[S-VC29] Monitoring and assessment of volcanic activities

[SVC29-05] The monitoring of continuous volcanic ash emissions at Sakurajima volcano using disdrometer ashfall observation

*Kosei Takishita¹, Tomofumi Kozono¹, Takahiro Miwa¹, Masato Iguchi² (1.National Research Institute for Earth Science and Disaster Resilience, 2.Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University)

Keywords:Continuous volcanic ash emission, Disdrometer, Sakurajima

Sakurajima is one of the most active volcanoes in Japan. Its typical eruption styles are vulcanian eruptions and continuous volcanic ash emissions (Poulidis et al., 2019). While vulcanian eruptions emit thousands of tons of volcanic ash impulsively, continuous volcanic ash emissions provide up to several tens of tons of volcanic ash per minute for several tens of minutes to hours, whose total masses are potentially comparable to that of vulcanian eruptions in case of the long duration. Therefore, the continuous volcanic ash emissions should be monitored as well as vulcanian eruptions to evaluate the activity of the Sakurajima volcano. The total ejected mass of vulcanian eruptions can be estimated from the combination of ground deformations and volcanic tremors (Iguchi, 2016). However, the mass of continuous volcanic ash emissions is conveniently estimated from the empirical relationship obtained from the vulcanian eruptions, and it has not been validated in the continuous volcanic ash emissions. Here, we try to detect continuous volcanic ash emissions and estimate ashfall load from them using disdrometers. Sakurajima Volcano Research Center, DPRI, Kyoto University, installed a disdrometer network at Sakurajima volcano to monitor ashfall, and the number of volcanic ash is counted every minute at each site (Iguchi et al., 2019). We extract the ashfall events that lasted for a certain period after excluding rainfall events from the data between January to June 2019.
From the comparison between the extracted ashfall and the seismogram measured at the Arimura volcanic observation tunnel, we found thet there exist 18 events in which the pulsatile amplitude change of the seismogram is linked by that of ashfall 5 to 15 minutes later. By comparing the integrated amplitude of the seismogram with the total ashfall load in the corresponding pulse, estimated via an empirical conversion from the particle number measurements by the disdrometer (Takishita et al., 2022), we revealed that there tend to be a positive correlation between the integrated amplitude and the ashfall load at a single site. Since the plume height and the tephra transport process of continuous volcanic ash emissions are relatively stable, the temporal change of ashfall load at a single site is suggested to be linked to that of mass eruption rate. Therefore, the results indicate that the correlations between the amplitude of the seismogram and the mass eruption rate, revealed from vulcanian eruptions, can be applied to continuous volcanic ash emissions. The disdrometers are expected to contribute to monitoring the relative temporal variations of mass eruption rate in the continuous volcanic ash emissions. The observation conditions needed for the detection of the pulsatile ashfall and the relationship between the time lag of the pulse and wind speed should be investigated in the future.