Tectonic and volcano-tectonic seismicity below Tungurahua volcano (Ecuador) between 2013 and 2018

Tungurahua (5023 m a.s.l.) is an andesitic stratovolcano located in Central Ecuador. The more recent eruptive cycle started in September 1999 and lasted until March 2016 with repeated phases of enhanced activity. Its activity included the occurrence of distinct eruptive phases separated by periods of quiescence, both lasting from few weeks to months. From October 2013 until March 2018, we operated at Tungurahua a temporary seismic network including up to 13 broadband stations. It complemented the permanent monitoring network operated by the Instituto Geofísico de la Escuela Politécnica Nacional (IG-EPN) and included stations up to 4275 m a.s.l. as well as stations on the remote Eastern flank.

Using IG-EPN catalogs and cross-correlation techniques, we identified several clusters of shallow and deep (volcano-)tectonic earthquakes. For these clusters, we manually picked a selection of larger events and used them to pick automatically other similar events. A visual inspection of the pickings was performed to confirm the absence of major biases. The comparison of P-phase times shows differences less than 0.1 s. Regarding S-phases, the cross correlation technique detected by far more S-phases per event, providing a general improvement in the location of events. Additionally we used seismic amplitudes and their decay as a function of distance to locate tremor and explosion quake sources during eruptive phases.

The seismicity below sea level defines 4 main clusters spread around the volcano between 2 and 10 km b.s.l.. The temporal evolution of these clusters displays a rather steady behavior for 3 of them and a swarm-type behavior for the fourth. Their relation with the eruptive phases is, however, unclear. Above sea level a single cluster of small volcano-tectonic events is observed about 2-3 km below the summit. This cluster displays a rather clear relationship with the eruptive phases and often preceded phases with strong explosive onsets. Most of tremor and explosion quake sources are found just above this cluster.

This study emphasizes the importance of dense, geographically well distributed networks, to identify seismic precursors and decipher volcanic plumbing systems.