Inversion tectonics, an old normal fault that acts as a reverse fault in the current stress field, is frequently observed in northeastern Japan (Tohoku District). Furthermore, new reverse faults that formed in the current stress field are distributed in this area; however, the conditions that control these fault activities remain unclear. To investigate the condition of fault activity and its regional variation in the current stress field, the stress field in Tohoku District and the likelihood of fault activities are estimated in this study using slip tendency (ST) analysis. In the eastern margin of the Japan Sea (EMJS) area, the reverse fault type of the stress field is dominant. Therefore, the maximum horizontal direction changes clockwise from E-W to NW-SE, from the northern to the southern region. In addition, it changes counterclockwise from NW-SE to EW from the Japan Sea area to the inland area. In the Tohoku inland area, the estimated direction of the maximum horizontal axis changed before and after the 2011 Tohoku-Oki earthquake. Before the 2011 Tohoku-Oki earthquake, it was E-W to WNW-ESE. Therefore, only the stress field before the 2011 Tohoku-Oki earthquake was used to calculate the ST values for seven events in the EMJS and four events in the Tohoku inland area. The results of the ST analysis showed eastward-dipping fault planes with low dip angles (approximately 30°–45°) and large ST values (approximately > 0.7). A large ST value indicates that the fault is favorable for slip in the stress field. A fault plane with a large ST value is consistent with the actual fault plane in the EMJS area. However, in the Tohoku inland area and southern part of the 1993 Hokkaido Nansei-Oki earthquake, the fault planes with large ST values were inconsistent with the actual fault plane, indicating that fault planes are unfavorable for slipping under the current stress field. These regional differences are consistent with the volcano distribution; therefore, the fluid supply from volcanic activity may help the fault slip under difficult stress conditions.
Research Article
Stress field in the northeastern Japan and its relationship with faults of recent earthquakes
https://doi.org/10.21203/rs.3.rs-3138136/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 07 Mar, 2024
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Fault activity
stress field
slip tendency analysis
northeastern Japan
inversion tectonics
In northeastern Japan (Tohoku District) (Fig. 1), the Pacific Plate subducts to Japan’s main island (Honshu) and converges in an east-west direction. In the eastern margin of the Japan Sea (EMJS) area, due to the eastward movement of the Amur Plate, the formation of a primary subduction zone is considered to have begun (e.g., Taira, 2001).
In such tectonic systems, the stress fields of the reverse fault type with an east-west compressional axis are widely distributed in Tohoku, Japan (e.g., Terakawa and Matsu’ura, 2010). Additionally, the current compressional stress fields are estimated to have begun approximately three million years ago during the Quaternary (e.g., Sato, 1994; Taira, 2001).
Several large earthquakes have occurred in the EMJS area from Hokkaido to Niigata (e.g., Okamura, 2010). Earthquakes in the EMJS area occur due to the east-west compressive stress associated with relative plate motion. Under the Pacific Ocean and along the coast, the vicinity of the plate boundary is thought to be under a compressional stress field due to the subduction of the Pacific Plate.
In the EMJS area, strain has been eliminated from several faults and folds distributed in the north-south direction (e.g., Earthquake Research Committee, 2003). These faults and folds are called strain-concentrated zones (intense deformation zones in the EMJS) (e.g., Okamura and Kato, 2002), which is the primary subduction zone (e.g., Kobayashi, 1983). Faults and folds in the EMJS area are thought to have been formed by a tensional stress field associated with the formation of the Japan Sea (e.g., Jolivet et al., 1994). Consequently, many high-dip-angle fault planes are widely distributed in the EMJS and Tohoku Districts of Japan (Okamura and Kato, 2002; Okamura, 2010). Current compressive stress fields are causing fault activity in this area; however, the fault activity is strongly controlled by old geological structures formed during the formation of the Japan Sea. Therefore, inversion tectonics is believed to occur, in which old normal faults (high-dip-angle fault planes) dip westward and act as reverse faults (Okamura et al., 1995). However, recent large earthquakes (e.g., the 1964 Niigata earthquake (Mjma 7.5) and the 2019 Yamagata-Oki earthquake (Mjma 6.7)) are confirmed to have occurred on a low-dip-angle plane with eastward dipping (Hurukawa and Harada, 2013; Earthquake Research Committee, 2019).
Based on these facts, the following two possibilities exist for fault activity in Tohoku, Japan: 1) inversion tectonics, in which an old normal fault plane with a high dip angle acts as a reverse fault, and 2) a reverse fault with a low dip angle formed in the present stress field.
The actual activated fault plane of large earthquakes in Tohoku District, Japan, including the EMJS area, has been studied using aftershock distribution, tsunami source models, and reflection surveys (e.g., Satake, 1986; Okamura et al., 1998; Tanioka et al., 1995; Yoshida et al., 2014; Sibson, 2009). Sibson (2009) summarized the inland earthquakes since 1896, and some of them were possibly unfavorably oriented faults under the assumption of the Andersonian (i.e., one of the principal axes, where the sigma-3 axis is vertical) stress field, which was coaxial with the fault strike. However, recent studies (e.g., Terakawa and Matsu’ura, 2010) have shown the heterogeneity of the stress field in northeastern Japan. Therefore, fault activities must be evaluated by considering the stress field in each focal area. Some studies have evaluated fault activity using a physical method called the slip-tendency (ST) method (Morris et al., 1996). Yukutake et al. (2015) investigated the applicability of the frictional reactivation theory to faults in Japan and found that faults adequately oriented to the stress fields had high slip ratios (> 1.0 m /1000 years) and large ST values (\(Ts> 0.7\)). Miyakawa and Otsubo (2015) evaluated ST values using the active fault database of Japan (National Institute of Advanced Industrial Science and Technology) for fault models and found that active faults in the Tohoku region generally had large ST values (\(Ts> 0.7\)). Furthermore, inactive faults that did not slip during the Quaternary showed low ST values (\(Ts< 0.6\)) (e.g., Miyakawa and Otsubo, 2017). However, some active faults (e.g., the Haramachi segment in eastern Fukushima) showed low ST values (\(approximately Ts=0.20\)), which are estimated to be caused by differences in the dip angle.
Cllettini and Trippetta (2007) used ST methods to investigate the relationship between aftershock sequences and the frictional reactivation theory in Colfiorito, Italy, and the 1999 Chi-Chi earthquake (Taiwan). They found that more aftershocks occurred on the fault plane with possible slip (\(0.6<ST<0.9\)) than on the optimal fault plane (\(ST=1\)). They considered fewer optimal fault planes in this region; therefore, stress release was performed on other planes.
