Crustal Structure of the Northern Chilean Forearc from Ambient Noise Tomography

Abstract:

In addition to being an excellent venue for investigating the tectonics of the Andean margin, northern Chile is of particular interest to seismologists because of its potential for an imminent megathrust earthquake. Such events often trigger destructive seismic activity in the populated forearc, as demonstrated for example in the aftermath of the 2010 Maule event. To investigate the nature of deformation in the forearc, we generated high resolution images of the subsurface from Rayleigh wave dispersion curves derived from cross correlation of ambient noise. The ambient noise data were recorded over a period of three years by 60 stations from three different networks of broad band stations. Because of the proximity of the stations to the Pacific Ocean, we estimated the bias in the estimated Green’s functions caused by the asymmetry of the noise distribution using a technique based on that described by Yao and van der Hilst (2009). Our results suggest that this bias can be as large as 5% for some station pairs. The unbiased times are then used to refine phase velocity maps, from which we derived transit times to generate a 3D image of shear wavespeed (Vs) from the surface to about 50 km depth. To first order, low-Vs anomalies correlate well with the geometry of the Atacama Bench Structure (western foreland basin) where leaching processes are related to large incisions in the Atacama Desert (north of 19ºS). In addition, high Vs anomalies correlate with the locations of fossil magmatic arcs developed as trench-parallel belts from the coast to the Altiplano. For example, high Vs correlates with the Jurassic-Cretaceous magmatic arc along the coast, the Paleocene-Oligocene magmatic arc in the central depression, and the Eocene-Oligocene magmatic arc in the Frontal Cordillera. A continuous seismic anomaly of low-Vs, located between 15 - 25 km depth, may be evidence of a weak and/or hydrated zone within the lower continental crust, related to slab-linked upper plate hydration processes and partial melting at the base of the crust. Finally, well located crustal seismicity correlates with the western Andean thrust structure that controls the western uplift of the Altiplano.