Examining the Relationship Between Electrical Activity and Jet Velocity During Vulcanian Eruptions at Sakurajima Volcano

Abstract:

Monitoring electrical signals from volcanoes is a promising tool for early detection of hazardous, ash-forming eruptions. A type of electrical activity known as vent discharges has potential to provide one of the earliest signs of an explosive eruption, before seismic or infrasound signals. However, there is a need for better constraints on the relationship between this early electrical activity and the dynamics of the eruption. Prior studies have shown that vent discharges are relatively small (<10 m in length), occur at low altitudes close to the vent, and produce a unique signature of very high frequency electromagnetic radiation called continual radio frequency. Vent discharges begin at the start of explosive eruptions and usually last at least few seconds. Previous work indicates that the duration of vent discharges may be related to the duration of ash emissions, and that the intensity of vent discharges may be related to eruptive intensity. Given these prior observations, our goal is to determine whether there is a relationship between the rate of vent discharges and the velocity of the eruptive jet. Using measurements from a 2015 field campaign at Sakurajima volcano in Japan, we investigate six well-monitored explosive events from Showa crater. The rate of vent discharges was determined using three Lightning Mapping Array stations, which were configured to detect impulsive radio emissions from lightning in the 66–72 MHz band. Plume dynamics and behavior were captured with infrared video using a FLIR SC600 camera. We used previous calculations of maximum jet velocity determined from the infrared plume measurements using an optical flow code. We found a positive relationship between the rate of vent discharges and jet velocity for five out of the six events studied. At the onset of each explosive eruption, the jet velocity and vent discharge rate both increase rapidly. Over a few seconds, the rate of vent discharges decreases as jet velocity slows down. A possible explanation for this relationship is that higher velocities in the near-source jet region lead to higher ash particle collision rates and more energetic collisions, thus leading to enhanced triboelectric charging.