Moeck et al. (2009) evaluated the reactivation of fractures in geothermal and hydrocarbon reservoir areas using ST methods in the northeast German Basin. The calculated ST value for the fault system in the volcanic rock layer had a maximum of 0.39, and it was difficult to slip. However, experiments with cyclic fluid injection confirmed that microseismicity occurred in the volcanic rock layer, suggesting that even with a small ST (i.e., the fault is unfavorable for slipping in the stress field), fault can be active owing to the increased fluid pressure.
Therefore, in this study, the likelihood of fault activity in Tohoku District, Japan, was estimated using Slip tendency (ST) methods and the nodal plane of the focal mechanism for the fault model of the previous earthquake. This is the first comprehensive evaluation of ST using nodal planes of the focal mechanisms of actual earthquakes in the Tohoku region. Notably, in the EMJS area and Tohoku District, Japan, it needs to be clarified whether the tectonic inversion fault or the new reverse fault is more favorable for slipping under stress conditions. Additionally, clarifying the conditions of fault slip, except for stress in regional variations, is necessary.
Previous studies have estimated the stress field in the EMJS area and Tohoku Districts of Japan. However, the stress field can be estimated using a small amount of focal mechanism data in the EMJS area (Terakawa and Matsu’ura, 2010) or by setting a uniform stress field over a wide area (e.g., Miyakawa and Otsubo, 2015). Therefore, the stress state on the fault plane must be evaluated for each earthquake focal area, particularly in the EMJS area. The stress field for each earthquake focal area was re-estimated using data from a new and broad time window that has not been considered in previous studies. Finally, the relationship between the obtained ST values, re-estimated stress field, and surrounding environment was discussed.
First, the regional stress field and then the ST values as the fault slipperiness, were estimated.
Stress fields were calculated using the stress-tensor inversion method developed by Michael (1984, 1987). F-net Moment Tensor data (National Research Institute for Earth Science and Disaster Resilience) was used to calculate the stress fields. When the moment tensor data was insufficient, the focal mechanisms given by Okada et al. (2015, 2022) were used. They estimated the P-wave initial motions from temporary and permanent stations by adopting the method of Hardebeck and Shearer (2002).
Next, fault activity was evaluated using ST methods (Morris et al., 1996). The ST value is the ratio of the shear stress to the normal stress on the fault plane, which can be expressed as follows:
$$\begin{array}{c}Ts= \frac{\left|{\sigma }_{s}\right|}{\mu \left|{\sigma }_{n}\right|} \#\left(1\right)\end{array}$$
where \(Ts\) is the ST value, \({\sigma }_{s}\) is shear stress on the fault, \({\sigma }_{n}\) is normal stress on the fault, and \(\mu\) is coefficient of the friction on the fault. \(Ts\) was normalized between 0 and 1. When \(Ts\) was large, the shear stress on the frictional resistance of the fault was also high. Therefore, it was easier for a fault to slip into a stress field.
The ST plug-ins for Coulomb (Neves et al., 2009) were used for calculating ST. Here, the principal stress directions of the stress tensor, stress ratio \(\varphi (=({\sigma }_{2}-{\sigma }_{3})/({\sigma }_{1}-{\sigma }_{3}\left)\right)\), fault shape (strike and dip), and coefficient of friction on the fault were required to calculate \(Ts\). For principal stress direction and stress ratio \(\varphi\), the results of estimated stress fields were used. For the fault shape, the nodal plane of the focal mechanisms and models estimated in previous research were used. The coefficient of friction was assumed to be 0.6 (Byerlee, 1978).
This study was conducted on the EMJS and Tohoku District’s inland areas (Fig. 1).
3.1. Stress fields in the EMJS area
To calculate the stress fields, the events that occurred between January 1st, 1997, and June 17th, 2019 were considered. The target area in the EMJS was lat. \(37\) to \(42 ^\circ \text{N}\), and lon. \(137\) to \(141 ^\circ \text{E}\). Additionally, events located at depths ranging from 0 to 30 km were used. Regarding the distribution of focal mechanisms, the reverse fault type was dominant in the study area (Fig. 2). The strike-slip fault type increased in the north compared to the lat. \(39 ^\circ \text{N}\). P-axis directions of the focal mechanisms tend to be more NW-SE in the south than in the lat. \(40 ^\circ \text{N}\) and E-W in the north than at lat. \(40 ^\circ \text{N}\). This boundary was located at lat. \(39 ^\circ \text{N}\) in depths of 10–20 km and became ambiguous at depths of 20–30 km. The P-axis directions tended to be NW-SE in the Japan Sea and E-W in inland areas.
First, the spatial variation in the stress field was estimated using regular gridding. The target area was divided into small sub-areas (\(2^\circ \times 2^\circ\)) to estimate the stress field (Figs. 3 and 4). Consequently, in most of the subareas, the reverse fault type was dominant, except in the southeastern subarea (Fig. 3, Subarea 10).
The maximum horizontal direction (\({\text{S}}_{\text{H}\text{m}\text{a}\text{x}}\)) rotated clockwise from E-W to NW-SE from the northern to southern parts. Additionally, \({\text{S}}_{\text{H}\text{m}\text{a}\text{x}}\) rotated counterclockwise from NW–SE to E-W from the Japan Sea area to the inland area. The uncertainties of \({\text{S}}_{\text{H}\text{m}\text{a}\text{x}}\) in the inland areas were approximately 20.26–101.47°, which is larger than those in the Japan Sea area (approximately 9.74–28.46°).
3.2. Slip Tendency in EMJS area
The likelihood of a fault slip was evaluated using the ST method. The ST values for seven large-to-moderate earthquakes were calculated and compared with actual fault activity.
The target events were the 1833 Shonai-Oki earthquake Mjma 7.7, 1939 Oga earthquake Mjma 6.8, 1940 Shakotan-Oki earthquake Mjma 7.5, 1964 Niigata earthquake Mjma 7.5, 1983 Nihonkai-Chubu earthquake Mjma 7.7, 1993 Hokkaido Nansei-Oki earthquake Mjma 7.8, and 2019 Yamagata-Oki earthquake Mjma 6.7 (Fig. 1). The three events (1833 Shonai-Oki earthquake Mjma 7.7, 1983 Nihonkai-Chubu earthquake Mjma7.7, and 1993 Hokkaido-Nansei-Oki earthquake Mjma 7.8) were estimated using fault models in previous studies (Aida, 1984; Kosuga et al., 1986; Satake, 1985; Tanioka et al., 1995; Aida, 1989); ST values were also calculated using these fault models.
