Locating sources of volcanic tremor and emergent events by seismic triangulation: Application to Arenal volcano, Costa Rica

Abstract : We address the issue of locating the sources of volcanic tremor and emergent events with a method requiring a limited amount of equipment. A network of several triangular seismic antennas made of vertical sensors is used. The slowness vectors are estimated at each array on a sliding window by inverting the time delays between the sensors calculated with the cross-spectral method. A probabilistic approach is adopted whereby each measure and its error are represented by a probability density function (PDF). A weighted summation of the PDFs is carried out in which the stable directions of propagation are enhanced. The effects of the structural heterogeneities are taken into account by introducing an additional error associated to a robust hyperbolic secant (sech)-type PDF. The resulting PDFs of the back-azimuth are combined to calculate a PDF of the source location. The maximum likelihood of this PDF is taken as an estimate of the source position and its spread is characterized by a covariance analysis. Data from an experiment carried out at Arenal volcano (Costa Rica) with four arrays are analyzed. The precision and robustness of the method are tested by exploring the influence of the array configuration and other parameters. The mean standard deviation on the position of the sources is 600 m for the tremor and 400 m for the explosions and long-period (LP) events. Several tremors, explosions and LP events are analyzed and their sources located. The seismogenic zone is located in a 600 m radius area centered on the active crater.