STRUCTURAL CHARACTERISTICS AROUND THE FRONTAL THRUST ALONG THE NANKAI TROUGH REVEALED BY RED RELIEF IMAGE MAPPING AND SEISMIC REFLECTION SURVEY

Great earthquakes with tsunamis with recurrence intervals of 100–200 years have occurred along the Nankai Trough near central Japan. To predict the exact height of the tsunami on the coast region generated by these large ruptures, it is important to estimate the vertical deformation that occurs on the seaward end of the rupture area.

Recent drilling results have also yielded evidence not only of splay faults that generate tsunamigenic rupture, but also new evidence of tsunamigenic rupture along the frontal thrust at the trench axis in the Nankai Trough.

In order to understand the deformation around the frontal thrust at the trench axis, a dense high-resolution seismic reflection survey was conducted by Japan Agency for Marine-Earth Science and Technology over 1500 km of line length.

Clear seismic reflection images of frontal and previous thrusts in the accretionary prism, trench-fill deposits and subducting Shikoku Basin, image deformation along the trench axis. The cumulative displacement along the frontal thrust and second thrust are measured from picked distinct reflectors in depth-converted profiles. The average value of cumulative displacement of the frontal thrust is more than 100 m within 2 km depth beneath the seafloor. The cumulative displacement of the second thrust has a similar trend to the frontal thrust. We also evaluate the seaward structure for understanding the future rupture distribution. The protothrust zone (PTZ) consisting of many incipient thrusts is identifiable in the portion of trough-fill sediments seaward of the frontal thrust. To image the spatial distribution of the PTZ, we merged topographic data using all seismic survey around the trough axis. In order to emphasize the characteristics of frontal thrust and PTZ, we construct the detailed red relief image map for focusing on the lineated slope of the PTZ at the trough axis. We identified the clear bathymetric lineament along the trough axis within the protothrust zone by this map.

This study is part of "Research project for compound disaster mitigation on the great earthquakes and tsunamis around the Nankai Trough region" funded by the Ministry of Education, Culture, Sports, Science, and Technology of Japan. This study is also partially supported by JSPS Grant-in-Aid for Young Scientists (B) 16K17824.