The stress field was estimated using only the focal mechanisms around each event to calculate the ST values using the events that occurred between January 1st, 1997, and December 31st, 2019 (Fig. 5). Each stress field showed a reverse fault type, and the \({\text{S}}_{\text{H}\text{m}\text{a}\text{x}}\) trends were consistent with the results of regular gridding in the previous section (section 3.1). Subsequently, the ST values were calculated for each nodal plane. The uncertainty in the stress fields was estimated by changing the input stress ratio within each stress ratio uncertainty (Fig. 6 and Table 1).
| Events | Fault model | Dip direction | Stress field | ST best | ST Error |
|---|---|---|---|---|---|
| Yamagata-Oki earthquake | F-net | East | 0.905 | 0.905 < ST < 0.906 | |
| West | 0.806 | 0.806 < ST < 0.807 | |||
| Niigata earthquake | Hirasawa (1995) | East | 0.944 | 0.944 < ST < 0.971 | |
| West | 0.691 | 0.660 < ST < 0.782 | |||
| Shakotan-Oki earthquake | Fukao and Furumoto (1995) | East | 0.953 | 0.944 < ST < 0.993 | |
| Satake (1986) | West | 0.759 | 0.673 < ST < 0.915 | ||
| Oga earthquake | Sato (1993) | East | 0.914 | 0.902 < ST < 0.930 | |
| West | 0.805 | 0.833 < ST < 0.805 | |||
| Nihonkai-Chubu earthquake | Satake (1985) | East | North | 0.915 | 0.914 < ST < 0.934 |
| West | 0.429 | 0.424 < ST < 0.443 | |||
| East | Middle | 0.850 | 0.836 < ST < 0.975 | ||
| West | 0.506 | 0.503 < ST < 0.514 | |||
| East | South | 0.934 | 0.908 < ST < 0.994 | ||
| West | 0.609 | 0.601 < ST < 0.622 | |||
| Hokkaido Nansei-Oki earthquake | Tanioka et al. (1995) MTRF | East | All | 0.601 | 0.597 < ST < 0.613 |
| West | 0.965 | 0.965 < ST < 0.975 | |||
| East | North | 0.619 | 0.612 < ST < 0.645 | ||
| West | 0.976 | 0.975 < ST < 0.991 | |||
| East | South | 0.611 | 0.603 < ST < 0.637 | ||
| West | 0.950 | 0.950 < ST < 0.964 | |||
| Tanioka et al. (1995) CMT | East | All | 0.947 | 0.943 < ST < 0.960 | |
| West | 0.431 | 0.412 < ST < 0.488 | |||
| East | North | 0.815 | 0.787 < ST < 0.996 | ||
| West | 0.371 | 0.370 < ST < 0.374 | |||
| East | South | 0.955 | 0.953 < ST < 0.964 | ||
| West | 0.440 | 0.409 < ST < 0.531 | |||
| ST Error is calculated by using the uncertainty of each stress ratio. The Bold numbers show the ST’s best value larger than 0.7. | |||||
Regarding the ST values of the nodal planes, all the eastward-dipping planes showed larger ST values than the westward-dipping planes, except for the 1993 Hokkaido-Nansei-Oki earthquake MTRF solution. For the eastward dipping planes of the earthquakes, except the 1993 Hokkaido-Nansei-Oki earthquake, ST values were larger than 0.7. These planes were favorable for slipping into the stress field. For the 2019 Yamagata-Oki earthquake and 1940 Shakotan-Oki earthquake, both planes showed ST values greater than 0.7.
Two types of fault models were proposed by Aida (1989) for the 1833 Shonai-Oki earthquake. Fault Model A was composed of two eastward-dipping planes: A-north and A-south. Fault Model B was composed of a westward-dipping plane. Based on Model A, the stress fields in three areas were estimated as: All (entire focal area), Middle 1 (focal mechanisms around the fault model without events occurring inland), and Middle 2 (focal mechanisms around the model with events occurring inland). Using the stress field All, each model exhibited an ST value of approximately 0.6 (Table 2). Using the stress fields Middle 1 or Middle 2, the eastward-dipping plane showed a large ST value, while using Middle 1, fault plane A-South showed an ST value larger than 0.7. These results indicate that model A is more favorable for slipping than model B.
| Events | Fault model | Dip direction | Stress field | ST best | ST Error | |
|---|---|---|---|---|---|---|
| Shonai-Oki earthquake | Aida (1989) | A-North | East | All | 0.604 | 0.598 < ST < 0.671 |
| A-South | East | 0.578 | 0.559 < ST < 0.623 | |||
| B | West | 0.603 | 0.603 < ST < 0.603 | |||
| A-North | East | Middle 1 | 0.676 | 0.571 < ST < 0.834 | ||
| A-South | East | 0.742 | 0.740 < ST < 0.744 | |||
| B | West | 0.461 | 0.461 < ST < 0.461 | |||
| A-North | East | Middle 2 | 0.619 | 0.588 < ST < 0.702 | ||
| A-South | East | 0.676 | 0.669 < ST < 0.693 | |||
| B | West | 0.536 | 0.534 < ST < 0.542 | |||
| Nihonkai-Chubu earthquake | Aida (1984) | North | East | North | 0.798 | 0.798 < ST < 0.798 |
| South | East | 0.949 | 0.948 < ST < 0.997 | |||
| North | East | Middle | 0.843 | 0.843 < ST < 0.905 | ||
| South | East | 0.783 | 0.724 < ST < 0.986 | |||
| North | East | South | 0.956 | 0.948 < ST < 0.987 | ||
| South | East | 0.827 | 0.773 < ST < 0.925 | |||
| Kosuga et al. (1986) | North | East | North | 0.770 | 0.769 < ST < 0.780 | |
| South | East | 0.766 | 0.752 < ST < 0.837 | |||
| North | East | Middle | 0.847 | 0.846 < ST < 0.885 | ||
| South | East | 0.758 | 0.757 < ST < 0.879 | |||
| North | East | South | 0.960 | 0.957 < ST < 0.977 | ||
| South | East | 0.900 | 0.873 < ST < 0.983 | |||
| Satake (1985) | North | East | North | 0.881 | 0.878 < ST < 0.902 | |
| South | East | 0.883 | 0.879 < ST < 0.933 | |||
| North | East | Middle | 0.925 | 0.924 < ST < 0.957 | ||
| South | East | 0.800 | 0.782 < ST < 0.961 | |||
| North | East | South | 0.991 | 0.989 < ST < 0.998 | ||
| South | East | 0.905 | 0.866 < ST < 0.994 | |||
Fault models of the 1983 Nihonkai-Chubu earthquake proposed in previous studies (Aida 1984; Kosuga et al., 1989; Satake 1985) were dipping in an eastward direction (Fig. 6, Fault models). Therefore, each model showed an ST value larger than 0.7 (Table 2) and the fault plane slipped because it was favorable for slipping into the stress field.
