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

Aitaro Kato,Shin’ichi Sakai,Satoshi Matsumoto,Yoshihisa Iio

doi:10.1038/s43247-020-00086-3

Aitaro Kato, Shin’ichi Sakai + Show 2 more

Open Access

https://doi.org/10.1038/s43247-020-00086-3

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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.

Highlights

Young faults display unique complexity associated with their evolution, but how this relates to earthquake occurrence is unclear
A detailed geomorphological study has clarified that conjugate active faults with WSW–ENE and NNW–SSE trends are distributed within the San-in shear zone, but that they are young growing faults with cumulative offsets of less than a few hundreds of meters[16]
Sub-vertical alignments can be identified in a crosssection along the fault-strike of the 2000 Tottori earthquake (Fig. 2c)

Summary

Introduction

Young faults display unique complexity associated with their evolution, but how this relates to earthquake occurrence is unclear. Fault complexity along young fault systems is poorly understood owing to a lack of highspatial resolution images of seismic sources and structural properties at seismogenic depths To address this issue, we deployed a spatially dense seismic array of 1000 vertical short-period sensors in and around the source area of the 2000 Tottori earthquake (Japan Meteorological Agency (JMA) magnitude Mj 7.3; Mw 6.8), and obtained continuous waveform data from April 2017 to April 2018 (13 months) (Fig. 1). No obvious surface traces of active faults linked to the 2000 Tottori earthquake have been identified (Fig. 2a), indicating that a major fault structure has not developed in the study area These observations suggest that the San-in shear zone is a young fault system with many faults that lie parallel and oblique to the shear zone[15], but there is little constraint on the small-scale structures within the shear zone. Cross−section (km) to be installed for more than 1 year, and above the aftershock area of a magnitude ~7 earthquake[17,18], to the best of our knowledge

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