Comparative interevent time statistics of degassing and seismic activity
at Villarrica Volcano (Chile)
Abstract
It is generally assumed that seismic activity at volcanoes is closely
connected to degassing processes. Intuitively, one would therefore
expect a good correlation between degassing rates and seismic amplitude.
However, both examples and counterexamples of such a correlation exist.
In this study on Villarrica volcano (Chile), we pursued a different
approach to relate gas flux and volcanic seismicity using 3 months of
SO$_2$ flux rate measurements and 12 days of seismic recordings from
early 2012.
We analyzed the statistical distributions of interevent times between
transient seismic waveforms commonly associated with explosions and
between peaks in the degassing time series.
Both event types showed a periodic recurrence with a mode of 20-25 s and
around 1 h for transients and degassing, respectively. The normalized
interevent times were fitted by almost identical log-normal
distributions. Given the actually very different time scales, this
similarity potentially indicates a scale-invariant phenomenon. We could
reproduce these empirical findings by modelling the occurrence of
transients as a renewal process from which the degassing events were
derived recursively with increasing probability since the previous
degassing event. In this model, the seismic transients could be either
produced by degassing processes within the conduit or by gas release at
the lava lake surface while the longer intervals of the degassing events
may be explained by accumulation of gas either in the magma column or in
the juvenile gas plume.
Additionally, we analyzed volcano-tectonic events, which behaved very
differently from the transients. They showed the clustered occurrence of
tectonic earthquakes.