Tanioka et al. (1995) proposed a fault model for the 1993 Hokkaido Nansei-Oki earthquake. This fault model was composed of three eastward-dipping planes and two westward-dipping planes (Fig. 6). The eastward dipping planes showed ST values larger than 0.7 (Table 3), and the westward dipping planes showed ST values smaller than 0.6. These results indicate that the eastward-dipping planes slipped because they were favorable for slipping into the stress field. However, the westward-dipping planes might have slipped due to the influence of other factors.
| Events | Fault model | Dip direction | Stress field | ST best | ST Error | |
|---|---|---|---|---|---|---|
| Hokkaido Nansei-Oki earthquake | Tanioka et al. (1995) | A | East | All | 0.947 | 0.943 < ST < 0.960 |
| B, C | East | 0.943 | 0.943 < ST < 0.961 | |||
| D, E | West | 0.513 | 0.497 < ST < 0.562 | |||
| A | East | North | 0.815 | 0.787 < ST < 0.996 | ||
| B, C | East | 0.935 | 0.933 < ST < 0.953 | |||
| D, E | West | 0.586 | 0.525 < ST < 0.821 | |||
| A | East | South | 0.955 | 0.953 < ST < 0.964 | ||
| B, C | East | 0.929 | 0.929 < ST < 0.957 | |||
| D, E | West | 0.507 | 0.497 < ST < 0.537 | |||
3.3. Stress fields in the Tohoku District’s inland area
To calculate the stress fields, the events that occurred between January 1st, 1997, and December 31st, 2018 were considered. The target area of this study in Tohoku District’s inland area was at lat. \(36.5 ^\circ \text{N}\) to \(41.5 ^\circ \text{N}\) and lon. \(139 ^\circ \text{E}\)\(142 ^\circ \text{E}\). The events that were only located at depths ranging from 0 to 30 km were used. To consider the influence of the 2011 Tohoku-Oki earthquake, the data were divided into two periods: ① January 1st, 1997–March 10th, 2011, and ② March 11th, 2011–December 31st, 2018.
Regarding the distribution of the focal mechanisms, only a few events occurred at depths of 20–30 km in all terms. In term ①, reverse fault type was dominant in the south of lat. \(40 ^\circ \text{N}\) (Fig. 7); the direction of the P-axis was from E-W to NW-SE. During term ②, some events occurred to the north of lat. \(40 ^\circ \text{N}\); the direction of the P axis was NE-SW, and the reverse fault type was dominant in this area. Additionally, the number of strike-slip types and normal fault types increased in term ② compared to term ①.
First, the spatial variation in the stress field was estimated using regular gridding, where the target area was divided into small regions (\(1^\circ \times 1^\circ\)). Consequently, in term ①, the stress field of the reverse fault type was dominant in this area. \({\text{S}}_{\text{H}\text{m}\text{a}\text{x}}\) direction was approximately E-W to WNW-ESE (Figs. 8 and 9). In term ②, \({\text{S}}_{\text{H}\text{m}\text{a}\text{x}}\)directions were ENE-WSW to NE-SW in the northern part and NW-SE in the southern part.
The stress ratio in term ① was 0.6–0.7; however, after the 2011 Tohoku-Oki earthquake (Term ②), it changed from 0.2 to 0.5. In the eastern part (east coast of the Miyagi and Fukushima prefectures), the uncertainty of the stress ratio became large in term ②, which may be attributed to various focal mechanisms.
3.4. Slip tendency in Tohoku District’s inland area
The likelihood of a fault slip was evaluated using ST methods. The ST values for four large-to-moderate earthquakes were calculated and compared with actual fault activity.
Our target events were the 1998 Shizukuishi earthquake Mjma 6.1, 2003 northern Miyagi earthquake Mjma 6.4, 2008 Iwate-Miyagi Nairiku earthquake Mjma 7.2, and the 2016 Oshika Peninsula earthquake Mjma 5.2 (Fig. 1).
The stress field was estimated using only the focal mechanisms that occurred between January 1st, 1997, and December 31st, 2019, around each event to calculate the ST values. Each stress field showed a reverse fault type, and the \({\text{S}}_{\text{H}\text{m}\text{a}\text{x}}\) trends were consistent with the results of the regular gridding in the section 3.3 (Fig. 5).
The data were divided into three terms for the 1998 Shizukuishi earthquake and the 2003 northern Miyagi earthquake: ①January 1st, 1997–March 10th, 2011; ② March 11th, 2011–December 31st, 2019; and ③ January 1st, 1997–December 31st, 2019. The 2008 Iwate-Miyagi Nairiku earthquake had a sufficient number of focal mechanisms to enable estimation of the stress field; therefore, they were divided into three terms: ① January 1st, 1997–June 13th, 2008; ② June 14th, 2008–March 10th, 2011; and ③ March 11th, 2011–December 31st, 2019. The 2016 Oshika Peninsula earthquake had only a few focal mechanisms; therefore, the events that occurred after the 2011 Tohoku-Oki earthquake were used. Additionally, only a few focal mechanisms were available for the F-net catalog; therefore, the focal mechanism data estimated from P-wave initial motions from Okada et al. (2015; 2022) were added to it.
Each event showed a reverse or strike-slip fault type. The \({\text{S}}_{\text{H}\text{m}\text{a}\text{x}}\) trends were consistent with the results presented in Section 3.
From the results of the 1998 Shizukuishi earthquake, the estimated stress field in area A showed a reverse fault type solution in term ① and \({\text{S}}_{\text{H}\text{m}\text{a}\text{x}}\) direction was E-W. The uncertainties of \({\sigma }_{2}\) and \({\sigma }_{3}\) overlapped. The terms ② and ③ showed a strike-slip fault type solution, with direction of \({\text{S}}_{\text{H}\text{m}\text{a}\text{x}}\) directions being ENE-WSW, and the uncertainties of \({\sigma }_{2}\) and \({\sigma }_{3}\) overlapped.
