Strain anomalies induced by the 2011 Tohoku Earthquake (Mw 9.0) as observed by a dense GPS network in northeastern Japan
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2012
Received: 26 December 2011
Accepted: 24 May 2012
Published: 28 January 2013
We have evaluated an anomalous crustal strain in the Tohoku region, northeastern Japan associated with a step-like stress change induced by the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0). Because the source area of the event was extremely large, the gradient of the observed eastward coseismic displacements that accompanied uniform stress change had a relatively uniform EW extension in northeastern Japan. Accordingly, the deformation anomaly, which is determined by subtracting the predicted displacement in a half-space elastic media from the observed displacement, should reflect the inhomogeneity of the rheology, or stiffness, of the crust. The difference of the EW extension anomaly between the forearc and backarc regions possibly indicates a dissimilarity of stiffness, depending on the crustal structure of the Tohoku region. The Ou-backbone range—a strain concentration zone in the interseismic period—shows an extension deficit compared with predictions. A low viscosity in the lower crust probably induced a relatively small extension. Meanwhile, the northern part of the Niigata-Kobe tectonic zone, another strain concentration zone, indicates an excess of extensional field. This is probably caused by a low elastic moduli of the thick sedimentation layer. The detection of strain anomalies in the coseismic period enables a new interpretation of the deformation process at strain concentration zones.
On the other hand, there is also the possibility that the strain concentration zone during the interseismic period is an indication of a small stiffness of the crust. The low seismic velocity beneath the OBR (Nakajima et al., 2001) suggests low elastic moduli there. It is also known that the northern part of the Niigata-Kobe Tectonic Zone (NKTZ) is covered by a thick sediment having a low seismic velocity (Kato et al., 2009). A region with smaller stiffness has the potential to accumulate a larger elastic strain during the interseismic period. The large elastic strain should then be released when a megathrust event occurs along the subducting plate boundary off the Pacific coast of Tohoku. These images are shown in Figs. 2(b) and 2(c).
It is difficult to distinguish the two possibilities for the origin of the strain concentration zone (inelastic relaxation of the lower crust, or a compliant elastic crust) from in-terseismic deformation, because both causes show a similar response to the interseismic slow loading. However, their behaviors against such quick unloading as associated with the Tohoku earthquake are opposite each other. Accordingly, coseismic EW extension anomalies can serve as a good measure to test the hypothesis of a strain concentration zone along the OBR and the northern part of the NKTZ. The coseismic deformation field induced by the Tohoku earthquake has been evaluated using the dense regional GPS network operated by Tohoku University in combination with GEONET.
2. Data and Method
Continuous GPS sites complementing the GEONET configuration were established to comprehensively monitor (a) the crustal deformation caused by the subduction of the Pacific plate, (b) the activity of volcanoes, and (c) the loading of inland active faults in the Tohoku region.
Estimated fault parameters.
To extract the strain anomaly caused by inhomogeneity of the subsurface structure, a strain change residual (SC-residual, the difference between the observed and calculated EW strain) is calculated using the observed and calculated coseismic strain field. The resultant EW component of the SC-residual is shown in Fig. 5(a). The further the distance from the source area, the smaller the coseismic stress change that is induced by an earthquake. Therefore, the amplitudes of the SC-residual along the Japan Sea coast can be smaller than those along the Pacific coast, even if both areas have the same level of rheological anomalies. To eliminate the effect of the geometrical spreading, the ratios of the observed coseismic strain changes to the calculated ones (SC-ratio) at each grid (every 0.01° in the present study) are also shown in Fig. 5(b).
3. Results and Discussion
3.1 General pattern in Tohoku region
In Figs. 5(a) and 5(b), it seems that the ED area is distributed along the Pacific coast and EE area along the Japan Sea coast. This pattern may raise the possibility of a difference in the deformability of the forearc (eastern part) and backarc (western part) of the crust. In the interseismic period, the strain rate field indicates a tendency for a relatively small contraction in the forearc region and a large contraction in the backarc region (Miura et al., 2004; Fig. 1). These are common features in both the coseismic and interseimic periods (small deformation in the forearc region and large deformation in the backarc region). In addition, seismic tomography also indicates a high-velocity field in the forearc region and a low-velocity field in the backarc region at the depth of 10 km (Nakajima et al., 2001). This ED and EE distribution pattern possibly indicates a regional-scale difference in the stiffness of the crust (forearc > backarc) over the Tohoku region.
3.2 Strain anomaly along the OBR
In Fig. 6, it is clear that a smaller SC-ratio is distributed along the OBR strain concentration zone more than that of the surrounding region. The observed crustal deformation (EW extension) in a part of the strain concentration zone along the OBR was up to 15% smaller than fault model predictions, which assume a uniform slip in an isotropic homogeneous elastic media. Consistent with the hypothesis of Hasegawa et al. (2005), who suggested a relaxation in the lower crust beneath the strain concentration zone, a blue area exists in both figures along the OBR between 38°N and 39.3°N, indicating that this region has relaxed more. This idea is shown in Fig. 2(a). In the smaller scale in the case of DDC = 15 km, although this does not give a good spatial coverage, we also confirm a clear ED region along the OBR strain concentration zone. This can be attributed to the condition of the crustal structure beneath the OBR.
