Volcanic Lightning as an Indicator of Ash Parameters: A Case Study of Sakurajima Volcano, Japan, May-June 2015

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Volcanic lightning is an emerging field of study for monitoring explosive, ashproducing eruptions. Volcanic ash is hazardous to aviation as well as local communities. Using volcanic lightning to monitor ongoing ash emissions, however, requires a better understanding of how volcanic lightning develops, and whether or not lightning can be used as an indicator for specific ash characteristics. Volcanic lightning is common at Sakurajima Volcano, Japan. During the summer of 2015 we deployed a 9 station Lightning Mapping Array (LMA), developed by New Mexico Institute of Mining and Technology, within 20 km of Sakurajima Volcano to detect the very high-frequency electromagnetic radiation generated by volcanic lightning. Of the eruptions analyzed so far the two main types of electrical signals recorded were near vent-lightning and continuous radio frequencies (CRF). In addition to the LMA data we collected samples of actively falling ash during a tenday observation period from May 29th-June 7th 2015. Eruptive events were also recorded on a Trillium broadband three-component seismometer. Ash samples were characterized in terms of componentry, plagioclase microlite number density, laser diffraction particle size analysis, and particle morphology using a PharmaVision 830. The ash parameters were compared with maximum vertical seismic amplitudes and the electrical discharge statistics of the respective explosive events. We have begun to determine the relationships that exist between quantifiable ash characteristics and volcanic electrification, especially the occurrence of CRF. We have found that CRF occurred during events where ash samples were composed of greater than 60% glass with less than 10% lithics. CRF was also present during eruptions whose samples have have high mean roundness but maintain a distribution tail of acicular grains. Finally, CRF occurred in more explosive events as defined by higher seismic amplitudes (>7um). We infer from these relationships that CRF is generated in highly explosive events by a combination of initial and secondary fragmentation of the ash as it is generated and travels up the conduit.

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Presented at the IAVCEI Scientific Assembly in August 2017 in Portland, OR