Varying seismic‐acoustic properties of the fluctuating lava lake at Villarrica volcano, Chile

Abstract

Villarrica volcano outgasses through an open lava lake, with bubbles ranging in size from submillimeter to several meters, the largest of which produce strombolian bursting events that are visible from the crater rim. Thousands of shallow strombolian events identified through seismic waveform cross correlation were found to produce discrete and repetitive long-period seismic and infrasonic signals. We identified variations of up to 0.7 s in seismic-acoustic arrival delay times between April and July 2010 at a station~750 m from the vent, which we interpret as due to fluctuations in the level of lava lake. During time periods interpreted as having high lava lake levels, based on reduced time delays, interevent times were also reduced, and average seismic amplitude measurements, seismic and acoustic event energies, and volcano acoustic-seismic ratios were all high as compared to times when the lava lake was lower. The crater is also a source of nearly continuous, monotonic infrasonic tremor. We found that the peak frequency of this infrasonic tremor, typically around 0.5-1.0 Hz, was inversely correlated with seismic-acoustic delay times and therefore an indicator of lava lake level. We use this correlation to propose a new model for infrasonic tremor generation, namely, using crater geometry to approximate a Bessel horn. We interpret the two clearest cycles of elevated seismicity and lava lake level as due to an increase in exsolved gas, resulting from an injection of volatile-rich magma or an overturn in a deeper magma reservoir.

Single-Station Seismo-Acoustic Monitoring of Nyiragongo's Lava Lake Activity (D.R. Congo)

“…Johnson and Aster (2005) obtained high and stable VASR values at Erebus volcano and attributed this observation to the lava lake activity associated with repetitive source processes at the surface of a stable lava lake. The observed decrease of the AS amplitude ratio in the case of Nyiragongo is not in agreement with such a constant trend and is better explained by variable VASR as observed at Villarrica volcano by Richardson et al (2014) during fluctuating lava lake episodes.…”

“…Two further attributes are of significance with respect to the lava lake level, namely the delay time of the infrasound arrival with respect to the seismic one and the frequency content of the infrasound events (Richardson et al, 2014;Johnson et al, 2018). Unfortunately, the delay time information, which is potentially useful for measuring the level of lava in a conduit (Ripepe et al, 2002;Johnson, 2007) is particularly affected by 6 km of propagation through the atmosphere (see section Identification of Repetitive Long-Period (LP) Seismo-Acoustic Events).…”

“…Several acoustic resonance models have been applied to other lava lake volcanoes (e.g., Bessel horn resonator at Villarrica in Richardson et al, 2014; Helmholtz resonator at Kilauea in Fee et al, 2010), the simplest one being the cylindrical geometry, or organ pipe, that could eventually fit to the shape of Nyiragongo's lava lake. However, the 1.2 km wide Nyiragongo crater is significantly more complex than a pipe or other standard geometries of resonators (see Figure 1B).…”

“…Other type of flow-induced resonance was described at the neighboring Pu'u O'o crater complex by Matoza et al (2010). Recently, Richardson et al (2014) showed that the lava lake fluctuations at Villarrica volcano may be inferred by the joint analysis of long-period seismo-acoustic single events and continuous tremor. This observation is therefore particularly relevant in the case of Nyiragongo for studying the activity of its lava lake.…”

Long period seismicity and very long period infrasound driven by shallow magmatic degassing at Mount Pagan, Mariana Islands

“…Expanding observations to include other data types also facilitates the interpretation of LP source processes. Studies correlating seismic and infrasound data have revealed important linkages between shallow LP seismicity and the generation of infrasound at other open vent, basaltic systems, including Shishaldin [ Petersen and McNutt , ], Kīlauea [ Fee et al , ], and Villarrica [ Richardson et al , ]. Some forms of degassing also often accompany LP events, and correlating seismic and infrasound data with plume observations or gas chemistry allows further refinement of source models [e.g., Petersen and McNutt , ; Fee et al , ], although few studies include quantitative information about plume composition and emission rate.…”