Non-linear dynamics in long-period event swarms at Tenerife (Canary Islands, Spain)

Dense swarms of long-period (LP) or hybrid events are a common occurrence on many volcanoes. In many cases they have been attributed to transient, high speed flow of ash-laden or steam-laden mixtures, through volcanic conduits presenting sudden width variations (Morrissey and Chouet, 1997).

In the active volcanic island of Tenerife (Canary Islands) at least twice, it was observed the occurrence of dense swarms of LP events. The first occurrence was on 2 of October 2016 while the second on 14 of June 2019. In both cases they consisted of hundreds of transient events evolving toward a continuous tremor at the end of the swarm.

Using a simple template matching technique, we isolated discrete transient events to apply a waveform clustering algorithm and subsequently to stack the waveforms of similar events. In both swarms most of the events resulted belonging to a single family.

Stacked waveforms were deconvolved from the original signals to obtain the source function of the whole swarm, including the volcanic tremor toward the end. This function revealed a hidden pattern within the tremor consisting of isolated pulses repeating at non-regular intervals.

We analyzed the time series of the source function computing the time-varying fractal dimension showing how it changed during the transition from discrete events to continuous tremor. We interpreted this variation in terms of a fluid dynamics model of an unstable choked flow (Morrissey and Chouet, 1997). In particular we interpret the sudden transition toward volcanic tremor as a genuine phase transition toward a chaotic regime, triggered by a continuous change in the pressure gradient within the conduit.

We believe our approach can be of general application in understanding the mechanism of similar LP-hybrid swarms in different volcanic contexts.