Source mechanism of Vulcanian eruption at Tungurahua Volcano, Ecuador, derived from seismic moment tensor inversions

Abstract

Source mechanisms of explosive volcanic eruptions are critical for understanding magmatic plumbing systems and magma transport. Tungurahua is a large andesitic stratovolcano where seismoacoustic data has been recorded over several years. In May 2010, an energetic eruption cycle began with a midsize Vulcanian explosion followed by swarms of explosive eruptions. The five-station seismoacoustic network recorded significant seismic and infrasonic signals from the explosions. Source mechanisms of 50 explosion earthquakes associated with Vulcanian explosions during this eruptive period are investigated here. The source centroid locations and geometries of explosive signals in the 10-2 s band were determined by full-waveform moment tensor inversion. The observed waveforms are well explained by a combination of volumetric moment tensor components and a single, vertical, downward force component. The source centroids are positioned about 1.5 km below and about 320 m north of the active crater. Eigenvalue and eigenvector analysis indicates that the source geometries can be described by a subhorizontal, thin ellipsoid representing a sill-like magma accumulation. Resultant source time histories show a repetitive sequence of inflation and deflation from event to event, indicating identical source processes frequently occurred over the period. The inflation/deflation in the deep source region may be the result of crack opening. Volatile bubble growth at depth opens a pathway for gases to escape and triggers shallow explosions at the summit crater. The associated downward single force is interpreted as an exchange of linear momentum between the source and the surrounding region during the escaping gas flow.

Rapid hazard assessment of volcanic ballistic projectiles using long-exposure photographs: insights from the 2010 eruptions at Tungurahua volcano, Ecuador

“…These velocities are significantly lower compared to Sub-Plinian eruptions [Robertson et al 1998]. In Hidalgo et al [2015], the January, May-June and November eruptions are named phases IV, V and VI and our photographs fall in the high-explosive activity (HEA) field that is interpreted as Vulcanian activity, supporting the seismo-acoustic analysis from Kim et al [2014]. Although the Vulcanian classification fit closely the May activity that exhibits discontinuous explosive activity, the January and November activities are characterized by continuous ash venting and pyroclastic fountaining.…”

Site effect determination using seismic noise from Tungurahua volcano (Ecuador): implications for seismo-acoustic analysis

“…However, a second mechanism where a deep source generates waves that propagate along the conduit, reach the summit and trigger a explosion (Nishimura & Chouet 2003) has been observed and modelled at Tungurahua volcano (Kumagai et al 2011;Kim et al 2014). After removing the site effects, Fig.…”

“…Due to the short time between explosions (minutes to hours) they were likely generated in a limited spatial region (e.g. Kim et al 2014). Computing the mean distance from the explosion locations to the cluster centre, and defining it as the median of the locations, we choose Q that produces a minimum cluster mean distance.…”

“…Although we do not know the velocity model of the stratified medium beneath each station, assuming that the recorded seismic noise stores site information, the site correction of the signals should produce amplitudes only related to high wave velocities (2000 m s −1 , as average, for Tungurahua). Additionally, the VASR computation must take into account the source location (Johnson & Aster 2005); in case of Tungurahua is common to find them several kilometres beneath the crater (Kumagai et al 2011;Kim et al 2014). The red triangles in Fig.…”

“…Furthermore, site corrections can lead to significantly improved source locations of volcanic explosions. We have assumed an isotropic source mechanism, but other source type could be generalized, as has been done in previous studies on Tungurahua volcano (Kumagai et al 2010;Kim et al 2014). In general, corrections for site effects will be important for any seismic analysis method that relies on true amplitudes and accurate amplitude ratios between components.…”