Temporally variable stress field from eastern Aomori to Tsugaru Strait
Our study results indicate that the stress field in regions C and S differs from that expected from subduction of the Pacific plate. In this section, we discuss the temporal variation in the stress field in these regions, in detail.
The stress field in regions C and S for the period before the Tohoku earthquake shows a reverse faulting stress regime with the maximum compressional principal stress (σ1) axis being sub-horizontal and trending N–S. Recent studies have identified a region with sub-horizontal σ1 axis trending N–S in the forearc region in NE Japan, and ascribed this orientation to the effect of bending of the upper-plate due to interplate coupling and/or the effect of subducting plate interface curvature (Imanishi et al. 2012, 2013; Yoshida et al. 2015, 2019). Yoshida et al. (2015) suggested that in parts of the forearc region, E–W compressional stress is low compared with N–S compressional stress because of upper-plate bending. The plate bending deformation produces E–W extensional stress, and thus N–S compressional stress is relatively larger than the E–W compressional stress. N–S trending σ1 axis in regions C and S can be similarly explained by the effect of upper-plate bending, because the regions are located at the northern margin of the forearc region in Honshu.
However, we also found that the stress field in regions C and S for the period after the Tohoku earthquake constitutes a reverse faulting stress regime with σ1 axis being sub-horizontal and trending ENE–WSW, suggesting that σ1 axis was rotated by ~ 90° as a result of the Tohoku earthquake. Since there is no significant difference in hypocenter distribution for the periods before and after the Tohoku earthquake in regions C and S (Fig. 2), this stress rotation cannot be explained as an artifact caused by spatial variation in the stress field or by a change in hypocenter distribution. Rather, the rotation in σ1 axis is explained by the effect of the Tohoku earthquake, as follows. Yoshida et al. (2012, 2019) reported that the stress field in inland areas of eastern Honshu, Japan, changed as a result of the Tohoku earthquake. The stress field in regions C and S is consistent with that in inland areas that were affected by coseismic slip of the Tohoku earthquake (Yoshida et al. 2012, 2019). Such a stress change caused by the 2011 Tohoku earthquake could have triggered some earthquakes with unfavorable fault orientation with the typical WNW–ESE compressional stress field in NE Japan. In addition, the strain distribution obtained from GEONET [GNSS Earth Observation Network System, operated by the GSI (Geospatial Information Authority of Japan)] shows that ENE–WSW contraction and NNW–SSE extension were predominant in the period after the Tohoku earthquake in northernmost Honshu, including regions C and S (e.g., Figure 10 in Geospatial Information Authority of Japan 2018a). To release such strains, the ENE–WSW compressional reverse fault earthquakes may have been caused. All of the above suggests that the stress field in regions C and S for the period after the Tohoku earthquake was influenced by coseismic static stress change and postseismic crustal deformation related to the Tohoku earthquake.
Yoshida et al. (2012) reported that the Tohoku earthquake resulted in a coseismic change in differential stress of < 1 MPa in the present study area. The strain change due to postseismic deformation (for the period up to 2017) is estimated to have been the order of 10−5 based on GNSS observations (Geospatial Information Authority of Japan 2018b), which corresponds to the order of 1 MPa change in the horizontal differential stress (σHmax − σHmin) assuming a rigidity of 30–40 GPa. We just show an order-estimation of the strain change here because the strain change estimated from GNSS station data varies with a subjective weighting parameter (e.g., “distance-decaying constant” proposed by Shen et al. 1996); but we can safely conclude that the change in the horizontal differential stress (σHmax − σHmin) due to the coseismic and postseismic deformation caused by the Tohoku earthquake should have been < 10 MPa. Thus, the value of (σ1 − σ2) before the occurrence of the Tohoku earthquake should have been also < 10 MPa because horizontal σ1 and σ2 axes in the region S exchanged their directions after the earthquake as shown in Fig. 3. As the stress ratio is estimated to have been ~ 0.3 for the period before the Tohoku earthquake, the differential stress (σ1 − σ3) is thought to have been less than tens of MPa. The obtained differential stress is consistent with the estimate of Yoshida et al. (2015, 2019).
In regions C and S, earthquakes are generated by movement on normal and strike–slip faults as well as reverse faults. Miyauchi (1985) identified geological folding structures in these regions that strike E–W as well as N–S. Bouguer anomaly patterns reveal small-scale structures with variable strike superimposed on large-scale N–S-striking structures (Fig. 4; Yamamoto 2005). Based on the information shown above, it is considered that fault planes with various orientations exist in these regions. The occurrence of various fault types of earthquakes under such low differential stress in these regions may be due to the presence of such weak fault planes.
Effect on the stress field of southwestward movement of the Kuril Forearc sliver in southern Hokkaido
Our results show that the stress regime (reverse faulting with σ1 axis oriented sub-horizontal and trending WNW–ESE) in region N was essentially unchanged between the periods before and after the Tohoku earthquake (Fig. 3). The results suggest that the effect of the subduction of the Pacific Plate is more dominant than the effect of the southwestward movement of the Kuril Forearc sliver (e.g., Kimura 1981, 1996; Seno 1985; Moriya 1986; Arita et al. 2001) on the stress field in this region even after the Tohoku earthquake.
On 6 September 2018, the Mw 6.6 Hokkaido Eastern Iburi earthquake (herein, the Iburi earthquake) occurred in the northeastern part of region N. Ohtani and Imanishi (2019) estimated the stress field around the northeastern edge of region N using the focal mechanism solutions of the aftershocks that occurred in the region indicated by a rectangle in Fig. 4. They found that this area is characterized by a reverse faulting stress regime with σ1 axis oriented sub-horizontal and trending ENE–WSW, and suggested that this stress regime has resulted from southwestward migration of the Kuril Forearc sliver and its collision with NE Japan.
The focal mechanism solutions used in the present analysis for region N are located mostly in the western part of the region. Therefore, it is likely that the stress field changes between the western and eastern parts of region N. The Bouguer anomaly shown in Fig. 4 also shows the difference in the structure between the western and eastern parts there. However, the details of the spatial variation in the stress are unclear, owing to insufficient data. To overcome this limitation, it will be necessary to use data from ocean-bottom stations (e.g., NIED 2019a) in future work.