Seismic Anisotropy in the Converging Lithospheric System of NW South America

Date: 4/25/2019

Time: 06:00 PM

Room: Grand Ballroom

We present a 3D Seismic Anisotropic model of the lithospheric system NW South America based on the tomographic inversion of travel time seismic data from regional earthquakes. The dataset was provided by the Colombian Geological Survey (SGC). It combines events recorded by 52 broadband stations in the region. We use a passive source tomography algorithm, which was previously implemented to study crustal and uppermost mantle in other subduction regions of the World. In general terms, the shallowest anisotropy tends to align with the main faulted zones of the crust. As one descends in depth, anisotropy tends to aligned perpendicularly to the west coast of Colombia, indicating the influence of the westward Nazca plate subduction under the South American plate. The evidence was compared with focal mechanisms of events with magnitudes larger than Mw >5.0. The two most significant features of our observations have to do with two significant negative velocity anomalies and the direction of the anisotropy vectors associated to both regions. The first anomaly matches with the location of the Bucaramanga seismic nest and the second one is located close to the known Cauca seismic nest. Negative anomalies are both probably associated with the presence of fluids. In Bucaramanga, our hypothesis aims to the dehydration of the Caribbean plate and its break-off. In the Cauca swarm, we propose that the anomaly is due to the rise of fluids due to the dehydration of the Nazca plate and its melting in the mantle during the subduction process. In both cases, mineral transformation occurs, and the mantle behaves like a fluid. We noticed a radial pattern that velocity vectors tend to form around these two points. This circular spin does not seem to correspond with a particular structural element, but rather the rotation of the fluid mantle inside the Earth. Even more striking, has to do with the vectors rotation, since it occurs in the counter-clockwise direction. Can this last observation be scientific evidence of the Coriolis Effect inside our planet?

Presenting Author: Carlos Vargas Jimenez

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