Source locations of volcanic earthquakes using DAS and fiber optic cables: Azuma and Sakurajima volcanoes, Japan 

Abstract

The DAS observation is quite useful to monitor the volcanic earthquakes and tremor at active volcanoes because fiber optic cables are not damaged by thunders and effused volcanic materials.  Also, highly dense distribution of measurement points along the fiber optic cable enables us to use correlation between observed waveforms to locate the volcanic earthquakes and tremor that are characterized with unclear P- and S-waves.  In the present study, we present some results of the source locations of volcanic earthquakes recorded by DAS at Azuma and Sakurajima volcanoes, Japan.  At Azuma volcano, Japan, we use the fiber optics cable deployed by the Ministry of Land and Transportation along the Bandai-Azuma sky road. The cables are set about 14 km long in a tube at a depth of about 50 m. The data are stored with a sampling frequency of 1000 Hz and the channel spacing of 10 m in July 2019. Extending the source locations done in Nishimura et al. (SR 2021), we precisely locate the source locations of volcanic earthquakes using relative hypocenter determination algorithm using a reference event with a known location. We apply complex principal component analysis to measure the arrival time difference of the seismic signals at nearby channels along the fiber optic cables. As a result, we succeeded in locating about 50 events during the period of about 1 month when no hypocenters are determined by a permanent seismic network.  At Sakurajima, we analyze the explosion earthquakes that are associated with vulcanian eruptions in November-December 2022. We use the fiber optic cables of about 4.4 km length extending from the coast to the mountain area, and continuously recorded the seismic signals with a sampling frequency of 200 Hz and channel spacing of 4.8m. Measuring the arrival time differences of the initial phases to coda waves continuing for minutes, we calculate the back azimuth and slowness of the seismic waves. The results show that the seismic waves arrive from the active crater area with slowness of about 1 s/km for about 20 s at 1-2 Hz and 60 s at > 2Hz. These results imply that seismic waves continuously emitted from a shallow region beneath the active craters. Also, even during no eruptions, the volcanic tremors are sometimes generated from the active craters. Our observations at Azuma and Sakurajima volcanoes indicate usefulness of the DAS observations for locating the sources of volcanic earthquakes and tremor that are not precisely located by using the data recorded at network stations.