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Abstract:
Volcanic tremor signals are usually observed before or during volcanic eruptions andmust
be monitored to evaluate the volcanic activity. A challenge in studying seismic signals of
volcanic origin is the coexistence of transient signal swarms and long-lasting volcanic
tremor signals. Separating transient events fromvolcanic tremors can, therefore, contribute
to improving upon our understanding of the underlying physical processes.
Exploiting the idea of harmonic–percussive separation in musical signal processing,
we develop a method to extract the harmonic volcanic tremor signals and to detect transient
events from seismic recordings. Based on the similarity properties of spectrogram
frames in the time–frequency domain, we decompose the signal into two separate spectrograms
representing repeating (harmonic) and nonrepeating (transient) patterns,
which correspond to volcanic tremor signals and earthquake signals, respectively.
We reconstruct the harmonic tremor signal in the time domain from the complex
spectrogram of the repeating pattern by only considering the phase components for
the frequency range in which the tremor amplitude spectrum is significantly contributing
to the energy of the signal. The reconstructed signal is, therefore, clean tremor
signal without transient events.
Furthermore, we derive a characteristic function suitable for the detection of transient
events (e.g., earthquakes) by integrating amplitudes of the nonrepeating spectrogram
over frequency at each time frame. Considering transient events like earthquakes,
78% of the events are detected for signal-to-noise ratio = 0.1 in our semisynthetic tests.
In addition, we compared the number of detected earthquakes using our method for
one month of continuous data recorded during the Holuhraun 2014–2015 eruption in
Iceland with the bulletin presented in Ágústsdóttir et al. (2019). Our single station event
detection algorithm identified 84% of the bulletin events. Moreover, we detected a
total of 12,619 events, which is more than twice the number of the bulletin events.