Automatic Reclassification of Volcano-Seismic Signals from Soufrière
Hills Volcano, Montserrat, 1996-2008
Abstract
Seismic activity during the eruption of Soufriere Hills volcano
comprised various transient signals, which were classified visually by
the Montserrat Volcano Observatory (MVO), considering waveforms recorded
at several stations. For 217,290 transients detected on the MVO digital
seismic network between 1996/10/21 and 2008/10/16, five main classes
have been identified: rockfall (ROC: 58%), hybrid (HYB: 19%),
long-period (LPE: 11%), lp-rockfall (LP-ROC: 5.8%), and
volcano-tectonic (VT: 3.1%). Temporal trends in the rate and energy
release of these different transients (in addition to swarms and tremor)
were key to short-term forecasting of eruptive activity. However, visual
classification is highly subjective and non-repeatable, and the
inconsistency of the catalog is a barrier to research. In a pilot study,
we automatically removed waveforms with dropouts, and manually verified
transient classifications until we had approximately 100 transients of
each class (total 522). We found ~21% of these
transients were incorrectly classified at MVO. Our re-labelled dataset
was then used as a starting point for supervised learning, using code
from http://github.com/malfante/AAA. This code was used by Malfante et
al. (2008) to classify 109,609 transients at Ubinas volcano with a
93.5% accuracy. They transformed each waveform into a set of 102
features: 34 features for each of three domains (time, spectral,
cepstral). We added 6 frequency features of our own, including band
ratios, peak frequency, median frequency, bandwidth, and frequency
change. The resulting 108-point vectors of features were then used for
modeling. The dataset is randomly divided 50 times into training and
testing datasets, to produce a robust model. One model is produced per
channel. We use the Random Forest Classifier algorithm from the
scikit-learn library. For each waveform, a probability is computed for
each class. Initial results are promising. Separate models for 3
channels yield accuracies of 76-80%. If the LP-ROC class is omitted
(following Langer et al, 2006), accuracy rises to 82-85%. If only VT
and LP classes are considered, accuracy is 96-99%. We intend to expand
our labelled dataset to 1000 events, add new features, build models for
each channel, and reclassify the catalog of 217,290 transients by a
weighted average of probabilities.