From the results of the ST evaluation, both nodal planes showed ST values larger than 0.7 (Fig. 10 and Table 4). The differences in the ST values were small (< 0.1) for most terms. Both nodal planes were favorable for slipping into the stress field. However, some other factors likely influenced the selection of the slip plane.
| Events | Fault model | Dip direction | Stress field | ST best | ST Error |
|---|---|---|---|---|---|
| Shizukuishi earthquake | F-net | East | Area A | 0.884 | 0.792 < ST < 0.895 |
| West | 1997–2011 | 0.885 | 0.883 < ST < 0.885 | ||
| East | Area A | 0.697 | 0.576 < ST < 0.752 | ||
| West | 2011–2019 | 0.839 | 0.589 < ST < 0.980 | ||
| East | Area A | 0.744 | 0.585 < ST < 0.791 | ||
| West | 1997–2019 | 0.891 | 0.572 < ST < 0.998 | ||
| East | Area B | 0.733 | 0.246 < ST < 0.968 | ||
| West | 1997–2011 | 0.650 | 0.375 < ST < 0.770 | ||
| East | Area B 2011–2019 | 0.746 | 0.692 < ST < 0.772 | ||
| West | 0.774 | 0.463 < ST < 0.956 | |||
| East | Area B 1997–2019 | 0.724 | 0.567 < ST < 0.825 | ||
| West | 0.786 | 0.491 < ST < 0.999 | |||
| Iwate-Miyagi Nairiku earthquake | F-net | East | Fnet 1997–2008 | 0.955 | 0.949 < ST < 0.963 |
| West | 0.704 | 0.685 < ST < 0.724 | |||
| East | Fnet 2008–2011 | 0.967 | 0.967 < ST < 0.968 | ||
| West | 0.679 | 0.678 < ST < 0.686 | |||
| East | Fnet 2011–2019 | 0.813 | 0.732 < ST < 0.910 | ||
| West | 0.578 | 0.356 < ST < 0.662 | |||
| East | Fnet + P 1997–2008 | 0.876 | 0.784 < ST < 0.982 | ||
| West | 0.660 | 0.636 < ST < 0.676 | |||
| East | Fnet + P 2008–2011 | 0.949 | 0.942 < ST < 0.960 | ||
| West | 0.707 | 0.704 < ST < 0.710 | |||
| East | Fnet + P 2011–2019 | 0.865 | 0.823 < ST < 0.952 | ||
| West | 0.540 | 0.527 < ST < 0.622 |
From the results of the 2003 northern Miyagi earthquake, the eastward-dipping plane showed a large ST value (> 0.7) in most terms (Fig. 10 and Table 6). The 2003 northern Miyagi earthquake followed the largest foreshock (Mjma 5.6) and the largest aftershock (Mjma 5.5) (Fig. 10). In the study area, the largest aftershock and foreshock occurred in the northern and central parts, respectively, while the main shock occurred in the southern part. Based on these locations, the study area was divided into three regions (North, Middle, and South) for the estimation of the stress field (Fig. 5). Each area represents a reverse fault type of the stress field, and accordingly, the ST values for each event were calculated. The mainshock and largest foreshock had larger ST values in the eastward-dipping plane. However, the largest aftershock in the westward-dipping plane, showed a large ST value (> 0.7), which was larger than that of the eastward-dipping plane (Fig. 10 and Table 5).
| Events | Fault model | Dip direction | Stress field | ST best | ST Error |
|---|---|---|---|---|---|
| northern Miyagi earthquake Mainshock | F-net | East | Area All | 0.960 | 0.959 < ST < 0.962 |
| West | 1997–2011 | 0.717 | 0.717 < ST < 0.725 | ||
| East | Area All | 0.790 | 0.732 < ST < 0.888 | ||
| West | 2011–2019 | 0.380 | 0.352 < ST < 0.884 | ||
| East | Area All | 0.975 | 0.971 < ST < 0.980 | ||
| West | 1997–2019 | 0.672 | 0.672 < ST < 0.687 | ||
| East | South | 0.981 | 0.980 < ST < 0.986 | ||
| West | 1997–2011 | 0.639 | 0.638 < ST < 0.645 | ||
| Largest foreshock | F-net | East | Middle 1997–2011 | 0.950 | 0.946 < ST < 0.964 |
| West | 0.673 | 0.673 < ST < 0.771 | |||
| Largest aftershock | F-net | East | North 1997–2011 | 0.508 | 0.506 < ST < 0.869 |
| West | 0.981 | 0.981 < ST < 0.982 | |||
| Oshika peninsula earthquake | F-net | Fnet and | 0.887 | 0.881 < ST < 0.895 | |
| P wave polarity | 0.824 | 0.824 < ST < 0.824 | |||
| P wave polarity | 0.877 | 0.863 < ST < 0.893 | |||
| 0.812 | 0.805 < ST < 0.819 |
| Area | Michael’s methods | Slip Instability methods | ||||||
|---|---|---|---|---|---|---|---|---|
| σ༑ azimuth plunge | σ2 azimuth plunge | σ3 azimuth plunge | φ min best max | σ༑ azimuth plunge | σ2 azimuth plunge | σ3 azimuth plunge | φ min best max | |
| All -༑ | 84 | 174 | -8 | 0.48 0.79 0.98 | 276 | 185 | 50 | 0.57 0.64 0.72 |
| 0 | 4 | 86 | 6 | 6 | 82 | |||
| All -༒ | 38 | 129 | -60 | 0.06 0.64 0.99 | 208 | 115 | 307 | 0.24 0.45 0.74 |
| 3 | 21 | 69 | 5 | 26 | 64 | |||
| All -༓ | 7 | 165 | -22 | 0.55 0.85 0.99 | 272 | 182 | 17 | 0.63 0.72 0.78 |
| 1 | 7 | 83 | 1 | 5 | 84 | |||
| North | 68 | 158 | -23 | 0.09 0.77 0.98 | 225 | 133 | 7 | 0.40 0.47 0.77 |
| 0 | 17 | 73 | 13 | 9 | 74 | |||
| Middle | -98 | 172 | 25 | 0.3 0.78 0.99 | 278 | 187 | 55 | 0.39 0.55 0.77 |
| 4 | 6 | 83 | 8 | 8 | 79 | |||
| South | 99 | 8 | -137 | 0.29 0.67 1.00 | 102 | 12 | 230 | 0.52 0.67 0.81 |
| 5 | 7 | 81 | 3 | 4 | 85 | |||
For the 2008 Iwate-Miyagi Nairiku earthquake, each term represented a reverse fault type of the stress field. \({\text{S}}_{\text{H}\text{m}\text{a}\text{x}}\) directions were WNW-ESE in terms ① and ②, and NE-SW in term ③. Uncertainty of \({\sigma }_{2}\) and \({\sigma }_{3}\) directions overlapped in term ①. In term ②, stress field had a strike slip component (Fig. 5). From the ST value results, the nodal plane with an eastward-dipping plane showed a large ST value (> 0.7) in all terms (Fig. 10 and Table 4). The eastward-dipping plane was favorable for slipping into the stress field.
