New velocity determinations of P waves and S waves for the seismic crust of the Cuyania terrane in the Andean backarc

Abstract

In this study a crustal model (1D) of P and S wave seismic velocities is independently obtained for the region located between 30°–32.5° S and 67°–68.5° W of the Andean, at about 450 km east from the trench.

To determine this crustal model 514 local earthquakes data were used. These earthquakes had local magnitudes between 0.8 < ML < 4.8 and had occurred between July 1996 and January 2014, and were reported by the permanent seismic network catalog of the Argentinean National Institute on Seismic Prevention (INPRES). For these earthquakes, the P and S wave arrival times were manually read in 37 seismic stations. Thus, it was possible to obtain the seismic location of each earthquake using weighing for quality and distance, and a seismic velocity model previously obtained by other studies.

Then, the seismic velocity structure was studied in detail using a travel time inversion technique. The iterative method took into account several starting models with their velocity varying with depth; the best results occur for a starting model built from gradients in velocities whose values are smaller than those of an available crustal model. A final crustal seismic velocity model was obtained after testing 36 combinations of crustal parameters with seven average number of inversions each time.

The obtained crustal model indicates intracrustal discontinuities in seismic velocities at depths of 3 km, 13 km, 27 km and 36 km. In addition, the greater discontinuity in seismic velocities is observed at 47 km depth approximately, which is interpreted as the Mohorovičić discontinuity on the base of previous studies and velocity values representative of crustal and upper mantle regions. The shallowest level is sensitive to sedimentary basins which contain more than 70% of the studied region. Discontinuities located at 13 km and 27 km depths show a good correlation with décol­lement levels shown by other geophysical studies. The results for wave velocities P (Vp) and for wave velocities S (Vs) agree with low Vp/Vs values in the lower crust. These new geophysical determinations are con­sistent with a lower crust of a higher increase in Vs in comparison with the lower increase observed in Vp, and a lack of seismicity. All together, these results agree with a mafic probably partially eclogitized lower crust.

The high Vp/Vs ratio for the upper crustal levels would indicate a higher fracturation of the Cuyania terrane consistent with a decrease in Vs values and earthquake generation in comparison with deeper crustal levels.

This best model enables to fit the both the P and S wave travel times, and thus allow improving accuracy of earthquake locations with epicenters within the region of study.

Since this study uses a densification of arrival P and S wave phases from crustal earthquakes located under the eastern part of the Andean Precordillera and the Western Sierras Pampeanas, results represent an indistinguishable basement that corresponds to the integrated Cuyania terrane.