5:15 PM - 6:45 PM
[SVC29-P05] Imaging Subsurface Structures Beneath Hachijojima Island Using Seismic Tomography with High-Density Observation Data
Keywords:Hachijojima, seismic tomography, manual picking, double difference, volcano seismology
Seismic observation was carried out on Hachijojima Island, an inhabited volcanic island in Japan. This temporary observation was conducted twice, with each session taking place within a 7-month period in 2019 and 2021. A total of 46 temporary stations were densely placed across the island, along with 9 permanent stations, to collect seismic data. The high-density seismic data recorded was subsequently used for seismic tomography study aimed at imaging the subsurface structures beneath the island.
During the analysis of waveforms, a total of 75 stations were utilized. These included 46 temporary stations, 9 permanent stations on Hachijojima Island, as well as 20 permanent stations situated on neighboring volcanic islands around Hachijojima. The arrival times of P- and S-wave were manually picked using the WIN System, a processing system for multi-channel seismic waveform data. Additionally, event hypocenters were directly determined by the hypomh program. This built-in program within the WIN System determines the hypocenter location by utilizing Bayesian statistics. During the survey timeframe, we identified a combined total of 179 local earthquakes, with 119 occurring in 2019 and the remaining 60 taking place in 2021. This led to a collection of 4637 P-arrival times and 3905 S-arrival times. Double Difference Tomography was utilized to determine the subsurface velocity structure. Previously, the initial reference velocity model for tomography inversion was derived from Velest program, which is designed to conduct inversion resulting in minimum 1-D seismic velocity.
According to the results of hypocenter location determination by hypomh and our manual picking analysis, most of the earthquake events did not occur directly below Hachijojima Island but were situated approximately 20-30 km northwest of the island. This suggests that these events were not likely triggered by magma activity but rather by other factors. Additionally, the 3-D tomography revealed notable features such as a high-velocity region at approximately four kilometers depth indicating preferential pathways through which magma from previous volcanic activity may have flowed underneath this region. The study also showed low-velocity regions in shallower areas around the central part of the island, suggesting these areas are mainly volcanic sediments.
During the analysis of waveforms, a total of 75 stations were utilized. These included 46 temporary stations, 9 permanent stations on Hachijojima Island, as well as 20 permanent stations situated on neighboring volcanic islands around Hachijojima. The arrival times of P- and S-wave were manually picked using the WIN System, a processing system for multi-channel seismic waveform data. Additionally, event hypocenters were directly determined by the hypomh program. This built-in program within the WIN System determines the hypocenter location by utilizing Bayesian statistics. During the survey timeframe, we identified a combined total of 179 local earthquakes, with 119 occurring in 2019 and the remaining 60 taking place in 2021. This led to a collection of 4637 P-arrival times and 3905 S-arrival times. Double Difference Tomography was utilized to determine the subsurface velocity structure. Previously, the initial reference velocity model for tomography inversion was derived from Velest program, which is designed to conduct inversion resulting in minimum 1-D seismic velocity.
According to the results of hypocenter location determination by hypomh and our manual picking analysis, most of the earthquake events did not occur directly below Hachijojima Island but were situated approximately 20-30 km northwest of the island. This suggests that these events were not likely triggered by magma activity but rather by other factors. Additionally, the 3-D tomography revealed notable features such as a high-velocity region at approximately four kilometers depth indicating preferential pathways through which magma from previous volcanic activity may have flowed underneath this region. The study also showed low-velocity regions in shallower areas around the central part of the island, suggesting these areas are mainly volcanic sediments.