Weak faults at megathrust plate boundary respond to tidal stress

Takashi Tonegawa,Suguru Yabe,Kazuya Shiraishi,Seiichi Miura,Shuichi Kodaira,Yasuyuki Nakamura,Yoshio Fukao,Masataka Kinoshita,Toshinori Kimura,Eiichiro Araki,Yoshinori Sanada

doi:10.1186/s40623-021-01414-3

Abstract

Lateral spatial variations of weak portions at the plate boundary in subduction zones have been estimated primarily by the distribution of slow earthquakes mainly occurring around seismogenic zones. However, the detailed depth profile of weak faults remains elusive. Here, we deployed six ocean bottom seismometers in the Nankai subduction zone, Japan, to observe reflections originated from drilling vessel Chikyu ship noise (hydroacoustic P wave) that was persistently radiated from a fixed position at the sea surface, and retrieved P-to-s (Ps) reflections from multiple dipping faults near the plate boundary. The Ps amplitudes were stacked and compared according to the degrees of tidal stresses, and high amplitudes were observed at high tide (compression). A migration technique shows that the locations where velocity contrasts fluctuate were estimated at both the megasplay fault and another fault between the megasplay fault and the top of the oceanic crust. This indicates that the physical properties of these faults are altered by tidal stress. The physical-property changes are attributed to fluid connections and isolations within the faults due to tidal stress fluctuations, inducing the variation of seismic anisotropy. Such a variation was confirmed by a three-dimensional numerical simulation for wave propagation with anisotropic medium. Our observation suggests that multiple weak faults are present around the plate boundary, and the obtained changes of fault physical properties may have implications for in-depth understanding of tidal triggering of earthquakes.

Highlights

Tidal stresses result from the deformation of the Earth and ocean loading due to the gravitational pull of the Sun and Moon, and these can trigger ordinary (Cochran et al 2004; Tanaka et al 2006; Ide et al 2016) and slow earthquakes (Rubinstein et al 2008; Nakata et al 2008; Lambert et al 2009; Ide 2010) by stress fluctuations on plate boundaries in subduction zones
Seismic interferometry is capable of detecting waves propagating between two receivers by cross-correlating the ambient noise recorded at both the receivers (Shapiro 2005; Brenguier et al 2007). Applying this technique to continuous seismic records observed at a near-vertical array of OBS04 and a borehole sensor (Kopf et al 2011, 2016) (Fig. 1b), we found that the downgoing P waves generated by the Chikyu ship noise were dominant during the non-drilling period, while the upgoing S waves from the drill-bit torque at depth could be observed in the drilling period (See details in “Materials and methods” section): Chikyu produces hydroacoustic P waves in the sea water by, e.g., internal instrumental and external thruster noises, from the fixed drilling position at the sea surface, and they are converted to downgoing P waves at the seafloor
Tidal response of reflections To investigate whether the amplitudes and travel times of Ps waves respond to tidal stress, we calculated the normal tidal stress on the megasplay fault (Yabe et al 2015) by Imaging of tidal responding boundaries Depth migrations of amplified differential Ps amplitudes (AD-Ps) between the high and low tides (Eq (4)) located where the velocity contrast fluctuates in the accretionary prism

Summary

Introduction

Tidal stresses result from the deformation of the Earth and ocean loading due to the gravitational pull of the Sun and Moon, and these can trigger ordinary (Cochran et al 2004; Tanaka et al 2006; Ide et al 2016) and slow earthquakes (Rubinstein et al 2008; Nakata et al 2008; Lambert et al 2009; Ide 2010) by stress fluctuations on plate boundaries in subduction zones. A scientific drilling project was carried out between October 2018 and March 2019 by the International Ocean Discovery Program (IODP) with the drilling vessel D/V Chikyu (hereafter Chikyu) (IODP expedition 358) (Fig. 1) at the C0002 site located off the Kii Peninsula in the Nankai subduction zone, Japan, beneath which.

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Discussion

Conclusion