ARTICLE https://doi.org/10.1038/s43247-020-00086-3 OPEN Conjugate faulting and structural complexity on the young fault system associated with the 2000 Tottori earthquake ✉ Aitaro Kato 1 , Shin’ichi Sakai1,2, Satoshi Matsumoto3 & Yoshihisa Iio 4 Young faults display unique complexity associated with their evolution, but how this relates 1234567890():,; 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 devel- oping fault system. 1 Earthquake Research Institute, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan. 2 Interfaculty Initiative in Information Studies, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. 3 Institute of Seismology and Volcanology, Kyushu University, 744 Moto-oka, Nishi-ku, ✉ Fukuoka 819-0395, Japan. 4 Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan. email: [email protected] COMMUNICATIONS EARTH & ENVIRONMENT | (2021) 2:13 | https://doi.org/10.1038/s43247-020-00086-3 | www.nature.com/commsenv 1 ARTICLE COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-020-00086-3 ault systems develop with an increase in total fault dis- continuous waveform data from April 2017 to April 2018 Fplacement. During the initial stage of development, tectonic (13 months) (Fig. 1). The Tottori earthquake took place along a strain is accommodated by pervasive brittle deformation left-lateral strike-slip fault within the San-in active shear zone, that can be associated with discontinuities such as bends, step- which is characterized by a high seismicity rate and a large shear overs, and conjugate faults1. However, as the total displacement strain rate of 1.0 × 10−7/year13–15. Along the San-in shear zone, a increases, slip gradually localizes into one major smooth fault2, conjugate seismic lineation with WSW–ENE and NNW–SSE eventually resulting to a release of tectonic strain along it. In a trends can be recognized from the JMA catalog (Fig. 1a). A young fault system, fault geometries, and structural properties Global Navigation Satellite System network has revealed that the are likely to change in a complex manner. Complicate interac- shear deformation zone is as wide as 50 km along the seismic tions between discontinuous faults and lateral structural prop- belt15. However, no major active faults have been identified at the erties may promote or inhibit fault nucleation and growth, and surface in the San-in region (Fig. 1a). A detailed geomorpholo- produce fault linkage and coalescence1. In addition, these gical study has clarified that conjugate active faults with interactions can control spatio-temporal pattern of seismic WSW–ENE and NNW–SSE trends are distributed within the activity, slip distribution, and the location of large earthquake San-in shear zone, but that they are young growing faults with rupture areas, which are important in seismic hazard cumulative offsets of less than a few hundreds of meters16. assessment3,4. Recent seismic and geodetic observations of fault Indeed, no obvious surface traces of active faults linked to the networks have revealed that multiple ruptures along conjugate 2000 Tottori earthquake have been identified (Fig. 2a), indicating and subparallel faults can occur simultaneously or in quick that a major fault structure has not developed in the study area. succession4–8. Crustal fluids spreading through a fault network These observations suggest that the San-in shear zone is a young can also enhance fault-slip9–12. However, fault complexity along fault system with many faults that lie parallel and oblique to the young fault systems is poorly understood owing to a lack of high- shear zone15, but there is little constraint on the small-scale spatial resolution images of seismic sources and structural structures within the shear zone. properties at seismogenic depths. The average spatial interval of the hyperdense seismic network To address this issue, we deployed a spatially dense seismic used in this study is ~1 km, covering the entire aftershock area array of 1000 vertical short-period sensors in and around the within a radius of ~15 km (Fig. 1b). High-density seismic arrays source area of the 2000 Tottori earthquake (Japan Meteorological have been deployed in other tectonic environments, but the Agency (JMA) magnitude Mj 7.3; Mw 6.8), and obtained network used in this study is the first high-density seismic array 5 a 10 c 4 the San-in shear zone 10 Mc = −0.6 3 10 2 10 1 10 Cumulative number 1 −2 −1 0123 Magnitude b M7.3 (2000) d M7.3 (2000) 35.4˚N 35.3˚N 35.2˚N Y 35.1˚N X km 0 10 133.1˚E 133.2˚E 133.3˚E 133.4˚E 133.5˚E 133.6˚E133.1˚E 133.2˚E 133.3˚E 133.4˚E 133.5˚E 133.6˚E Fig. 1 Tectonic setting and hyperdense seismic network. 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. 2 COMMUNICATIONS EARTH & ENVIRONMENT | (2021) 2:13 | https://doi.org/10.1038/s43247-020-00086-3 | www.nature.com/commsenv COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-020-00086-3 ARTICLE 133.3˚E 133.4˚E 35.4˚N 1 M7.3 (2000) Fig. 4 510 24 23 22 0 21 35.3˚N 20 19 18 −5 17 16 15 Distance along fault-strike (km) −10 14 13 c 12 11 10 S30E0 5 10 15 N30W 09 Depth (km) 08 07 036912 15 06 35.2˚N Depth (km) 05 04 km 03 M2 M1 M0 M-1 02 a 0 5 01 0123 Slip (m) b W35S E35N 04 08 12 16 20 24 03 07 11 15 19 23 02 06 10 14 18 22 0 5 10 Depth (km) 01 05 09 13 17 21 15 −50 5 Cross−section (km) Fig. 2 Map and cross-sectional views of persisting aftershocks. a Epicentral map of the relocated hypocenters, which are shown as circles that are scaled to earthquake magnitude and colored to hypocentral depth. Blue star denotes the epicenter of the 2000 Tottori earthquake. Each blue arrow indicates conjugate vertical lineation. A small red rectangle around the slice 19 denotes a tiny seismic cluster in Fig. 4. Purple arrows in the inset show the orientation 14 of the maximum compressive stress (σ1) . Red line is the surface trace of an active fault. b Depth sections of the relocated hypocenters along the profiles shown in (a). c Seismicity cross-section for events within 2.5 km of a profile along the mainshock fault plane (blue circles). Background color denotes the coseismic-slip distribution of the 2000 Tottori earthquake inverted using strong motion waveforms and GPS data27. to be installed for more than 1 year, and above the aftershock area (Fig. 1c). Dense, well-covered ray-paths from the many sources of a magnitude ~7 earthquake17,18, to the best of our knowledge. provide valuable opportunities to (1) precisely map the aftershock Using the waveform data retrieved from the hyperdense network, distribution following the 2000 Tottori earthquake (Fig. 1d), (2) we obtained arrival time data for 4033 micro-earthquakes with delineate fine-scale P-wave velocity structures with a spatial − magnitudes greater than the completeness magnitude of ML 0.6 resolution down to 500 m, (3) explore the spatial and temporal COMMUNICATIONS EARTH & ENVIRONMENT | (2021) 2:13 | https://doi.org/10.1038/s43247-020-00086-3 | www.nature.com/commsenv 3 ARTICLE COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-020-00086-3 evolution of a small seismic cluster, and (4) understand the In contrast, many relocated events on the northwest side of the multiscale complexity of this young fault system at depth. source area have conjugate vertical alignments (blue arrows in Fig. 2a). The data show that the fault zone exhibits conjugate faulting over multiple length scales. The most prominent Results conjugate fault plane extends toward W20S from the main trend, Geometric complexity of faults and velocity structures. Nearly with a horizontal length of ~5 km along strike.