Lithospheric velocity model in the flat slab region of Argentina from joint inversion of Rayleigh-waves and teleseismic receiver functions

AMMIRATI, J.P.; ALVARADO, P.

Cordoba

Congreso; XIX CONGRESO GEOLÓGICO ARGENTINO; 2014

AGA

Receiver Function (RFs) analysis is a powerful technique for imaging earth structure (Langston, 1979; Owens et al., 1984). It uses travelling waveforms from distant earthquakes (with epicentral distance from ~3000 km to ~10000 km) that will generate reverberations (P-wave to S-wave conversions) when crossing discontinuities in seismic velocities. Considering several earthquakes with a good azimuthal distribution, RFs registered at a three-component seismological station can be inverted to obtain a lithospheric velocity model beneath the station. The technique, however presents a velocity-depth trade-off, which drives to a non-unique model because RFs do not depend on the absolute seismic velocities but rather on relative velocity contrasts between two layers (Ammon et al., 1990). To solve for a more robust model we have combined RFs with surface wave analysis (Julià et al., 2000). Surface wave dispersion is not sensitive to S-wave velocity contrasts like RFs but it is however, sensitive to the average shear-wave velocity which makes it well adapted for studying the long period variations of the lithospheric seismic velocities. In this study we used a joint inversion of RFs and Rayleigh-waves to investigate the velocity structure beneath the SIEMBRA network, a 40-broadband-seismic-station array deployed between 2008 and 2009 in the Pampean flat slab region of Argentina (Fig. 1). Preliminary results show a better defined lithospheric structure and agree with averaged previous images based on other techniques (ex. Zapata, 1998; Fromm et al., 2004; McGlashan et al., 2008; Perarnau et al., 2010; Gans et al., 2011; Ammirati et al., 2013). Thus, our results indicate: 1) The presence of several mid-crustal discontinuities probably related with terrane accretionary history; 2) A relatively high seismic velocity in the lower crust well-suited with the hypothesis of partial eclogitisation; 3) The presence of the subducting oceanic crust lying at a depth of approximately 100 km with different thickness. This Pampean flat slab velocity model will help to better characterize regional seismicity.