1. Magnitude and Interevent Time Statistics of Strombolian Activity of Villarrica Volcano and Inference Regarding the Flow Regime.
- Author
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Lehr, Johanna and Rabbel, Wolfgang
- Subjects
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MAGNITUDE estimation , *BASALT , *VOLCANISM , *VOLCANIC ash, tuff, etc. , *VOLCANIC eruptions - Abstract
The different flow regimes of two‐phase gas‐in‐liquid flow—as it occurs in the shallow conduit of basaltic volcanoes—are characterized by distinct frequency‐size distributions of the liquid and gaseous slugs. Assuming that the ascent of gaseous bubbles is indicated by seismic events, we explore the possibility of inferring the flow regime from the frequency distributions of magnitudes and interevent times. Our data set consists of 20,000 volcanic seismic events recorded in early March 2012 at Villarrica Volcano (Chile), which are commonly attributed to Strombolian activity. One crucial factor is the completeness of the catalog in terms of detectable amplitudes, which we assess using a stochastic simulation of the network output based on statistical properties of the ambient seismicity. Magnitudes, at which we consider the catalog complete, show an exponential occurrence. Yet, the simulation approach indicates that low magnitude events occur indeed more sparsely than expected for an exponential distribution, and that the magnitude distribution does not obey the Gutenberg‐Richter law. Interevent times are log‐normally distributed, which implies a preferred recurrence interval. The distributions are consistent with a slug flow regime. They correlate weakly with the preceding magnitude, suggesting that slugs coalesce while ascending. Plain Language Summary: A common type of volcanic seismic events at Villarrica is thought to result from gas bubbles, which ascend through the liquid magma in the conduit and produce mild volcanic explosions at the surface. We investigated the statistical occurrence of the time between these events and of their size. In contrast to normal earthquakes, the events recur more periodically and the number of smaller events does not increase exponentially, hence confirming their different nature. Laboratory experiments showed that bubbles flowing in a liquid develop characteristic patterns of size and distance between them, which depend on the gas supply rate and the flow ability of the liquid. We qualitatively compared the statistical distributions of event sizes to bubble sizes and the time between the events to the distance between gas bubbles. The result suggests a flow pattern where the gas bubbles span the entire width of the conduit and are several times longer than wide. Such batches of gas are known to produce the typical volcanic activity of Villarrica. Key Points: Seismic transient events are commonly seen as ascending gas slugs but other mechanisms, for example, brittle failure, are conceivableAmplitudes and time between events are approximately log‐normally distributed and thus different from tectonic earthquakesComparison with slug size distributions in gas‐in‐liquid flow suggests a sustained slug flow regime [ABSTRACT FROM AUTHOR]
- Published
- 2021
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