The stress field was transpressive during the 2016 Oshika Peninsula earthquake (Fig. 5), and the direction of \({\text{S}}_{\text{H}\text{m}\text{a}\text{x}}\) was N-S. The results of the ST analysis showed that both nodal planes had large ST values (> 0.7) (Fig. 10 and Table 5); however, the differences in ST values were small (< 0.1). Both nodal planes were favorable for slipping into the stress field, and other factors might have influenced the selection of the slip plane.
4.1. Reliability of stress fields
The stress fields in the 2003 northern Miyagi earthquake Mjma 6.4 and the 2016 Oshika Peninsula earthquake Mjma 5.2were estimated. However, the estimated stress fields had wide uncertainties and were not stable. Therefore, for these two earthquakes, the stress fields were re-estimated using the slip instability method (Vavryčuk et al., 2013; Vavryčuk, 2014) and the results were compared.
Michael’s method (Michael, 1987) assumes the following three conditions: 1) the same stress tensor acted on the target area; 2) earthquakes occurred on existing faults in various directions; and 3) the slip vector direction is equal to the shear stress direction on the fault.
The focal mechanism has two nodal planes: the fault plane and the auxiliary plane. To apply Assumption 3 of the Wallace-Bott hypothesis, the nodal plane with the fault must be determined, which is only possible by distinguishing the fault plane using other information (e.g., coseismic surface faults and aftershock distributions). In Michael’s method, the fault plane is randomly selected from the two nodal planes of each focal mechanism. The incorrect selection of auxiliary surfaces can lead to biased results and unstable solutions with large uncertainties. Vavryčuk et al. (2013) defined fault instability \(I\) based on Lund and Slunga (1999) as follows:
$$\begin{array}{c}I=\frac{\tau -\mu \left(\sigma -{\sigma }_{1}\right)}{{\tau }_{c}-\mu \left({\sigma }_{c}-{\sigma }_{1}\right)} \#\left(2\right)\end{array}$$
The fault plane can be determined from the two nodal planes using \(I\), and the stress field can be estimated with a narrower confidence range than that in Michael’s method.
To assess the effect of the fault plane selection on the stress tensor inversion, Vavryčuk’s method was applied to re-estimate the stress field of the 2003 northern Miyagi earthquake Mjma 6.4 and 2016 Oshika Peninsula earthquake Mjma 5.2.
a. 2003 northern Miyagi earthquake M jma 6.4For all periods and ranges, the results did not differ significantly from those obtained using Michael’s method (Figs. 5 and 11, and Table 6). The results re-estimated by Vavryčuk’s method had smaller uncertainties in \({\sigma }_{1}\), \({\sigma }_{2}\), and \({\sigma }_{3}\) directions (Fig. 11) than those in the results estimated by Michael’s method. The optimal values of the stress ratio (\({\varphi }_{\text{B}\text{e}\text{s}\text{t}}\)) and optimal \({\text{S}}_{\text{H}\text{m}\text{a}\text{x}}\) direction (\({\text{S}}_{\text{H}\text{m}\text{a}\text{x} \text{B}\text{e}\text{s}\text{t}}\)) were slightly changed, but were all within the error range of the results estimated by Michael’s method.b. 2016 Oshika Peninsula earthquake M jma 5.2Using only the P-wave polarity data, the stress field showed a transpressive type, which was closer to the reverse fault type (Fig. 11 and Table 7). Including the F-net data, the stress field was transpressive, which was similar to the result estimated using Michael’s method. The uncertainty of each principal-stress direction was smaller than that estimated using Michael’s method; \({\varphi }_{\text{B}\text{e}\text{s}\text{t}}\) and \({\text{S}}_{\text{H}\text{m}\text{a}\text{x} \text{B}\text{e}\text{s}\text{t}}\) did not change significantly. The re-estimated stress fields were almost the same as those estimated using Michael’s method.| |
SATSI |
Slip Instability |
||||||
|---|---|---|---|---|---|---|---|---|
| |
σ༑ azimuth plunge |
σ2 azimuth plunge |
σ3 azimuth plunge |
φ min best max |
σ༑ azimuth plunge |
σ2 azimuth plunge |
σ3 azimuth plunge |
φ min best max |
| P |
-5 |
87 |
-101 |
0.01 0.24 0.66 |
7 |
277 |
136 |
0.16 0.29 0.39 |
| 3 |
31 |
59 |
3 |
4 |
85 |
|||
| F + P |
-2 |
92 |
-100 |
0.03 0.29 0.65 |
4 |
99 |
268 |
0.16 0.32 0.39 |
| 6 |
38 |
52 |
6 |
42 |
48 |
|||
| The parameter on left and right sides of the table are estimated by Michael’s method and slip instability method, respectively. The range of each area is given in Fig. 5. |
||||||||
The ST values were calculated using the re-estimated stress fields (Fig. 12 and Table 8). Focusing on the ST values, the results of the 2016 Oshika earthquake showed almost the same results as the previous one (section 3.4), except for using only P-wave polarity data (Oshika-P wave). The uncertainty of the ST value became smaller than that obtained from the stress tensor inversion results using Michael’s method (Michael’s stress field). This may be because the uncertainty of the stress ratio was smaller than that of the previous one. The Oshika-P wave showed results different from Michael’s stress field. Therefore, the stress fields estimated by the slip instability methods showed different types of stress fields estimated using Michael’s method. The difference in the stress field type may have affected ST values.
