Temporal stress variations along a seismogenic megasplay fault in the subduction zone: An example from the Nobeoka Thrust, southwestern Japan

Abstract

AbstractThe Nobeoka Thrust, an ancient megasplay fault in the Shimanto Belt, southwestern Japan, contains fault rocks from the seismogenic zone, providing an accessible analog of active megasplay faults in deep subduction settings. In this study, the paleostress along the Nobeoka Thrust was analyzed using multiple inversion techniques, including k‐means clustering of fault datasets acquired from drillcores that intersected the thrust. The six resultant stress orientation clusters can be divided into two general groups: stress solutions with north–south‐trending σ1 axes, and those with east–west‐trending σ1 axes. These groups are characterized by the temporal changes for the orientations of the σ1 and σ3 principal stress axes that involve alternation between horizontal and vertical. The findings are probably due to a change in stress state before and after earthquakes that occurred on the fault; similar changes have been observed in active tectonic settings, such as the 2011 Tohoku‐Oki earthquake (Japan).