Imaging the Crust and Upper Mantle of Northern Chile Using Ambient Noise Tomography
Wednesday, 17 December 2014
Northern Chile is recognized as a seismic gap along the interplate contact, associated with the subduction of the Nazca plate beneath the South American plate. In March 2014, part of this region near the town of Pisagua experienced anomalous seismic activity, which motivated the deployment of 26 broadband seismometers by ONEMI-CSN (Oficina Nacional de Emergencia del Ministerio del Interior y Centro Sismológico Nacional) just prior to and following the magnitude 8.2 April 1st earthquake. Incoming data from those stations, supplemented by data from 14 broadband permanent stations of the Integrated Plate boundary Observatory Chile (IPOC) and 9 temporary Iquique Local Network (ILN) stations operating through 2012-2013 are analyzed to create a 3D elastic shear-wave velocity model of the upper 40 km beneath northern Chile using ambient noise tomography. The vertical component of continuous ambient noise is whitened and cross-correlated between 202 station pairs to construct empirical Green’s functions (EGFs) of Rayleigh waves. EGFs with signal-to-noise ratios above 2.0 dB are used to recover group and phase velocities of the fundamental mode between 5 and 40 second periods. The resulting group and phase velocity maps are inverted on a grid with concentrated spacing in the area surrounding Iquique (with a dense network of stations) to generate a 3D shear wave velocity model. The model shows that the upper 40 km beneath this area are characterized by stark contrasts in shear wave velocities marking the major faults, going from 2.5 km/s to 4 km/s in some areas. The shear wavespeeds range between 2-5 km/s throughout the model.