5:15 PM - 6:45 PM
[SSS09-P08] Crustal structure investigation by seismic survey in the eastern Nankai Trough
Keywords:Nankai trough, crustal structure, seismic survey, reflection profile, subduction zone
In the Nankai Trough, where large earthquakes and slow earthquakes occur repeatedly associated with the plate subduction, it is necessary to investigate the crustal structure over a wide area and clarify the detailed structure with physical properties in three dimensions in order to better understand the relationship between various seismic activities and geological structures. JAMSTEC has continued to investigate the crustal structure by reflection seismic surveys and wide-angle seismic surveys with newly implemented high-density survey lines since 2018. In this presentation, we report on seismic surveys conducted in the eastern Nankai Trough in 2022 and 2023. The area from off the Kumano-nada to off the Enshu-nada includes the area where the 1944 Showa Tonankai earthquake and other large earthquakes have occurred. In recent years, slow earthquakes have been observed repeatedly, but their distribution is not uniform. The seafloor topography and the structure of accretionary complexes vary greatly in this area. The subduction of the oceanic crust highly-undulated with like the Paleo-Zenisu Ridge, which is subducting obliquely to the trough axis, may affects the structural development of accretionary complexes and seismic activity.
The survey cruises consisted of a series of operations: (1) installation of ocean-bottom seismographs (OBSs), (2) reflection and wide-angle seismic surveys, and (3) recovery of OBSs. We conducted surveys of (1) YK22-14 by R/V Yokosuka, (2) KM22-10 by R/V Kaimei, and (3) a chartered vessel cruise in FY2022, and (1)-(2) KM23-13 by R/V Kaimei, and (3) a chartered vessel cruise in FY2023. The multi-channel seismic (MCS) reflection data were acquired with a single hydrophone streamer cable (4.5 km or 5.5 km) and a large-volume air gun array (10,600 cu.in. in total) fired every 50 m. The survey lines were designed mainly in the direction generally orthogonal to the trough axis to cover the target area at 4 km intervals together with the past surveys, and several lines roughly parallel to the trough axis. However, there are still some areas where data acquisition has progressed only at intervals of 8 km to 12 km, and we plan to continue the survey to fill in the survey lines at the same density in the future. On the other hand, for the wide-angle seismic surveys, we also used several past survey lines approximately perpendicular to the trough axis. We installed 50 four-component OBSs on each survey line at intervals of two additional OBSs between the installation points in the previous surveys, and recorded the seismic signals generated from the same large-volume air gun array fired every 200m. The purpose of increasing the density of OBSs on the survey lines is to apply full waveform inversion to estimate the seismic velocity structure in the crust at higher resolution than that derived by conventional first-arrival tomography.
Newly acquired MCS data and some of existing MCS data are processed in a unified manner by applying recent standard data processing and imaging techniques. After applying pre-processing including noise and multiple reflection suppression, de-ghosting, de-bubbling, etc., pre-stack migration is applied in time and depth domains to generate subsurface cross sections delineating the detailed geological structure of the entire survey area. As a result of the comprehensive investigation, the geometry of the top surface of the regional subducting oceanic crust, deformation structures within the accretionary complex, and sedimentary basins were confirmed to vary along the trough axis. The accretionary complex is generally formed by the repetition of multiple imbricate thrusts. In some areas, sedimentary sequences with less deformation than the surrounding area and flower structures caused by regional strike-slip faulting were observed in the accretionary complex. Future analyses combining detailed interpretation of the geological structures based on the dense reflection profiles and physical property estimation based on seismic wave velocities from the wide-angle seismic data are important to clarify the accretionary complex formation its influence on seismic activity associated with oblique subduction of the undulated oceanic plate.
The survey cruises consisted of a series of operations: (1) installation of ocean-bottom seismographs (OBSs), (2) reflection and wide-angle seismic surveys, and (3) recovery of OBSs. We conducted surveys of (1) YK22-14 by R/V Yokosuka, (2) KM22-10 by R/V Kaimei, and (3) a chartered vessel cruise in FY2022, and (1)-(2) KM23-13 by R/V Kaimei, and (3) a chartered vessel cruise in FY2023. The multi-channel seismic (MCS) reflection data were acquired with a single hydrophone streamer cable (4.5 km or 5.5 km) and a large-volume air gun array (10,600 cu.in. in total) fired every 50 m. The survey lines were designed mainly in the direction generally orthogonal to the trough axis to cover the target area at 4 km intervals together with the past surveys, and several lines roughly parallel to the trough axis. However, there are still some areas where data acquisition has progressed only at intervals of 8 km to 12 km, and we plan to continue the survey to fill in the survey lines at the same density in the future. On the other hand, for the wide-angle seismic surveys, we also used several past survey lines approximately perpendicular to the trough axis. We installed 50 four-component OBSs on each survey line at intervals of two additional OBSs between the installation points in the previous surveys, and recorded the seismic signals generated from the same large-volume air gun array fired every 200m. The purpose of increasing the density of OBSs on the survey lines is to apply full waveform inversion to estimate the seismic velocity structure in the crust at higher resolution than that derived by conventional first-arrival tomography.
Newly acquired MCS data and some of existing MCS data are processed in a unified manner by applying recent standard data processing and imaging techniques. After applying pre-processing including noise and multiple reflection suppression, de-ghosting, de-bubbling, etc., pre-stack migration is applied in time and depth domains to generate subsurface cross sections delineating the detailed geological structure of the entire survey area. As a result of the comprehensive investigation, the geometry of the top surface of the regional subducting oceanic crust, deformation structures within the accretionary complex, and sedimentary basins were confirmed to vary along the trough axis. The accretionary complex is generally formed by the repetition of multiple imbricate thrusts. In some areas, sedimentary sequences with less deformation than the surrounding area and flower structures caused by regional strike-slip faulting were observed in the accretionary complex. Future analyses combining detailed interpretation of the geological structures based on the dense reflection profiles and physical property estimation based on seismic wave velocities from the wide-angle seismic data are important to clarify the accretionary complex formation its influence on seismic activity associated with oblique subduction of the undulated oceanic plate.