| Events |
Fault model |
Dip direction |
Stress field |
ST best |
ST Error |
|---|---|---|---|---|---|
| northern Miyagi earthquake Mainshock |
F-net |
East |
Area All |
0.876 |
0.876 < ST < 0.876 |
| West |
1997–2011 |
0.834 |
0.834 < ST < 0.835 |
||
| East |
Area All |
0.759 |
0.745 < ST < 0.806 |
||
| West |
2011–2019 |
0.316 |
0. 272 < ST < 0.427 |
||
| East |
Area All |
0.939 |
0.939 < ST < 0.939 |
||
| West |
1997–2019 |
0.756 |
0.755 < ST < 0.756 |
||
| East |
South |
0.977 |
0.977 < ST < 0.977 |
||
| West |
1997–2011 |
0.677 |
0.677 < ST < 0.679 |
||
| Largest foreshock |
F-net |
East |
Middle 1997–2011 |
0.894 |
0.894 < ST < 0.895 |
| West |
0.777 |
0.767 < ST < 0.799 |
|||
| Largest aftershock |
F-net |
East |
North 1997–2011 |
0.489 |
0.457 < ST < 0.500 |
| West |
0.984 |
0.983 < ST < 0.987 |
|||
| Oshika peninsula earthquake |
F-net |
|
Fnet and |
0.946 |
0.945 < ST < 0.950 |
| |
P wave polarity |
0.753 |
0.752 < ST < 0.758 |
||
| |
P wave polarity |
0.709 |
0.653 < ST < 0.814 |
||
| |
|
0.639 |
0.624 < ST < 0.664 |
d. Summary of comparisons
The re-estimated stress fields had a smaller uncertainty and more stability than those estimated using Michael’s method. However, the re-estimated stress fields were almost identical to those estimated using Michael’s method. The change in the results for the 2016 Oshika earthquake may be attributed to the small number of mechanism solutions. For the 2003 northern Miyagi earthquake, the stress fields estimated using Michael’s method were sufficiently reliable.4.2. Synthetic slip
The rake angle (calculated rake) on the fault plane of the estimated stress field was calculated and then compared with the rake angle of the fault model (model rake). To calculate the rake, a synthetic slip program developed by Neves et al. (2009) was used. To compare the calculated rake with the model rake, the following rake difference was defined:
$$\begin{array}{c}model rake-calculated rake=rake difference \#\left(3\right)\end{array}$$
If the actual and estimated stress fields are similar, the model differences are expected to be close to zero. The relationship between the model differences and ST values was also confirmed.
Overall, the ST values were concentrated around 0.4–1.0 and rake differences around \(0\)–\(45^\circ\) (Fig. 13). The 1998 Shizukuishi earthquake exhibited a rake difference of approximately \(80^\circ\). Earthquakes in the Tohoku inland area had larger rake differences than those in the EMJS area. Moreover, even if the rake difference was the same on the east- and west-dipping planes, the east-dipping planes tended to show a higher ST value.
Next, the Tohoku inland area was focused, considering three earthquakes (the 1998 Shizukuishi earthquake, 2003 northern Miyagi earthquake, and 2008 Iwate-Miyagi Nairiku earthquake). the whole term was divided into two terms: ① before the 2011 Tohoku-Oki earthquake and ② after the 2011 Tohoku-Oki earthquake. After the Tohoku-Oki earthquake, the rake differences tended to increase (Fig. 14), while before the Tohoku-Oki earthquake, the rake differences were distributed around \(20^\circ\), and \(60\)–\(70^\circ\). Therefore, if the uncertainty of the stress field is large, the rake difference increases.From the results of the stress field estimation in this study, the tendency of the stress fields was found to changed before and after the 2011 Tohoku-Oki earthquake. This was attributed to the apparent change in the stress field caused by the stress change on the hanging wall side of the plate boundary (Okada et al., 2011; Okada et al., 2015; Yoshida et al., 2018). These changes in the stress field also affect the rake difference. In the 2008 Iwate-Miyagi Nairiku earthquake, the tendencies of the stress field, ST value, and rake difference did not change after 2008.
4.3. Stress change: Influence of the 2011 Tohoku-Oki earthquake
In this study, the change in the stress field trend before and after the 2011 Tohoku-Oki earthquake was confirmed using the stress ratio change. The stress change was calculated using the Coulomb program (Toda et al., 2005; Lin and Stein, 2004). The friction coefficient in the fault gouge was estimated to be 0.1–0.2 (Lockner et al., 2011); therefore, the coefficient of friction (µ) used was 0, 0.1, and 0.4. Hayes (2011) used a fault model of the Tohoku-Oki earthquake, where the effective vertical stress was calculated from the lithostatic and hydrostatic pressures as follows:
$$\begin{array}{c}effective vertical stress= lithostatic pressure- hydrostatic pressure\\ =\left(2.84\times {10}^{3}\times 9.8\right)-\left(1.00\times {10}^{3}\times 9.8\right)\sim180bar/km\#\left(4\right)\end{array}$$
The results of the stress ratio change were output in the focal mechanism solutions. These focal mechanisms exhibited stress fields. Michael’s method was used to estimate the stress field (output stress field) by using the output mechanism solution in the same area as that of the input stress field. Subsequently, the results of the output stress field were compared with those of the input stress field.
Using this procedure, the conditions under which the stress field changes were confirmed before and after the Tohoku-Oki earthquake, which were thought to be the effects of the stress disturbances caused by large-scale earthquakes. All results showed similar types of stress fields; therefore, the stress ratio and the \({S}_{Hmax}\) direction were focused when comparing the stress fields.
The cases where the stress ratio and \({S}_{Hmax}\) become more closer to stress fields after the Tohoku-Oki earthquake were confirmed under the following conditions:
In the case of the 1998 Shizukuishi earthquake, when \({S}_{2}\)= 45 bar/km, \({S}_{1}\)= 67.5 bar/km, \({S}_{3}\) = 43.8 bar/km, and \(\mu\) = 0, \({S}_{Hmax}\) direction became close to after the Tohoku-Oki earthquake (see Additional File 1). Also, when \({S}_{2}\)= 45 bar/km and \(\mu\) = 0.4, \({S}_{Hmax}\) direction was smaller than before the Tohoku-Oki earthquake, except for \({S}_{1}\)= 90.0 bar/km and \({S}_{3}\) = 42.6 bar/km.
In the case of the 2003 northern Miyagi earthquake, \({S}_{Hmax}\) direction became closer to that after the Tohoku-Oki earthquake in the next three conditions: ① \({S}_{2}\)= 45 bar/km, \({S}_{1}\) = 49.5 to 67.5 bar/km, \({S}_{3}\) = 1.3 to 36.3 bar/km, and \(\mu\) = 0.1, ② \({S}_{2 }\)= 45 bar/km, \({S}_{1}\) = 47.3 to 67.5 bar/km, \({S}_{3}\)= 1.3 to 40.6 bar/km, and \(\mu\) = 0, and ③ \({S}_{2}\)= 45 bar/km, \({S}_{1}\) = 47.7 to 67.5 bar/km, \({S}_{3}\)= 1.3 to 39.8 bar/km, and \(\mu\)= 0.4 (see Additional File 2).
