PRELIMINARY RESULTS OF THE ISHIDO THRUST (OUT-OF-SEQUENCE THRUST) DRILLING IN BOSO PENINSULA, CENTRAL JAPAN: STUDYING EARTHQUAKE RUPTURE TO SHALLOW PORTION OF SUBDUCTION ZONE

Since plate tectonics paradigm was developed, one of the important aspects of accretionary complexes has been considered to be the sites some of the world's largest earthquakes. Moreover, they are fundamental components of mountain belts. Although earthquake ruptures in shallow portion of plate boundary have recently been identified (e.g. Tohoku, Nankai, etc.), their mechanisms and nature of the slip are under controversial, which is key to understanding the initial phase of orogeny associated with plate subduction. Geology in the southern part of the Boso Peninsula, central Japan, is composed of young and non-metamorphosed accretionary complexes covered by trench-slope and forearc-basin deposits. Two accretionary complexes are divided: 1) the Early to Middle Miocene and 2) the Late Miocene to Early Pliocene accretionary complex. Based on the vitrinite reflectance, the maximum burial depth were estimated as 2-4 km (65-90˚C) and less than 1 km (about 20 ˚C), respectively. The Ishido Thrust, the boundary thrust extends to the source region of the largest aftershock of the 1923 Taisho (Moment Magnitude Mw 7.5; main shock Mw 7.9) and 1703 Genroku earthquakes (Magnitude M 8.2), is therefore unique example representing the shallow portion of an ancient out-of-sequence thrust. The fault core is composed of black-colored thin (<1 mm) pseudotachylite and fault gouge. Therefore, this thrust experienced earthquake rupture propagation in the past.

To examine the lateral variation of earthquake rupture to shallow portion of subduction zone, we conducted coring campaign for the Ishido Thrust (October 2017-April 2018): we cored 5 sites within ~10 km along the thrust. We successfully penetrated the thrust in three sites, and retrieved primary shear zone in two sites (three holes). In this presentation, we are going to report the structural characteristics of the cored samples.