Combining experimental volcanology, petrology and geophysical monitoring techniques

In general, an understanding of the complex processes acting before
and during volcanic eruptions is approached from various different
sides, e.g. laboratory experiments on fragmentation and/or bubble
burst eruption mechanisms, petrological analysis of the eruptive
products and various geophysical monitoring and source localization
techniques. Each of these techniques can deliver valuable insights
by adding pieces of information about the physical processes that
drive the volcanic activity. However, often studies are focussing
on a single aspect of the process, without setting the results in a
more general context. Often, this strategy is absolutely valid,
when the focus is laid on a single piece in the complex chain of
processes taking place in volcanic eruptions. This must fail when
the results aim to suggest a valid model for the combined
observations at volcanoes using the above described techniques. The
resulting models of volcanic source mechanisms and eruptive
features can therefore lead to biased assumptions. This study aims
to close this gap between laboratory experiments, petro-chemical
analysis and modern geophysical monitoring and source localization
techniques in a case study of Mt. Yasur (Vanuatu) volcano. The
presented laboratory experiments on explosive volcanic eruptions
upon rapid decompression show that decompression rate is the dening
parameter in the experiments and that a scaling to large-scale
processes is valid. Furthermore, a model is presented that
correlates measured particle velocities to decompression rate and
initial gas-overpressure. This model is used to estimate source
volumes and overpressures at Volcan de Colima (Mexico) and Mt.
Yasur (Vanuatu). A petrographically and geochemically
characterization of Mt. Yasurs eruptive products suggests a shallow
magma-mingling process at both of Mt. Yasurs active craters,
perhaps due to rejuvenation of material slumped from the crater
walls into an open conduit system. A study on the time-reversal
imaging technique and its ability to detect the details of finite
rupture (or time-variant) processes shows that the limitations of
TR imaging start where the source stops being point-localised with
respect to the used wavelength. Inversion of the source mechanisms
of Strombolian explosions at Mt. Yasur are performed using a
multi-parameter dataset consisting of seismic, acoustic and
Doppler-radar data. Time-reversal imaging and moment tensor
inversion are used to invert the source location of the seismic
long-period (f < 1Hz) signals, which is supposed to refl ect
fluid movement at depth. The source is located in the north-east of
the crater region in a depth of several hundred meters.
Furthermore, the source volume of the radiated infrasound signals
is estimated from fundamental resonance frequencies. The results
showed that the maximum particle velocity measured with the Doppler
radar correlates nicely with the estimated source volumes lengths.
The inverted seismic moment does not show any correlation with the
estimated slug sizes, i.e. the slug size does not map in seismic
moment. This is an important information, as it states that a
larger source volume does not necessarily produces a larger seismic
moment. From these combined results, a common feeder system for all
active craters at Mt. Yasur is proposed. The differences in event
recurrence rate at the three active craters are believed to be
controlled by either the conduit geometry or variations in
degassing or cooling rate. Strombolian-type eruptions at Mt. Yasur
are suggested to be due to the burst of gas slugs with lengths and
overpressures comparable to volcanoes showing similar eruptive
patterns. The results illustrate the importance of combined studies
that overcome the limitations of single disciplines. In this way, a
more comprehensive view of volcanic eruptions and the associated
observations is possible. Such a multi-disciplinary approach will
contribute to a better understanding of volcanic processes and the
associated hazards.