11:00 AM - 1:00 PM
[HCG25-P06] Seismic stratigraphy and shallow subsurface structure in the Kii Strait, SW Japan
Keywords:Kii Strait, Median Tectonic Line active fault system, coastal area, reflection seismic survey, seismic stratigraphy
The National Institute of Advanced Industrial Science and Technology (AIST) has conducted comprehensive survey on Japanese coastal marine and land geology for the purpose of assessing seismic hazard risks and industrial locations. The Kii Strait, SW Japan, has been targeted since 2020 to investigate the Median Tectonic Line (MTL) active fault system and its influence on the development of the Quaternary strata in the region. Systematic geological and geophysical data have been acquired during campaigns including gravity measurements, reflection seismic survey and bottom sediment sampling in the marine realm as well as a boring onland. Here we provide preliminary results and discussion on seismic stratigraphy and characteristic geological structures of shallow submarine strata in the Kii Strait, revealed by reflection seismic survey.
The Kii Strait, extended ~50 km in both N–S and E–W directions, is located at the eastern end of the Seto Inland Sea. The region is surrounded by Tokushima, Wakayama and Hyogo (Awaji Island) prefectures, facing toward the Philippin Sea at its southern opening. Water depths of the study area range from approximately 10 m to 80 m. A boomer and a hydrophone array were towed from a small vessel (14 t) for the multichannel reflection survey, and sub-bottom acoustic reflection data were obtained along in total ~780 km survey lines.
Subsurface strata in the study area are divided into at least three acoustic units A to C in descending order, according to the concept of depositional sequence in Mitchum et al. (1977). As the acoustic basements of the study area, the Unit C is restrictedly distributed near the shore. This unit generally underlies the Unit B scattered like "buried islands", although in some places it exposes at the sea floor. The Unit B is distributed over the entire study area with its thickness ~100 msec or more in Two-way traveltime (TWT). It is exposed at the sea floor at some deeper water depths below ~60 m. This unit shows subparallel and poorly continuous internal reflections. The lower boundary of the unit B is in general obscured, although onlap patterns onto the upper surfaces of the Unit C are recognized around nearshore areas. The Unit B is potentially subdivided further by distinguishing internal acoustic facies and tracing characteristic reflection surfaces in more detail. The Unit A is widely distributed with its maximum thickness ~50 msec in TWT and is generally pinching-out seaward. This unit has parallel and continuous internal reflections. The lower boundary of the Unit A represents onlap/downlap surface on the underlying units B or C. Those units A, B and C are tentatively correlated to the contiguous onland strata, representing the Holocene (Alluvium), Pleistocene and pre-Quaternary basement rocks, respectively. The acoustic reflection boundary between units A and B thus represents erosional surfaces during the Last Glacial Maximum and correlative conformable lithological bounadries.
In the northern area, prominent NE–SW trending reverse faults are recognizable. The Holocene activity of these faults, belonging to the MTL active fault system, has been inferred from the observation that some of these shallow faults accompany deformations or displacements of internal reflection surfaces of the Unit A. Further efforts to construct a detail age model of the acoustic units would reveal the Quaternary history of fault activities and development of the MTL active fault system in the study area.
Reference: Mitchum, R.M.Jr., Vail, P.R. and Thompson, S.Ⅲ (1977), AAPG Memoir 26, 53–62.
The Kii Strait, extended ~50 km in both N–S and E–W directions, is located at the eastern end of the Seto Inland Sea. The region is surrounded by Tokushima, Wakayama and Hyogo (Awaji Island) prefectures, facing toward the Philippin Sea at its southern opening. Water depths of the study area range from approximately 10 m to 80 m. A boomer and a hydrophone array were towed from a small vessel (14 t) for the multichannel reflection survey, and sub-bottom acoustic reflection data were obtained along in total ~780 km survey lines.
Subsurface strata in the study area are divided into at least three acoustic units A to C in descending order, according to the concept of depositional sequence in Mitchum et al. (1977). As the acoustic basements of the study area, the Unit C is restrictedly distributed near the shore. This unit generally underlies the Unit B scattered like "buried islands", although in some places it exposes at the sea floor. The Unit B is distributed over the entire study area with its thickness ~100 msec or more in Two-way traveltime (TWT). It is exposed at the sea floor at some deeper water depths below ~60 m. This unit shows subparallel and poorly continuous internal reflections. The lower boundary of the unit B is in general obscured, although onlap patterns onto the upper surfaces of the Unit C are recognized around nearshore areas. The Unit B is potentially subdivided further by distinguishing internal acoustic facies and tracing characteristic reflection surfaces in more detail. The Unit A is widely distributed with its maximum thickness ~50 msec in TWT and is generally pinching-out seaward. This unit has parallel and continuous internal reflections. The lower boundary of the Unit A represents onlap/downlap surface on the underlying units B or C. Those units A, B and C are tentatively correlated to the contiguous onland strata, representing the Holocene (Alluvium), Pleistocene and pre-Quaternary basement rocks, respectively. The acoustic reflection boundary between units A and B thus represents erosional surfaces during the Last Glacial Maximum and correlative conformable lithological bounadries.
In the northern area, prominent NE–SW trending reverse faults are recognizable. The Holocene activity of these faults, belonging to the MTL active fault system, has been inferred from the observation that some of these shallow faults accompany deformations or displacements of internal reflection surfaces of the Unit A. Further efforts to construct a detail age model of the acoustic units would reveal the Quaternary history of fault activities and development of the MTL active fault system in the study area.
Reference: Mitchum, R.M.Jr., Vail, P.R. and Thompson, S.Ⅲ (1977), AAPG Memoir 26, 53–62.