Explosive multiples preceding the growth of a new lava dome: Volcán de Colima, México, January–February 2016

Abstract

Explosive multiples (EM) represent the continuous sequencies of small-amplitude explosive events (members of multiples), which are characterized by low-height emissions of gas and ash. In this article, the seismic signatures of EMs, recorded in January–February 2016 at Volcán de Colima, México, by broadband seismic station at 4 km from the crater before the new lava dome growth, are discussed. The EM signals were recorded during the vent-clearing period; they practically disappeared with appearance of a new lava dome. Video images, recorded simultaneously with EMs, demonstrated that the EM signals were associated with the venting of low-height ash plumes. The durations of 24 selected EM seismic signatures varied from 1.5 to 8 min. The sequences of EM consisted of 3 to 7 explosive impulses, members of the EMs. Comparison with Vulcanian explosions, which were recorded simultaneously with EMs during vent-clearing period before the lava dome growth and were followed with pyroclastic flows and igneous rockfalls (PFIRs), demonstrates that the energy of the first members of EMs sequences was lower than the energy of Vulcanian explosions. The EMs were originated at more shallow depths comparing with Vulcanian explosions. Our study allows to see a feedback type relationship between the process of the vent-clearing by Vulcanian explosions, followed with PFIRs, and simultaneous explosive ash-venting by EMs with exit of gas and ash. As was noted by Zobin et al. (2021), the volume of PFIRs, following the Vulcanian explosions during vent-clearing period, decreased in time before the exit of a new lava dome. At the same time, there was recorded a simultaneous contrary tendency of increasing in durations of the EM seismic signatures in time (coefficient of correlation 0.55). Therefore, while the Vulcanian explosions worked digging large masses of rocks, which formed then PFIRs, the sequences of more shallow EMs, generated along the purified faults of crack system of the lava cap, blowed out clogged cavities clearing a way for lava exit with ash-venting. When the volume of explosive generated PFIRs became smaller, indicating the appearance of well-purified faults in the crater, the sequences of EMs became longer, increasing in the number of their explosive members occurring along these faults. They began intensive ash-venting of the fault cavities clearing way for lava exit. This feedback collaboration of two processes during vent-clearing period gives us an option for modelling of the preparation of the way for exit of the lava body into the crater.