Seismic refraction tomography in San Luis, headward Coca River erosion zone

Abstract

In the context of the headward erosion of the Coca River, we have carried out a seismic tomography experiment in the San Luis town at the river bank. San Luis is located directly over the eroded debris avalanche, but this resists mass movement, probably due to the presence of volcanic breccia deposits.

We have developed a procedure to attack seismic tomography’s regularization problem. This procedure consists in using regularization by smoothing. The smoothing parameter is chosen whit the L-curve criterion and gives the optimal solution. To draw the L-curve, we have varied the vertical and horizontal smoothing parameters and plotted the root mean square error of the model versus the three norms between the a priori and the resulting models. We have tested the Euclidean, taxicab, and infinity norms in our procedure to better detail the optimal solution. We have tested the regularization strategy in a synthetic model with a similar dimension to the problem in our survey.

The results of tomography allow describing the geology strata structure below the village. Two stratigraphic units resulting from debris avalanche deposits are overlayed in a bedrock unit that seems to give structural strength to prevent river erosion in San Luis. We have identified this bedrock with the volcanic breccia at 20 m depth. The breccia P wave seismic velocity obtained with the tomography is 1.2 km/s.

The parameters of velocity and structure resulting from our seismic survey allow a hazard diagnostic and future solutions for the San Luis settlement.