Swarms of microearthquakes associated with the 2005 Vulcanian explosion sequence at Volcán de Colima, México

The swarms of microearthquakes, that appeared at Volcán de Colima, México before and after its large 2005 Vulcanian explosions, are discussed. The study of 966 microearthquakes is based on the seismic records of short-period seismic station EZV4 situated at a distance of 1.7 km from the crater. Three samples of microearthquakes were selected: the seismic records associated with a single 2005 September 16 large explosion, and the seismic records associated with two sequences of large explosions, the 2005 March 10 and 13 explosions and the 2005 May 30 and June 2, 5 and 7 explosions. These explosions were seven of 15 large explosions (E ≥ 10¹¹ J) that occurred during the 2005 explosive sequence. The microearthquake waveforms were identified as the records of rockfalls and microexplosions. The explosive microearthquakes represent from 84 to 89 per cent of the total number of microearthquakes with the readable waveforms. The dominant frequencies of the explosive microearthquakes were 2.8 Hz for the 2005 March explosion sequence and 2005 September 16 explosion, and 1.3, 2.0 and 2.8 Hz for the 2005 May-June explosion sequence. Energy of microexplosions that generated the microearthquake waveforms ranged from 10⁴ to 7 × 10⁷ J. The force component, corresponding to the largest microexplosive events, was estimated at a level from 3.1 × 10⁷ to 3.6 × 10⁸ N. The appearance of microearthquakes before large volcanic explosions and the sharp increase in their rate of appearance some hours before an explosion makes them a useful instrument for volcano monitoring.