[S-SS30_29PO1] New perspective of great earthquakes along subduction zones

[SSS30-P27] Stress estimation of Kure OSTs, Shimanto accretionary complex

*Yusuke KOMETANI¹ (1.yamaguchiuniversity)

Keywords:subduction zone, ancient seismogenic, calcite, twin density

Stress must be concentrated at front of seismogenic fault during rupture propagation. The level of this stress concentration depends on rupture propagation velocity, fault length, thickness of process zone and strength of host rock. However, few quantitative analysis was reported in natural fault due to difficulty of stress estimation. The calcite-twin piezometer, enables stress estimation from elastic rebounded rock, was proposed based on discrete element method simulation and tri-axial rock experiments (Sakaguchi et al., 2011).The Shimanto accretionary complex is ancient subduction zone and some fossil seismogenic faults were reported. Among them, pseudotachylyte bearing Kure OSTs cuts Cretaceous Shimotsui, Nonokawa Formation and Kure Melange. This Kure OSTs is composed of echelon formed small faults with thin damaged zone, and burial depth of the host rock is estimated as below 3 km in depth. We obtained three rock samples, applicable for calcite-twin piezometer. The highest value of estimated stress was approximately 420MPa. This is much higher value than the other seismogenic fault in Shimanto accretionary. The Okitsu Fault, formed deeper depth of approximately 4 km, have suffered lower stress of 350 MPa at fault center (Sakaguchi et al., 2011). This indicates that much higher stress was concentrated at shallow Kure OST than deep Okitsu Fault. We propose two models to make high stress at shallow portion. Long crack length from deep to surface causes high stress concentration at shallow portion. Other model causes high stress due to narrower fault zone than the Okitsu Fault. Stress may tend to concentrate at narrower process zone of Kure OSTs than wide process zone of Okitsu Fault.