17:15 〜 19:15
[SCG45-P16] Co-seismic ductile deformation in the carbonate-hosted active faults
キーワード：Co-seismic ductile deformation, Frictional heating, Ryukyu Limestone, Yonabaru Fault, EBSD analysis
Unambiguous geological signatures of seismic slip such as pseudotachylyte are rare, particularly in carbonate-hosted faults where thermal decomposition of carbonate minerals occurs at temperatures lower than those required for frictional melting. As a result, pseudotachylyte formation is unlikely, necessitating the identification of alternative indicators of seismic slip in carbonate rocks, which is a key sedimentary rock in seismically active regions.
This study examines one of the active faults on Miyako Island (Yonabaru Fault), located in the southern Ryukyu Arc, which displace the Pleistocene Ryukyu Limestone and is expected to generate a magnitude 6.9 - 7.2 class earthquake. The fault zone is 50 m thick and consists of multiple sub-vertically oriented slip zones and fractures. Kinematic data indicate normal faulting within a northeast-southwest extensional regime. The slip surfaces are characterized by mirror-like polished and lineated features, with slip zones averaging 2 mm in thickness, consisting of limestone fragments embedded in a fine-grained calcite matrix (~ 4.2 µm).
Scanning Electron Microscopy (SEM) reveals wear grooves and particle coalescence on the slip surfaces, while Electron Backscatter Diffraction (EBSD) analysis of the calcite matrix indicates a crystallographic preferred orientation (CPO), with the [0001] axis inclined relative to the shear direction. The grain orientation spread (GOS) and kernel average misorientation (KAM) reveals that the calcite matrix experienced dislocation activity and static recrystallisation. We suggests that co-seismic frictional heating induces ductile deformation, recrystallisation and grain sintering. These findings demonstrate that carbonate-hosted faults can preserve microstructural records of co-seismic slip events at shallow crustal depths.
This study examines one of the active faults on Miyako Island (Yonabaru Fault), located in the southern Ryukyu Arc, which displace the Pleistocene Ryukyu Limestone and is expected to generate a magnitude 6.9 - 7.2 class earthquake. The fault zone is 50 m thick and consists of multiple sub-vertically oriented slip zones and fractures. Kinematic data indicate normal faulting within a northeast-southwest extensional regime. The slip surfaces are characterized by mirror-like polished and lineated features, with slip zones averaging 2 mm in thickness, consisting of limestone fragments embedded in a fine-grained calcite matrix (~ 4.2 µm).
Scanning Electron Microscopy (SEM) reveals wear grooves and particle coalescence on the slip surfaces, while Electron Backscatter Diffraction (EBSD) analysis of the calcite matrix indicates a crystallographic preferred orientation (CPO), with the [0001] axis inclined relative to the shear direction. The grain orientation spread (GOS) and kernel average misorientation (KAM) reveals that the calcite matrix experienced dislocation activity and static recrystallisation. We suggests that co-seismic frictional heating induces ductile deformation, recrystallisation and grain sintering. These findings demonstrate that carbonate-hosted faults can preserve microstructural records of co-seismic slip events at shallow crustal depths.