Transition from passive degassing to slow growth of a new lava dome as derived from seismic signatures: Volcán de Colima, México, January–February 2016

The seismic signatures, recorded during transition from passive degassing to slow growth of a new lava dome in January–February 2016 at Volcán de Colima, gave us the information allowing reconstruct this process. The study is based on seismic data collected by the seismic network of Colima University (RESCO). Processing of seismic signals was performed using the unfiltered records of vertical component of digital broadband station EZ5 situated at a distance of 4 km from the crater. Two types of seismic signals, with explosions followed by pyroclastic flows and incandescent rockfalls (PFIRs) (type 1) and without PFIRs (type 2), define the vent-clearing and a dome growing processes. Totally were selected 154 seismic signals which represented these two types of waveforms. Events of type 1 dominated during January and the first days of February. Events of type 2 began to appear actively beginning from 4 to 6 February. The process, generating Vulcanian explosions and corresponding seismic signals, is considered to follow two-stage conceptual model which consists of the ascent of fragmented magma to the surface (stage 1), and a subsequent explosion (stage 2). Explosive waveforms of the events of both types demonstrated the similarity in the range of seismic energy of the explosive phases and their spectral frequency. Similarity in distributions of durations of the phases, corresponding to the depths of initial ascent of fragmented magma to the surface before explosion, indicated the generation of both types of explosions at the same fragmentation zone within the conduit. Existence of two-phase and three-phase explosive waveforms showed the complex collection of the magma bubbles within fragmentation zone. It was noted a tendency of decreasing of the spectral amplitudes of PFIR signals in time during the period of study, which illustrated the development of the vent-clearing process and defined the period of a birth of new lava dome in the crater of volcano. Initial growth of a new lava dome may be considered within the time interval from 4 to 6 to 24 February, during a break in a vent-clearing process. The full filling of the crater with magma continued for about 8 months.