Conjugate faulting and structural complexity on the young fault system associated with the 2000 Tottori earthquake

Aitaro Kato, Shin’ichi Sakai, Satoshi Matsumoto(Institute of Seismology and Volcanology, Kyushu University) & Yoshihisa Iio (Disaster Prevention Research Institute, Kyoto University)
Communications Earth & Environment volume 2, 13 (2021) https://www.nature.com/articles/s43247-020-00086-3

Abstract

　Young faults display unique complexity associated with their evolution, but how this relates to earthquake occurrence is unclear. Unravelling the fine-scale complexity in these systems could lead to a greater understanding of ongoing strain localization in young fault zones. Here we present high-spatial-resolution images of seismic sources and structural properties along a young fault zone that hosted the Tottori earthquake (Mw 6.8) in southwest Japan in 2000, based on data from a hyperdense network of ~1,000 seismic stations. Our precise micro-earthquake catalog reveals conjugate faulting over multiple length scales. These conjugate faults are well developed in zones of low seismic velocity. A vertically dipping seismic cluster of about 200 m length occurs within a width of about 10 m. Earthquake migrations in this cluster have a speed of about 30 m per day, which suggests that fluid diffusion plays a role. We suggest that fine structural complexities influence the pattern of seismicity in a developing fault system.

a Map of seismicity and surface traces of active faults (gray lines) in and around the San-in shear zone, SW Japan. The shear zone is characterized by a high seismicity rate and a large right-lateral shear strain rate. Blue rectangle denotes the location of (b). b Locations of seismic stations (red squares) in the hyperdense network and the relocated earthquakes (blue circles). Red star denotes the epicenter of the 2000 Tottori earthquake determined by JMA with CMT solution determined by the National Research Institute for Earth Science and Disaster Resilience (NIED). Black line is the surface trace of an active fault. c Plot of cumulative number of analyzed earthquakes as a function of magnitude. d Map of the grid used in the tomography analysis (crosses) and the relocated earthquakes (blue circles), overlaid on top of the terrain. The horizontal grid spacing near the mainshock fault plane is 0.5 km.