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

摘要

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.