S U M M A R YWe have investigated the wavefield properties of the seismic signals generated by the explosions of Volcán de Colima (México). 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 Volcán 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.

The block-lava effusion at Volcµn de Colima, MØxico began on November 20, 1998, after 12 months of seismic activity, and ended about 80 days later. Three types of seismic events were observed during the lava effusion. Volcano-tectonic earthquakes occurred mainly at the very beginning and after the termination of lava effusion. Explosion earthquakes occurred frequently during the period of the maximum rate in lava effusion. The remainder of the seismic signals were associated with pyroclastic flows and rockfalls from the lava dome. These latter signals increased sharply in number at the onset of lava effusion. The rate of occurrence remained high when the lava discharge rate decreased but gradually decreased after the termination of lava effusion. Maximum daily durations of seismic signals are proportional to the daily volumetric output of lava, indicating the dependence of the number of pyroclastic flows on the rate of lava output. A log-log plot of seismic signal duration vs. number of events with this duration displays a linear relationship. The short-period seismic signals can be divided into three categories based on duration: short events with durations less than 100 s; intermediate events with durations between 100 and 250 s; and long events with durations longer than 250 s. We infer that long events correspond to pyroclastic flows with mean deposit volume $210 5 m 3 , and intermediate events represent pyroclastic flows with mean deposit volume $110 3 m 3 .

This paper presents an analysis of seismicity associated with the volcanic activity of Volcàn de Colima (México) and recorded in the period

No abstract