Spatiotemporal distributions of interplate coupling in Tohoku, northeast Japan, for 14 years prior to the 2011 Tohoku-oki earthquake inverted from GNSS data
Introduction
Off the east coast of Tohoku district, the oceanic Pacific (PAC) plate is subducting beneath the continental North American (NA) plate at a convergence rate of approximately 8.3 cm/yr in a west-northwest direction (DeMets et al., 2010) along the Japan Trench (Fig. 1). The crustal deformation caused by interplate coupling has been recognized by continuous GNSS observations. In this study, the so-called slip deficit rate is hereafter referred to as interplate coupling. In 1996, the Geospatial Information Authority (GSI) of Japan initated the operation of a continuous observation system called GEONET (GNSS Earth Observation Network System); currently more than 1300 land-based stations have been placed throughout the Japanese archipelago at intervals of approximately 20 km. Time series data observed at GNSS stations are useful for monitoring crustal deformation caused by volcanoes and earthquakes. Additionally, geodetic data have contributed to the understanding of the seismotectonics of the Japanese Islands, as evidenced by the use of fault models and the estimation of interplate coupling based on continuous observational data (e.g., Ozawa et al., 1999; Nishimura et al., 1999; Ito et al., 2000).
The Mw 9.0 megathrust Tohoku-oki earthquake occurred at the boundary of the NA and PAC plates on 11 March 2011, and its rupture area was widespread from off the coast of Iwate Prefecture to off the coast of Ibaraki Prefecture. To understand the generation mechanism of large megathrust earthquakes such as the 2011 Tohoku-oki earthquake, it is important to know the state of interplate coupling prior to the earthquake. Since the initiation of GEONET operation, many studies have estimated interplate coupling off the Tohoku east coast using GNSS data (e.g., Ito et al., 2000; Mazzotti et al., 2000; Hashimoto et al., 2009). Nishimura et al. (2004) estimated yearly interplate coupling off the Tohoku coast using horizontal and vertical displacement data from April 1995 to March 2002. The results show that there was no significant temporal change in interplate coupling during this period, except for the area at the estimated afterslip location of the 1994 Sanriku-haruka-oki earthquake (Mw 7.7–7.8). Suwa et al. (2006) estimated the spatial variation in interplate coupling off the Tohoku coast using the horizontal and vertical displacements of GNSS continuous observations from 1997 to 2001 and found that strong interplate coupling was distributed from off the coast of Miyagi and Aomori Prefectures to Tokachi and that interplate coupling reached a depth of approximately 100 km on the plate boundary. It is important to use not only the horizontal data but also the vertical data of the displacement rates at GNSS stations for estimating interplate coupling. Ikuta et al. (2012) analyzed the horizontal and vertical displacements of GNSS time series data in the Tohoku district from 21 March 1996 to 1 March 2011 and estimated the yearly distribution of interplate coupling for the 15 years prior to the Tohoku-oki earthquake. As a result, they found that the interplate coupling distribution on the northeastern plate boundary was nearly stable before 2003, and a strong coupling with a rate of approximately 8.5 cm/yr was obtained between 36°N and 40°N. A common characteristic of Ikuta et al. (2012) and previous studies is that no interplate coupling, namely, zero slip deficit rate, or aseismic slip was observed in the area at approximately 40°N, mainly at a depth of 50 km. From 2003 onward, strong interplate coupling is observed between 36°N and 40°N, but the strength of the interplate coupling varies by year. It is also shown that this strongly coupled area coincides with the slip area of the 2011 Tohoku-oki earthquake.
In addition, Nishimura (2012) estimated the spatial distribution of interplate coupling during two-year periods from October 1997 to October 2008, compared these distributions and showed that a decrease in coupling occurred off the coast of Fukushima Prefecture before the Tohoku-oki earthquake. Mavrommatis et al. (2014) analyzed GNSS data from 21 March 1996 to 6 February 2011, and Yokota and Koketsu (2015) analyzed GNSS data from 21 March 1996 to 8 March 2011 before the 2011 Tohoku-oki earthquake. They found that a decadal long-term decrease in interplate coupling has occurred at deeper positions on the plate boundary to the south of 39°N, and an increase in interplate coupling at deeper positions on the plate boundary to the north of 39°N since approximately 2003. Iinuma (2018) examined the spatiotemporal variations in horizontal and vertical displacement gradients using GNSS data from 24 March 1997 to 26 December 2016 and considered the area with small displacement gradients to indicate weakened interplate coupling. The results showed that interplate coupling weakened in 1997, 2001, 2005, and 2009 from 36.5°N to 38.5°N and that the northern limit of this region moved northward.
