Analysis of the seismic wavefield properties of volcanic explosions at Volc ´an de Colima,Mexico: insights into the source mechanism

Abstract

We have investigated thewavefield properties of the seismic signals generated by the explosions of Volc´an de Colima (M´exico). We have analysed these properties to understand the initial mechanism that triggered the explosive events. Our study is focused on the direct waves coming from the crater area. Thus, we have analysed a set of moderate volcanic explosions at Volc´an de Colima that was recorded by a small aperture seismic array over two periods: October 2005 and April 2006. We can distinguish two types of explosions, Vulcanian and ash-free events. Both types of explosions share the same characteristics, that is a long-period signal (not related to any type of emission) before the arrival of high frequency phases, and a later high frequency signal directly related to ash or gas emission. We have applied the Zero Lag Cross Correlation technique to obtain backazimuth and apparent slowness of the incoming waves. We have also applied polarization analysis to the record of every detected volcanic explosion. By comparing the results of both of these analyses, we have been able to identify the dominant wave types that comprise the seismic wavefield and infer in time and space a possible primary source mechanism that would trigger the volcanic explosions. We have observed an apparent slowness variation of the first onset of the long-period (LP) signal with a possible upward migration of the source; the depth of the source has been identified at a range between 2.6 and 3.3 km below the crater, associated with the range of measured apparent velocities relative to the first onset of the LP signal.