11:15 〜 11:30
[SCG54-08] 海陸自然地震観測による鬼界カルデラの地震波速度構造
キーワード：鬼界カルデラ、地震観測、地震波トモグラフィ
Kikai submarine caldera is located to the south of the Kyushu island, and its last eruption occurred at 7.3 ka. The 7.3-ka Akahoya super-eruption devastated the lives of the Jomon people, especially in southern Kyushu. Post-caldera volcanic activity was also active and formed Iwo-Dake, Inamura-Dake, and Showa Iwo-Jima Island. However, both the location and spatial extent of the magma chamber is still unknown due to the sparse observation network.
To clarify the magma supply system beneath the Kikai submarine caldera, we have started the passive seismic observations both offshore and onshore since Autumn of 2020. We deployed 25 short-period ocean bottom seismometers (SPOBSs) and 12 broadband ocean bottom seismometers (BBOBSs) in and around the Kikai submarine caldera during the cruise of R/V KAIREI. Onshore seismic stations were deployed in five islands; Take-Shima, Satsuma Iwo-Jima, Kuro-Shima, Tanegashima, and Yakushima. All the land stations are equipped the broadband seismometer and three of them have telemeter systems.
One SPOBSs popped up by itself and were washed onto the coast in June 2021. The recovery cruise for SPOBS was conducted in July 2021. Although five SPOBS could not be recovered, the other 19 SPOBSs were recovered in 2021 by R/V KAIMEI. After that, one of five OBSs were recovered in 2023 by ROV Hyper-dolphin equipped on R/V Shinsei-Maru. All BBOBSs were successfully recovered in 2023 by R/V Shinsei-Maru, and the most of onshore stations were retreated in the end of 2022. In addition, the R/Vs Kaimei, Kaiyo Maru No. 2, and Fukae Maru installed SPOBS and conducted refraction surveys using artificial seismic sources in 2021(Nagaya et al. 2023). The data from these SPOBS and permanent land stations (including two temporary stations) from JMA, NIED, Kyoto and Kagoshima universities are also included in the analysis.
Now we are conducting the first arrival phase picking on each temporal stations based on the JMA unified catalog for the period when SPOBS data available. Using some of these data, we attempted to estimate the structure using seismic tomography (Zhang and Thurber, 2006). The 3D structure from a previous study by Yamamoto et al. (2020) was assumed as the initial velocity structure, and part of the data used in this previous study was also used in the analysis.
As a result of the structure estimation, the P-wave low-velocity zone (LVZ) with a spatial scale of about 10 km square was imaged within a depth range of about 10~20 km near the center of the caldera. This LVZ may be related to the shallow LVZ detected by Nagaya et al. (2023) by refraction study, which is less than 10 km deep, 20 km horizontal, and 8 km thick. Our results also show that the center of the LVZ slightly shifts to the north at 40 km depth. In addition, the S-wave LVZ between 50 km and 150 km depth within the mantle wedge were imaged. Since the root of this LVZ locates just above the active intraslab seismicity zone, we consider that they are related to the upwelling melt or fluid flow from the slab dehydration.
Acknowledgement:
We used observation data from the JMA unified Catalog and the JVDN system for Satsuma-Iwo Island. We would also like to thank the captains, crews, and observation technicians of the "KAIREI," "KAIMEI," "KAIYOMARU No. 2," "FUKAE MARU," and "SHINSEI MARU" for their assistance with land observations and with offshore observations, and Mishima Village, Nishino-omote City, and Yakushima Town for their assistance in land observations. This research was supported by JSPS Grant-in-Aid for Scientific Research 20H00199.
To clarify the magma supply system beneath the Kikai submarine caldera, we have started the passive seismic observations both offshore and onshore since Autumn of 2020. We deployed 25 short-period ocean bottom seismometers (SPOBSs) and 12 broadband ocean bottom seismometers (BBOBSs) in and around the Kikai submarine caldera during the cruise of R/V KAIREI. Onshore seismic stations were deployed in five islands; Take-Shima, Satsuma Iwo-Jima, Kuro-Shima, Tanegashima, and Yakushima. All the land stations are equipped the broadband seismometer and three of them have telemeter systems.
One SPOBSs popped up by itself and were washed onto the coast in June 2021. The recovery cruise for SPOBS was conducted in July 2021. Although five SPOBS could not be recovered, the other 19 SPOBSs were recovered in 2021 by R/V KAIMEI. After that, one of five OBSs were recovered in 2023 by ROV Hyper-dolphin equipped on R/V Shinsei-Maru. All BBOBSs were successfully recovered in 2023 by R/V Shinsei-Maru, and the most of onshore stations were retreated in the end of 2022. In addition, the R/Vs Kaimei, Kaiyo Maru No. 2, and Fukae Maru installed SPOBS and conducted refraction surveys using artificial seismic sources in 2021(Nagaya et al. 2023). The data from these SPOBS and permanent land stations (including two temporary stations) from JMA, NIED, Kyoto and Kagoshima universities are also included in the analysis.
Now we are conducting the first arrival phase picking on each temporal stations based on the JMA unified catalog for the period when SPOBS data available. Using some of these data, we attempted to estimate the structure using seismic tomography (Zhang and Thurber, 2006). The 3D structure from a previous study by Yamamoto et al. (2020) was assumed as the initial velocity structure, and part of the data used in this previous study was also used in the analysis.
As a result of the structure estimation, the P-wave low-velocity zone (LVZ) with a spatial scale of about 10 km square was imaged within a depth range of about 10~20 km near the center of the caldera. This LVZ may be related to the shallow LVZ detected by Nagaya et al. (2023) by refraction study, which is less than 10 km deep, 20 km horizontal, and 8 km thick. Our results also show that the center of the LVZ slightly shifts to the north at 40 km depth. In addition, the S-wave LVZ between 50 km and 150 km depth within the mantle wedge were imaged. Since the root of this LVZ locates just above the active intraslab seismicity zone, we consider that they are related to the upwelling melt or fluid flow from the slab dehydration.
Acknowledgement:
We used observation data from the JMA unified Catalog and the JVDN system for Satsuma-Iwo Island. We would also like to thank the captains, crews, and observation technicians of the "KAIREI," "KAIMEI," "KAIYOMARU No. 2," "FUKAE MARU," and "SHINSEI MARU" for their assistance with land observations and with offshore observations, and Mishima Village, Nishino-omote City, and Yakushima Town for their assistance in land observations. This research was supported by JSPS Grant-in-Aid for Scientific Research 20H00199.