5:15 PM - 7:15 PM
[SSS08-P06] Three-dimensional travel-time tomography around the Ata caldera, Kyushu, Japan, based on seafloor and onshore seismic observation
Keywords:Ata caldera, 3-D travel-time tomography
The Ata caldera is one of the major calderas in Japan, which is located at the southern tip of Kyushu. The northern Ata caldera is under the central part of Kagoshima Bay and to be source of the Ata pyroclastic flow (~110 ka). The southern Ata caldera is located near the mouth of Kagoshima Bay including parts of the southern Satsuma Peninsula and the southern Osumi Peninsula. The Ata-Torihama tephra is presumed to have erupted at 240 ka of the southern Ata caldera. The location and structure of both northern and southern Ata calderas, however, remains poorly defined because large parts of the calderas are an offshore area. The objectives of our project are to elucidate the 3-D velocity structure of the crust around the Ata caldera and to contribute to understanding the magma supply system beneath the Ata caldera.
In order to clarify the detailed caldera structure of the Ata caldera, we have been conducting temporary seafloor and onshore seismic observation in southern Kyushu since 2021. In terms of the seafloor observation, we deployed 6–7 short-period (4.5 Hz) ocean bottom seismometers (OBSs) from the central part of Kagoshima Bay to the mouth of the bay for about 4 months every year. In terms of the onshore observation, we deployed temporary stations with a 4.5-Hz seismometer so that we could fill the observation gaps left by existing telemetered seismic network. Currently, the number of temporary onshore stations is 22, allowing for almost year-round observation.
For the 3-D seismic travel-time tomography analysis, we first determined hypocenters of earthquakes with a magnitude of approximately 2.0 or larger in southern Kyushu based on manual phase picking of arrival times of P- and S-phases using seismic waveform data obtained from both the temporary seismic stations mentioned above and the telemetered stations. The event list is derived from a routine earthquake catalog of Kagoshima University. We assumed a 1-D velocity structure estimated by using the VELEST (version 3.1) package (Kissling et al., 1994) from the 2021 data for the velocity structure used in the hypocenter determination and the initial model for tomography analysis. We also introduced a station correction method to P- and S-phase picks for the location. Then, we conducted a 3-D seismic tomography using absolute travel-time data. We adopted the FMTOMO package (Rawlinson et al., 2006) which performs the theoretical travel-time calculation and the ray tracing by a robust wavefront tracking. The velocity grid spacing is set to 0.05 degrees in the horizontal and 5 km in depth.
Although the temporary seismic observation is still ongoing, we have performed several trial analyses. In this presentation, we will show a preliminary result of the 3-D travel-time tomography using the data from 2023–2024. At the same time, we will also present results of trade-off analysis for selection of optimal damping and smoothing parameters.
Acknowledgements:
This work is conducted through “Cooperation Research Project for Crustal Structure in South Kyushu” with Kyushu Electric Power Co., Inc. The OBSs were deployed and recovered by the T/V Nansei-maru, Kagoshima University. We would like to thank all the crews and students. We would also like to thank JMA, NIED, and ADEP for allowing us to use continuous waveform data.
In order to clarify the detailed caldera structure of the Ata caldera, we have been conducting temporary seafloor and onshore seismic observation in southern Kyushu since 2021. In terms of the seafloor observation, we deployed 6–7 short-period (4.5 Hz) ocean bottom seismometers (OBSs) from the central part of Kagoshima Bay to the mouth of the bay for about 4 months every year. In terms of the onshore observation, we deployed temporary stations with a 4.5-Hz seismometer so that we could fill the observation gaps left by existing telemetered seismic network. Currently, the number of temporary onshore stations is 22, allowing for almost year-round observation.
For the 3-D seismic travel-time tomography analysis, we first determined hypocenters of earthquakes with a magnitude of approximately 2.0 or larger in southern Kyushu based on manual phase picking of arrival times of P- and S-phases using seismic waveform data obtained from both the temporary seismic stations mentioned above and the telemetered stations. The event list is derived from a routine earthquake catalog of Kagoshima University. We assumed a 1-D velocity structure estimated by using the VELEST (version 3.1) package (Kissling et al., 1994) from the 2021 data for the velocity structure used in the hypocenter determination and the initial model for tomography analysis. We also introduced a station correction method to P- and S-phase picks for the location. Then, we conducted a 3-D seismic tomography using absolute travel-time data. We adopted the FMTOMO package (Rawlinson et al., 2006) which performs the theoretical travel-time calculation and the ray tracing by a robust wavefront tracking. The velocity grid spacing is set to 0.05 degrees in the horizontal and 5 km in depth.
Although the temporary seismic observation is still ongoing, we have performed several trial analyses. In this presentation, we will show a preliminary result of the 3-D travel-time tomography using the data from 2023–2024. At the same time, we will also present results of trade-off analysis for selection of optimal damping and smoothing parameters.
Acknowledgements:
This work is conducted through “Cooperation Research Project for Crustal Structure in South Kyushu” with Kyushu Electric Power Co., Inc. The OBSs were deployed and recovered by the T/V Nansei-maru, Kagoshima University. We would like to thank all the crews and students. We would also like to thank JMA, NIED, and ADEP for allowing us to use continuous waveform data.