Subdivision of long-period and volcanic tectonic events in the Kilauea volcano, Hawaii and implication for diverse volcanic processes

Abstract:

Long-period (LP) and volcanic tectonic (VT) events have been observed in many volcanic systems around the world. Traditionally, they are classified by a metric called frequency index which is defined as a ratio of mean spectral amplitude between high (e.g. 5-15 Hz) and low (e.g. 1-5 Hz) frequency bands. However, the frequency index alone is unlikely to fully describe the spectral features resulted from complex interactions between multiple-phase volcanic materials (i.e. gas, fluid and rocks). Here we use cluster analysis to classify seismic events based on their overall spectral shape in 1-15 Hz. We apply the analysis to the seismic events in the Kilauea volcano at Hawaii around the 2018 eruption episode. Our results show that, in addition to dominant frequency, spectral width (broadband vs. harmonic) is another critical feature that distinguishes different event clusters. Considering dominant frequency and spectral width together, the events can be divided into five groups, each displaying distinct spatiotemporal patterns around the 2018 Kilauea eruption. Specifically, the harmonic-type events with dominant frequency ranging from 1 to 6 Hz are all located beneath the Kilauea caldera, whereas the broadband-type events (with dominant frequency at about 3, 6, and 9 Hz) show relatively diffusive distribution around the volcanic region. Our results reveal that, similar to volcanic tremor, LP and VT events also have both harmonic and broadband subtypes, which reflect the diverse volcanic processes associated with magmatic transportation and/or brittle failure of surrounding rocks.