The objectives of this study are 1) to obtain the decadal long-term spatiotemporal tectonic crustal deformation in the Tohoku district as precisely as possible by analyzing GNSS data for 14 years just prior to the 2011 Tohoku-oki earthquake; 2) on the basis of the extracted tectonic crustal deformation, to estimate the spatiotemporal distribution of interplate coupling by performing time-dependent inversion analysis, and 3) to elucidate the temporally changing processes of interplate coupling to reach the main shock of the Tohoku-oki earthquake.
Section snippets
Data
In this study, we used daily coordinates (F3 solution; Nakagawa et al., 2009), which are the results of the final analysis of the GEONET observations provided by the GSI. We determined the tectonic deformation caused by the interplate coupling between the PAC and NA plates observed at the GNSS stations in the Tohoku district by analyzing the time series of the GEONET data at 186 stations during the period from 21 March 1996 to 7 March 2011. The reference frame of the estimated coordinates is
Time series analysis of GNSS data
We estimated interseismic surface displacements by removing steps of antenna exchange, coseismic steps, annual and semiannual variations, postseismic transient deformations, and common-mode errors from the GNSS time series data, and assumed that they represent deformation due to interplate coupling of the two plates. In this section, we describe our procedure in detail.
We fitted the following equation to time series data of each component at an arbitrary observation station:
Tectonic crustal deformations caused by interplate coupling
In Fig. 2(a)-(n), the directions of the horizontal displacement rates at the eastern stations on land are generally consistent with the convergence direction of the PAC plate with respect to the NA plate (DeMets et al., 2010) for the entire period, but the displacement rates decreased and rotated northward gradually toward the western stations. In the following section, we describe the characteristics of displacement rates for each region in detail, dividing the amounts of displacement rates
Comparison with coseismic slip distribution of the 2011 Tohoku-oki earthquake
First, we compare the coseismic slip distributions associated with the 2011 Tohoku-oki earthquake (e.g., Simons et al., 2011; Lay et al., 2011; Iinuma et al., 2012) with the preearthquake interplate coupling distribution obtained in this study (Fig. 5). Simons et al. (2011) estimated 60 m as the maximum amount of coseismic slip, which was obtained using land-based GNSS data and teleseismic waveform data. Lay et al. (2011) also obtained the same maximum slip amount from the inversion of seismic
Conclusions
In this study, by removing coseismic steps, steps caused by antenna exchanges, annual and semiannual variations, postseismic transient deformations associated with the eight large earthquakes, and common-mode errors from the original GNSS time series data during the period from 21 March 1996 to 7 March 2011, we obtained tectonic crustal movements in northeastern Japan due to the interplate coupling between the PAC and NA plates as precisely as possible. The tectonic crustal deformations were
CRediT authorship contribution statement
Daiki Abe: Conceptualization, Software, Validation, Formal analysis, Writing – original draft. Shoichi Yoshioka: Methodology, Validation, Writing – review & editing, Visualization, Supervision, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank T. Yabuki for sharing the source code for geodetic data inversion, and Y. Nishino and M. Tanaka for sharing the source code for GNSS time series analysis. We also thank the editor, G. Houseman, an anonymous reviewer, H. Hirose, F. H. Ortega Culaciati, and M. Matsu'ura for their constructive comments to improve the manuscript. We used GNSS data and hypocenter data from the Geospatial Information Authority of Japan and Japan Meteorological Agency, respectively. All the figures were
References (39)
- et al.
Interplate coupling in Northeast Japan deduced from inversion analysis of GPS data
Earth Planet. Sci. Lett.
(2000) - et al.
Anomalous deepening of a seismic belt in the upper-plane of the double seismic zone in the Pacific slab beneath the Hokkaido corner: possible evidence for thermal shielding caused by subducted forearc crust materials
Earth Planet. Sci. Lett.
(2010) - et al.
Two decades of spatiotemporal variations in subduction zone coupling offshore Japan
Earth Planet. Sci. Lett.
(2016) - et al.
Afterslip of the plate interface following the 1978 Miyagi–Oki, Japan, earthquake, as revealed from geodetic measurement data
Tectonophysics
(2001) Information theory and an extension of the maximum likelihood principle
Likelihood and the Bayes procedure
- et al.
ITRF2005: a new release of the International Terrestrial Reference Frame based on time series of station positions and Earth Orientation Parameters
J. Geophys. Res.
(2007) - et al.
Months-long thousand-kilometre-scale wobbling before great subduction earthquakes
Nature
(2020) - et al.
Geologically current plate motions
Geophys. J. Int.
(2010) - et al.
Interplate seismogenic zones along the Kuril–Japan trench inferred from GPS data inversion
Nat. Geosci.
(2009)