15:45 〜 16:00
[SCG48-17] Spatial variations in rift structure and associated seismicity in the southern Okinawa Trough
キーワード：沖縄トラフ、背弧リフト、貫入構造、地震活動
The Okinawa Trough is an active back-arc basin that has developed behind the Ryukyu subduction zone in southwest Japan and is currently in a transitional process from continental rifting to oceanic spreading. In its southernmost part, a variety of tectonic phenomena, such as dense normal fault development, seafloor volcanisms and hydrothermal venting, have been confirmed to occur in the narrow (~100-km-wide) basin. In addition, the seismicity rate associated with the back-arc rifting is rather high and large earthquakes with a magnitude greater than 5 have been produced frequently over the last decades. Despite these highly active tectonic processes arising from back-arc rifting, however, geophysical data for subsurface structure are scarcely documented.
In 2023-2024, we conducted integrated marine geophysical surveys in the southern Okinawa Trough using the R/V Hakuho-maru (cruise ID KH-23-11). This is a follow-up of our previous experiment in 2021 (KH-21-3) and the both cruises aim at understanding tectonic controls on the rifting and volcanic processes by focusing on two linear bathymetric depressions considered as the rift axes (the Yaeyama Rift and the Yonaguni Rift). Same as the 2021 experiment, we newly acquired multichannel seismic reflection data using the GI guns with a total air volume of 710 cubic inches and the 1200-m-long streamer cable with 48 channels. We collected the seismic reflection data on a total of 10 lines that cross the Yaeyama Rift and the Yonaguni Rift and also cover volcanic regions between the rift axes and the Ryukyu arc.
This presentation provides structural constraints on on-axis intrusions, fault distribution and potential volcanic activities at shallow depths. The preliminary conclusions are as follows:
1. The seismic reflection images across the Yaeyama Rift reveal along-trough structural variations (width and depth) of kilometer-wide on-axis intrusions (recognized as a seismically transparent body that breaches horizontally stratified sedimentary layers). This observation may reflect local heterogeneities of the rifting and intrusion processes. On the other hand, such narrow intrusive structures are missing around the Yonaguni Rift, but the sedimentary layers are highly deformed at some locations south of the rift axis, which imply hydrothermal/volcanic activities.
2. While the Yaeyama Rift exhibits a narrow (~10-km-wide) deformation zone, the Yonaguni Rift obviously accommodates active normal faulting in much broader (>20 km) areas.
3. The Yaeyama Rift often hosts large earthquakes on the western and eastern edges of the rift axis, but only a few in the central part. In contrast, shallow seismicity including seismic swarms occurs regionally and ubiquitously around the Yonaguni Rift.
4. All the results mentioned above highlight variations in structure and stress field between the Yaeyama Rift and the Yonaguni Rift and may reflect the difference of lithospheric strength associated with thermal regime.
In 2023-2024, we conducted integrated marine geophysical surveys in the southern Okinawa Trough using the R/V Hakuho-maru (cruise ID KH-23-11). This is a follow-up of our previous experiment in 2021 (KH-21-3) and the both cruises aim at understanding tectonic controls on the rifting and volcanic processes by focusing on two linear bathymetric depressions considered as the rift axes (the Yaeyama Rift and the Yonaguni Rift). Same as the 2021 experiment, we newly acquired multichannel seismic reflection data using the GI guns with a total air volume of 710 cubic inches and the 1200-m-long streamer cable with 48 channels. We collected the seismic reflection data on a total of 10 lines that cross the Yaeyama Rift and the Yonaguni Rift and also cover volcanic regions between the rift axes and the Ryukyu arc.
This presentation provides structural constraints on on-axis intrusions, fault distribution and potential volcanic activities at shallow depths. The preliminary conclusions are as follows:
1. The seismic reflection images across the Yaeyama Rift reveal along-trough structural variations (width and depth) of kilometer-wide on-axis intrusions (recognized as a seismically transparent body that breaches horizontally stratified sedimentary layers). This observation may reflect local heterogeneities of the rifting and intrusion processes. On the other hand, such narrow intrusive structures are missing around the Yonaguni Rift, but the sedimentary layers are highly deformed at some locations south of the rift axis, which imply hydrothermal/volcanic activities.
2. While the Yaeyama Rift exhibits a narrow (~10-km-wide) deformation zone, the Yonaguni Rift obviously accommodates active normal faulting in much broader (>20 km) areas.
3. The Yaeyama Rift often hosts large earthquakes on the western and eastern edges of the rift axis, but only a few in the central part. In contrast, shallow seismicity including seismic swarms occurs regionally and ubiquitously around the Yonaguni Rift.
4. All the results mentioned above highlight variations in structure and stress field between the Yaeyama Rift and the Yonaguni Rift and may reflect the difference of lithospheric strength associated with thermal regime.