In this OBR strain concentration zone, there is a variation of the strain distribution (Figs. 5 and 6) along north to south. The SC-ratio becomes larger going north. The area to the north of 39.0°N is located between two reverse active faults (the Kitakami-teichi-seien fault and the Yokote-bouchi-toen fault) with less active volcanoes. Therefore, this may imply a higher viscosity due to low temperature in its lower crust than that of the south ED region. A large strain concentration during the interseismic period, and a larger extension associated with the Tohoku earthquake, indicates a small stiffness of the crust in this area. Because this area is not a sedimentation region, a small stiffness is not the case for the upper crust. A large EW extension is possibly the result of a low elastic modulus in the lower crust. This corresponds to Fig. 2(b). However, in this strain concentration zone, the number of GPS sites in the north area is less than that of south area (Fig. 5(c)). Therefore, there is another possibility that the effect outside the strain concentration zone could be contaminated in the northern part.
3.3 Other characteristic region
Several characteristic areas are shown in Fig. 5. The most interesting area is the northern part of the NKTZ. This is another well-known strain concentration zone in central Japan (Fig. 1, Sagiya et al., 2000). The EE region is a result of a response of the northeastern part of the NKTZ to a steplike stress perturbation. Although both the OBR and the northeastern part of the NKTZ behave as a strain concentration zone under long-term slow loading, they responded differently to step-like stress changes induced by the Tohoku earthquake. The northern part of the NKTZ (Niigata basin) is formed by Miocene sedimentation. Therefore, its elastic modulus is thought to be smaller than other regions. The seismic velocity structure shows that the surface layer of small P-wave velocity (<5 km/s) extends to 10 km in depth (Kato et al., 2009). A detailed discussion necessitates an examination of the deformation processes in the respective regions using numerical modeling, such as a finite element method that considers an inhomogeneous structure. The evidence suggests that the OBR and northeastern part of the NKTZ strain concentration zones may have been formed under a different crustal rheology.
The EE regions also appear at the eastern and western side of the ED region in the OBR. The former is considered to be a branch of the strain concentration zone along the OBR (Miura et al., 2004). These areas correspond to sedimentary basins (Sendai and Yamagata basins). The same mechanism as in the case of the NKTZ can be applied to the eastern branch of the strain concentration zone. A small stiffness in the upper crust is expected (Fig. 2(c)).
3.4 Dependency of coseismic fault model
Another fault model is a non-uniform coseismic slip model estimated by Iinuma et al. (2011) (Figs. 7(c) and 7(d)). The SC-residual and SC-ratio indicates the existence of an ED region in the forearc and an EE region in the backarc. Therefore, the regional-scale variation in stiffness is also reflected in the same way as the result of simple rectangular fault model cases. The SC-ratio in the OBR strain concentration zone (Fig. 7(d)) shows a low value compared with the surrounding region. The variation of this strain anomaly pattern is not affected by the different fault models.
We have examined the crustal response, in the Tohoku region, to a step-like stress change induced by the 2011 To-hoku earthquake. The observed crustal deformation (EW extension) in a part of the strain concentration zone along the OBR was up to 15% smaller than predictions based on a source model, which assumes a uniform slip on two rectangular faults, in an isotropic homogeneous elastic media. In the Tohoku region, the ED area is distributed in the forearc region, while the EE area is distributed in the backarc region. This distribution pattern possibly indicates a regional variation in stiffness according to the crustal structure. The response of a southern part of the strain concentration zone along the OBR to an instantaneous unloading caused by the earthquake was consistent with that expected of a region for a model in which stresses in the lower crust and/or the uppermost mantle have been inelastically relaxed under a long-term slow loading (Hasegawa et al., 2005). On the other hand, the EE region in the northern part of the OBR possibly reflects a low elastic stiffness in the lower crust rather than a flow in the lower crust. In addition, although both the northeastern part of the NKTZ and the OBR behave as the strain concentration zones under a long-term slow loading, their responses to instantaneous unloading look different. This finding suggests that the crust and/or uppermost mantle of the northeastern part of the NKTZ and the OBR have different rheological characteristics. In conclusion, strain concentration zones exhibit a variety of responses against step-like unloading. This evidence represents a new constraint to the rheology model of the crust and uppermost mantle beneath northeast Japan.
The GPS data were obtained from a research project conducted by the Japanese Nuclear Energy Safety Organization to establish evaluation techniques for seismogenic faults, the Geospatial Information Authority of Japan, and the Mizusawa VLBI Observatory of the National Astronomical Observatory of Japan. We thank Dr. J. Muto for the discussion of rheological structure in the Tohoku region. We are grateful to two anonymous reviewers and an editor, Prof. Dr. A. Hasegawa, for useful discussions and comments to improve this paper. Most of the figures were generated using the GMT software (Wessel and Smith, 1998).
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