DOI: 10.25349/d9k60w
title: Supplementary Movies: High-rate very-long-period seismicity at Yasur volcano, Vanuatu: Source mechanism and decoupling from surficial explosions and infrasound

description: Yasur volcano, Vanuatu is a continuously active open-vent
 basaltic-andesite stratocone with persistent and long-lived eruptive
 activity. We present results from a seismo-acoustic field experiment at
 Yasur, providing locally dense broadband seismic and infrasonic network
 coverage from 27 July to 3 August 2016. We corroborate our seismo-acoustic
 observations with coincident video data from cameras deployed at the
 crater and on an unoccupied aircraft system (UAS). The waveforms contain a
 profusion of signals reflecting Yasur's rapidly occurring and
 persistent explosive activity. The typical infrasonic signature of Yasur
 explosions is a classic short-duration and often asymmetric explosion
 waveform characterized by a sharp compressive onset and wideband frequency
 content. The dominant seismic signals are numerous repetitive
 very-long-period (VLP) signals with periods of ~2 to 10 sec. The VLP
 seismic events are "high-rate", reoccurring near-continuously
 throughout the dataset with short interevent times (~20 to 60 sec). We
 observe variability in the synchronization of seismic VLP and acoustic
 sources. Explosion events clearly delineated by infrasonic waveforms are
 underlain by seismic VLPs. However, strong seismic VLPs also occur with
 only a weak infrasonic expression. Multiplet analysis of the seismic VLPs
 reveals a systematic progression in the seismo-acoustic source decoupling.
 The same dominant seismic VLP multiplet occurs with and without surficial
 explosions and infrasound, and these transitions occur over a time-scale
 of a few days during our field campaign. We subsequently employ template
 matching, stacking, and full-waveform inversion to image the source
 mechanism of the dominant VLP multiplet. Inversion of the dominant VLP
 multiplet stack points to a composite source consisting of either a
 dual-crack (plus forces) or pipe-crack (plus forces) mechanism. The
 derived mechanisms correspond to a point-source directly beneath the
 summit vents with centroid depths in the range ~900-1,000 m below
 topography. All mechanisms suggest a northeast trending crack dipping
 relatively shallowly to the northwest and indicate a VLP source centroid
 and mechanism controlled by a stable structural geologic feature beneath
 Yasur. We interpret the results in the framework of gas slug ascent
 through the conduit responsible for Yasur explosions. The VLP mechanism
 and timing with infrasound (when present) are explained by a
 shallow-buffered top-down model in which slug ascent is relatively
 aseismic until reaching the base of a shallow section. Slug disruption in
 this shallow zone triggers a pressure disturbance that propagates downward
 and couples at the conduit base (VLP centroid). If the shallow section is
 open, an explosion propagates to the surface, producing infrasound. In the
 case of (the same multiplet) VLPs occurring without surficial explosions
 and weak or no infrasound, the decoupling of the dominant VLPs at
 ~900-1,000 m depth from surficial explosions and infrasound strongly
 indicates buffering of the terminal slug ascent. This buffering could be
 achieved by a variety of conditions at or directly beneath the vents, such
 as a high-viscosity layer of crystal-rich magma, a debris cap from
 backfill, a foam layer, or a combination of these. The dominant VLP at
 Yasur captured by our experiment has a source depth and mechanism
 separated from surface processes and is stable over time. 
description: The data folder contains four mp4 files and a README file
 describing them.