Mechanism of non-eruptive deflation at Sakurajima volcano, Southwest Japan

Analysis of volcanic ash samples from Non-Eruptive Deflation (NED) events at Sakurajima volcano enabled us to examine the underlying mechanism and eruptive sequence. We conducted continuous sampling using SATSUMA, an automatic volcanic ash sampler, and successfully obtained multiple samples during NED events, which are typically accompanied by very small amounts of ejecta. The results revealed that, although the volcanic ash particle componentry of NEDs is rich in black, angular lava fragments similar to those observed in typical Vulcanian eruptions, it is systematically enriched in vesicle-rich particles with low crystallinity. Geophysical and geochemical observations indicate that ground deformation associated with deflation is synchronized with fluctuations in volcanic gas emissions during NED events. This behavior is consistent with the tendency for NEDs to produce small but volatile-rich ejecta. It is inferred that conduit choking occurs during NED events, analogous to that in typical Vulcanian eruption, albeit on a much smaller scale. The continued gas supply subsequently leads to a pressure increase and facilitates cap rock failure, ultimately resulting in an NED. To derive volcanic ash particle component ratios that were previously determined subjectively, we employed an objective classification method based on the waveforms of microscopic visible spectra of individual particles. The results generally agreed with the microscopic classification when fewer than five types were considered; however, classification into 10 types performed poorly. For Sakurajima volcanic ash, classification that incorporates information beyond spectral data, such as shape parameters and groundmass structure, are necessary to fully reproduce the results of manual classification.