneous velocity inversion, hypocentre determination and station correction were performed using the VELEST code. In order to obtain better velocity models (VP and VS) and VP/VS ratio, different tests were carried out in all processes. The final VP model presented in this paper incorporates considerations made in other seismological studies. This model accurately determined velocities between 10 and 65 km depth. The upper mantle velocity (VP) was defined in 8.05 km/s. From the Wadatti diagram, which represents a line whose slope is related to the wave velocities and the Poisson´s coefficient, a VP/VS value of 1.732 was obtained. Alternative VP/VS values were also considered to invert VP, VS and VP/VS simultaneously. The minimum detailed model of VP, VS and VP/VS decreased the RMS by 25% relative to the initial models, implying an improvement in the uncertainty of locations. The new and precise locations of crustal earthquakes allowed defining clusters within Cuyania, a tectonic terrain that collided to Gondwana in the Early Paleozoic. Although it is known that foreland seismicity is mainly concentrated in this terrain, this work shows that there are well-defined zones within Cuyania that are more seismically active. Earthquakes of intermediate depth (upper mantle) are strongly agglomerated in the flat-slab zone of the subducting Nazca Plate. The discontinuities present in our model suggest upper, middle and lower crustal structuring, as well as a marked contrast between the sediments and the underlying basement. It was possible to interpret an upper crust consisting of sediments and basement to a depth of 25 km, a middle crust between 25 and 45 km and a lower crust between 45 and 55 km. We also defined the Mohorovičić (Moho) discontinuity at 55 km depth. The crustal velocities are in agreement with previous seismological studies.