5:15 PM - 6:45 PM
[MGI31-P07] ReCoRD program for Understanding the Formation Process and Physical Property Distribution of the Upper Prism in the Japan Trench (ReC23-02)
Keywords:Japan Trench, Upper prism, Physical properties, Formation process, Deformation history
The Japan Trench is an erosional subduction zone, where the shallow part of the plate boundary fault was thought to have neither strain accumulation nor slip propagation due to the non-asperity nature of the shallow plate boundary. However, the 2011 off the Pacific coast of Tohoku earthquake demonstrated that the shallow part has the potential to generate a large slip during an earthquake.
IODP Exp. 343 (JFAST), which conducted drilling in the shallow part of the plate boundary at the Japan Trench, found that the plate boundary fault had been developed in low-friction pelagic claystone and that strain had been accumulated in the frontal prism of the hanging wall and stress was released during the earthquake. On the other hand, because of limited sample recovery from the upper prism during the JFAST, structural development processes and material characteristics of the upper prism have not been clarified.
The aims of this study are to elucidate the formation process of the upper prism of the Japan Trench plate subduction zone, which is the host of strain accumulation in great earthquakes, and to clarify the temporal- and spatial- physical and structural characteristics of the upper prism. For this purpose, we investigate the material characteristics and structural development of the sediments before and after subduction of the Japan Trench. The target boreholes are at one input site (Site 436) and four upper prism sites (Sites 434, 439, 440, and C0019). The input site recovered chronologically continuous siliceous sediments from the Holocene to the Eocene and chert from the Cretaceous. At Sites 434 and 440, located in the frontal side of the upper prism, siliceous mudstones up to the Upper Miocene were sampled and correlated in age with the pre-subduction sediments. A number of folds and reverse faults have been identified in these core samples, suggesting that they suffered deformation associated with plate subduction. On the other hand, core recovery at Site C0019 is concentrated near the plate boundary fault (648.0 - 821.5 mbsf), and core samples from the shallow upper prism are limited to 176.5 – 186.0 mbsf. Since physical properties were measured by simultaneous logging at this site, it would be possible to determine the lithology and infer the structure of the unrecovered depths at Site C0019 by measuring the MSCL properties of the DSDP core samples. Site 439 is the only site that penetrated an unconformity that appears as an acoustic basement, and Cretaceous sediments were recovered from the lowest part of the borehole. Since no clear reflective surface is observed below the unconformity, it is possible that these sediments dominantly comprise the upper prism and that their physical properties might control the strain distribution within the hanging wall of the plate boundary fault.
In this presentation, we will mainly present the results of a new continuous core analysis using cores acquired at the DSDP.
IODP Exp. 343 (JFAST), which conducted drilling in the shallow part of the plate boundary at the Japan Trench, found that the plate boundary fault had been developed in low-friction pelagic claystone and that strain had been accumulated in the frontal prism of the hanging wall and stress was released during the earthquake. On the other hand, because of limited sample recovery from the upper prism during the JFAST, structural development processes and material characteristics of the upper prism have not been clarified.
The aims of this study are to elucidate the formation process of the upper prism of the Japan Trench plate subduction zone, which is the host of strain accumulation in great earthquakes, and to clarify the temporal- and spatial- physical and structural characteristics of the upper prism. For this purpose, we investigate the material characteristics and structural development of the sediments before and after subduction of the Japan Trench. The target boreholes are at one input site (Site 436) and four upper prism sites (Sites 434, 439, 440, and C0019). The input site recovered chronologically continuous siliceous sediments from the Holocene to the Eocene and chert from the Cretaceous. At Sites 434 and 440, located in the frontal side of the upper prism, siliceous mudstones up to the Upper Miocene were sampled and correlated in age with the pre-subduction sediments. A number of folds and reverse faults have been identified in these core samples, suggesting that they suffered deformation associated with plate subduction. On the other hand, core recovery at Site C0019 is concentrated near the plate boundary fault (648.0 - 821.5 mbsf), and core samples from the shallow upper prism are limited to 176.5 – 186.0 mbsf. Since physical properties were measured by simultaneous logging at this site, it would be possible to determine the lithology and infer the structure of the unrecovered depths at Site C0019 by measuring the MSCL properties of the DSDP core samples. Site 439 is the only site that penetrated an unconformity that appears as an acoustic basement, and Cretaceous sediments were recovered from the lowest part of the borehole. Since no clear reflective surface is observed below the unconformity, it is possible that these sediments dominantly comprise the upper prism and that their physical properties might control the strain distribution within the hanging wall of the plate boundary fault.
In this presentation, we will mainly present the results of a new continuous core analysis using cores acquired at the DSDP.