Analysis of seismograms associated with a debris flow at Sakurajima volcano, Japan, observed with the DAS system

Debris flows occur frequently in volcanic regions and cause serious damage to nearby residents and infrastructure. Hence, it is essential to detect debris flows quickly and understand their mechanisms for mitigating the hazard. At Sakurajima volcano, Japan, which has one of the highest incidences of debris flows in Japan due to frequent rainfall and explosive eruptions, we conducted an observation along the Nojiri river, located in the southwestern part of Sakurajima, during November 11 and December 9 in 2022 utilizing an existing fiber-optic cable with a length of about 4.4 km and a sampling frequency of 200 Hz. During the observation period, we succeeded in recording seismic signals generated by the debris flow on November 29.

The duration and predominant frequency of the seismograms are similar to those associated with debris flows at other volcanoes, and are distinguished from those associated with pyroclastic flows and floods. Because the seismograms show unclear onsets and continuous oscillations for more than about 15 minutes, we apply two methods to track the debris flow. One method is to determine the propagation direction of the seismic wave from the arrival time difference determined by the waveform correlations. Another method uses the spatial distribution of the seismic wave amplitudes recorded at individual DAS channels. Assuming isotropic radiation for high-frequency components of seismograms and a constant wave velocity, we evaluate the strength of seismic waves generated along the river. The results using both methods track the debris flow migrating downstream with increasing time. The obtained passage time of the debris flow at each dam along the river roughly coincides with the time when the debris flow passage was recorded by the video camera. The debris flow velocity ranged 1.1-4.6 m/s with the average of 4.2 m/s. These results show that DAS observation is quite useful for quantitatively evaluating the spatio-time distribution of debris flows along the river and for understanding the mechanism of debris flow. Also, we can conduct debris flow monitoring continuously at volcanoes without being affected by the weather or the debris flow itself because the fiber-optic cable is buried underground.