Monitoring of volcanic earthquakes and tremor using DAS at Sakurajima volcano, Japan

The Sakurajima volcano in Japan is one of the most active volcanoes in the world.  The volcano has erupted in Vulcanian style since the 1950s from three craters at the summit.  We started continuous DAS observations in September 2025.  Two fiber cables are embedded in FEP pipes at a depth of approximately 50 cm below the road surface: one cable is deployed along the Nojiri River (ca. 4.4 km long) and another one is deployed under the road that circles the volcano island (ca. 38 km long).  We record many explosion earthquakes accompanied by Vulcanian eruptions, eruption tremors associated with continuous volcanic ash emissions, harmonic tremors characterized by multiple spectral peaks, volcano-tectonic earthquakes (VTs), and regional tectonic earthquakes.  In addition, we record the debris flows occurring along the Nojiri River.  Because most of these volcanic earthquakes, except VTs, are characterized by unclear onsets of P- and S-waves, their source locations are determined from phase differences between nearby channels of DAS data using cross-correlation function (CCF) and complex principal component analyses. The amplitude source location (ASL) is also applied to the DAS data.  As a result, explosion earthquakes, eruption tremors, and harmonic tremors were inferred to be generated at shallow depths of the active craters from the slowness determined from DAS data along the Nojiri River.  Debris flows are tracked using ASL and/or phase difference data.  Template matching using explosion earthquakes was applied to automatically detect Vulcanian eruptions from very small to large scales. These data analyses of the detection and source location codes are planned to be mounted on an edge computer connected to an interrogator (ONYX, SINTELA), and the results are transferred to our laboratory for real-time monitoring.