At Stromboli Volcano, Italy, very long period (VLP) seismic signals and Strombolian eruptions have been attributed to the unsteady flow of gas slugs through the shallow plumbing system followed by explosive slug bursting at a free surface. In data from a 2018 seismo‐acoustic deployment, ∼92% of events in two main VLP multiplets do not coincide in time with impulsive infrasonic signals (the expected signal of explosive slug bursting); we term these “silent VLPs.” The lack of infrasonically detected explosions relative to repeating VLPs does not support the commonly invoked “gas slug” model. We propose that VLPs may be generated when gas bubbles move into a weak semi‐solid plug in the uppermost portion of the conduit. The plug then acts as a mechanical filter in which pathways vary and guide or trap ascending gas slugs, allowing for passive (silent) gas release and explosive escape mechanisms decoupled in time from VLPs.
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 broad-band seismic and infrasonic network coverage from 2016 July 27 to August 3. 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 ∼2–10 s. The VLP seismic events are ‘high-rate’, reoccurring near-continuously throughout the data set with short interevent times (∼20–60 s). 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 timescale 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 ∼900–1000 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 ∼900–1000 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.
more » « less- NSF-PAR ID:
- 10364664
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Geophysical Journal International
- Volume:
- 230
- Issue:
- 1
- ISSN:
- 0956-540X
- Format(s):
- Medium: X Size: p. 392-426
- Size(s):
- ["p. 392-426"]
- Sponsoring Org:
- National Science Foundation
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Abstract Very long period (VLP) seismic events (with dominant periods of 15 to 40 s), observed from 2007 to 2018 at the summit of Kīlauea Volcano, Hawai‘i, arise from resonant oscillations in the shallow magma plumbing system. Utilizing an oscillation model developed in the companion paper (Liang et al., 2020), we perform Bayesian inversions on seismic data from four representative VLP events separately for the parameters of the shallow conduit‐reservoir system, exploring both sphere and crack reservoir geometries. Both sphere and crack geometries are preferentially located ∼1–2 km beneath the northeast edge of Halema‘uma‘u crater and produce similar fits to the data. Considering a reasonable range for reservoir storativity, magma density, and density contrast between the top and bottom of the conduit, we favor a spherical reservoir with a radius of 0.8 to 1.2 km and a short conduit of less than a few hundred meters. For this geometry, buoyancy from density stratification in the conduit provides the dominant restoring force for the VLP oscillation. Viscosity is constrained within an order of magnitude for each event (e.g., approximately 2 to 23 Pa s for one event versus 27 to 513 Pa s for another). Changes in VLP period
T and quality factorQ can be explained by changes in viscosity, density stratification, and/or conduit/reservoir geometry. In particular, observed fluctuations inQ over short time intervals (e.g., hours) with minimal changes inT apparently require rapid changes of magma viscosity by over an order of magnitude, assuming geometry remains unchanged, possibly reflecting changes in volatile content, bubble concentration, or conduit flow regime.
