Fault geometry and segmentation of the MTL active fault system in the Iyo_|nada Sea, western Shikoku, in Japan

The Median Tectonic Line (MTL) active fault system is one of the most active fault system in Japan, which is an east-west trending, 190 km-long fault system and consists of several rupture segments in Shikoku. A long active fault system such as the MTL active fault system may not rupture along its entire length in a single earthquake but instead consists of multiple seismic segments that rupture independently of one another. Therefore, the identification of the active segment for an each earthquake is very important subject for estimating ground motion. We investigated the detailed submarine topography, subsurface structure and fault activity of the MTL active fault system in the Iyo-nada Sea (Iyo-nada MTL active fault system), in Shikoku, Japan, by using echo sounder, single-channel seismic profiler and all-core boring in 2000 and 2001, in order to reveal the fault distribution and activity. We obtained the detailed fault trace with geometric discontinuities such as en echelon steps, bends, changes in strike, and gaps in this study area. These data are very precise compared with previous data, and permit us to consider fault segmentation and continuity to the Beppu Bay fault system and Iyo fault zone. The high-resolution core analysis revealed three or more seismic events and recurrence interval with 2500 to 3500yrs. The main fault trace and Holocene activity revealed in this study are apparently different from these of the Beppu Bay fault system. This result suggests that activity of these two fault systems depend on different tectonic setting one another. In contrast, the Iyo fault zone that distributes on land continues to the MTL active fault system in the Iyo-nada Sea as forming positive flower structure accompanied with compressional bend. We tried to conduct fault segmentation for the Iyo-nada MTL active fault system based on the 3-D fault geometry because we can easily consider the relationship between the surface trace and the subsurface structure of fault. Moreover, we also took into account of gravity data that may reflect basement structure, since it is suggested that basement structure may control the surface fault geometry, in recently. The Iyo-nada MTL active fault system is divided into three segments, which are the Iyo segment, the Iyo-nada segment and the Hoyo-kaikyo segment, on the basis of the 3-D fault geometry. These segment boundaries indicate the extensional right lateral steps with basin structure, respectively. In particularly, the extensional step at the eastern Iyo-nada Sea constructs pull-apart basin. We recognized these extensional right steps as the _gExtensional Jog_h in this study. These extensional jogs correspond to the changing parts of gravity anomaly trend. Therefore, deep crustal structure may be closely related with the fault geometry and segmentation.