Nuevas determinaciones de velocidades de ondas P y ondas S para la corteza sísmica del terreno Cuyania en el retroarco andino

VENERDINI, A.; SANCHEZ, G.; ALVARADO, P.; BILBAO, S.; AMMIRATI, J.B.

REVISTA MEXICANA DE CIENCIAS GEOLóGICAS

CENTRO GEOCIENCIAS UNAM

Lugar: México; Año: 2016 vol. 33 p. 59 - 71

1026-8774

In this study a crustal model (1D) of P and S wave seismic velocitiesis independently obtained for the region located between 30°?32.5° Sand 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 andhad occurred between July 1996 and January 2014, and were reportedby the permanent seismic network catalog of the Argentinean NationalInstitute on Seismic Prevention (INPRES). For these earthquakes, theP and S wave arrival times were manually read in 37 seismic stations.Thus, it was possible to obtain the seismic location of each earthquakeusing weighing for quality and distance, and a seismic velocity modelpreviously obtained by other studies.Then, the seismic velocity structure was studied in detail using atravel time inversion technique. The iterative method took into accountseveral starting models with their velocity varying with depth; the bestresults occur for a starting model built from gradients in velocities whosevalues are smaller than those of an available crustal model. A final crustalseismic velocity model was obtained after testing 36 combinations ofcrustal parameters with seven average number of inversions each time.The obtained crustal model indicates intracrustal discontinuities inseismic 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 depthapproximately, which is interpreted as the Mohorovičić discontinuity onthe base of previous studies and velocity values representative of crustaland upper mantle regions. The shallowest level is sensitive to sedimentarybasins which contain more than 70% of the studied region. Discontinuitieslocated at 13 km and 27 km depths show a good correlation with décollementlevels shown by other geophysical studies. The results for wavevelocities P (Vp) and for wave velocities S (Vs) agree with low Vp/Vsvalues in the lower crust. These new geophysical determinations are consistentwith a lower crust of a higher increase in Vs in comparison withthe 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 ahigher fracturation of the Cuyania terrane consistent with a decreasein Vs values and earthquake generation in comparison with deepercrustal levels.This best model enables to fit the both the P and S wave traveltimes, and thus allow improving accuracy of earthquake locations withepicenters within the region of study.Since this study uses a densification of arrival P and S wave phasesfrom crustal earthquakes located under the eastern part of the AndeanPrecordillera and the Western Sierras Pampeanas, results represent anindistinguishable basement that corresponds to the integrated Cuyaniaterrane.