In the case of the 2008 Iwate-Miyagi Nairiku earthquake, when \({S}_{2}\)= 180 bar/km and \(\mu\) = 0, \({S}_{Hmax}\) direction and stress ratio became close to after the Tohoku-Oki earthquake in \({S}_{1}\)= 181.8 to 270.0 bar/km and \({S}_{3}\)= 131.5 to 179.0 bar/km (see Additional File 3). When \({S}_{2}\) = 180 bar/km and \(\mu\) = 0.1, \({S}_{Hmax}\) direction showed a smaller value than that before the Tohoku-Oki earthquake in \({S}_{1}\) = 181.8 to 216.0 bar/km and \({S}_{3}\)= 131.5 to 160.6 bar/km. Also, the stress ratio became close in \({S}_{1}\)= 234.0 to 270.0 bar/km and \({S}_{3}\) = 150.9 to 131.5 bar/km. When \({S}_{2}\)= 180 bar/km and \(\mu\) = 0.4, the stress ratio became close to after the Tohoku-Oki earthquake in \({S}_{1}\)= 198.0 to 270.0 bar/km and \({S}_{3}\) = 131.5 to 170.3 bar/km.
Under these conditions, a stress field similar to that observed after the Tohoku-Oki earthquake was confirmed. Therefore, it is possible that the stress field changed after the Tohoku-Oki earthquake due to the stress disturbance caused by large-scale earthquakes under these conditions. Notably, this could be a possible interpretation; however, the temporal change was enhanced and apparently caused within the heterogeneous stress field and high pore fluid pressure (e.g., Smith and Dieterich, 2010; Terakawa et al., 2013; Terakawa and Matsu’ura., 2022). In any case, the effects of large earthquakes that cause changes in the stress field over a wide area can be excluded.
4.4. Comparing the result with previous studies
In this section, the results of this study are compared with those of some previous studies.
The results of the stress fields in the Tohoku inland area were consistent with those of previous studies (e.g., Terakawa and Matsu’ura, 2010; Miyakawa and Otsubo, 2015; Yukutake et al., 2015; Yoshida et al., 2015; Miyakawa and Otsubo, 2017; Terakawa and Matsu’ura 2022; Uchide et al., 2022). Terakawa and Matsu’ura (2010) estimated the stress fields in the EMJS area using fewer NIED moment tensor solutions than those used in current study. From their results, the direction of maximum compression was estimated to be E-W in the north of \(39 ^\circ\)N and WNW-ESE or NW-SE in the south of \(39 ^\circ\)N. Terakawa and Matsu’ura (2022) estimated centroid moment tensor (CMT) data from 1997 to 2020 and confirmed that the stress direction prior to the 2011 Tohoku-Oki earthquake was stable for approximately 14 years. However, the stress direction changed after the 2011 Tohoku-Oki Earthquake. They concluded that the 2011 Tohoku-Oki earthquake caused these significant changes. Uchide et al. (2022) estimated the focal mechanism solution and stress field using data from NIED, Hinet, JMA (Japan Meteorological Agency), and GSJ/AIST (Geological Survey of Japan, AIST) stations. From their results, \({\text{S}}_{\text{H}\text{m}\text{a}\text{x}}\) showed a N-S direction along the Sanriku coast and an E-W direction in other Tohoku District areas. After the 2011 Tohoku-Oki earthquake, the \({\text{S}}_{\text{H}\text{m}\text{a}\text{x}}\) direction rotated clockwise and anticlockwise in the central and eastern parts of the Tohoku District area, respectively. Particularly in the southern part of the Tohoku District’s inland area, the direction of the shortening strain (e.g., Miura et al., 2004) and our result of \({\text{S}}_{\text{H}\text{m}\text{a}\text{x}}\) direction was approximately the same. The detailed strain distribution in the northern part was unknown; however, the direction of relative motion between the Amur Plate, corresponding to the western side of the EMJS area, and the North American Plate (Heki et al., 1999) or Okhotsk Plate (Wei and Seno, 1998), corresponding to the eastern side of the EMJS area, was estimated to be approximately E-W. This is consistent with the direction of maximum compression in the northern EMJS in this study.
In this study, the estimated ST values in the EMJS and Tohoku inland areas were compared with the actual fault planes and ST values estimated in previous studies (e.g., Aida, 1984; Aida, 1989; Earthquake Research Committee, 2019; Hatori, 1969; Hurukawa and Harada, 2013; Kosuga et al., 1986; Nakanishi et al., 1993; Ohta et al., 2008; Okada et al, 2003; Okada et al., 2012; Satake, 1985; Satake, 1986; Satake and Abe, 1983; Sato, 1993; Sato et al., 1986; Tanioka et al., 1995; Umino et al., 1985; Umino et al., 1998; Yoshida et al., 2014; Yoshida et al., 2016) (Fig. 15). In the EMJS area, the fault planes that were thought to be active exhibited high ST values. In contrast, in the Tohoku inland area, fault planes that actually slip tend to had small ST values.
Miyakawa and Otsubo (2015) discussed the effects of pore fluid pressure on ST. Magma is one of the origins of underground pore water. Therefore, the volcanic distribution and ST values in Tohoku, Japan, were compared using the map of active volcanoes in Japan (https://www.data.jma.go.jp/svd/vois/data/tokyo/STOCK/bulletin/catalog/appendix/v_active.html) and the list of Quaternary volcanoes (https://gbank.gsj.jp/volcano/index.htm).Figure 15 shows the distribution of volcanoes and the ST values of this study. In the Tohoku inland area, the results were selected using the stress field before the Tohoku-Oki earthquake. More volcanoes are distributed in the Tohoku inland area than in the EMJS area.
The 1998 Shizukuishi earthquake and 2008 Iwate-Miyagi Nairiku earthquake were characterized by the existence of volcanoes near the mainshocks. Additionally, around the 2003 northern Miyagi earthquake, low-seismic-velocity regions were distributed in the deep parts, suggesting the existence of fluid (Okada et al., 2010). This suggests that high pore fluid pressure may be one of the factors that cause the activation of high-west-dipping faults in the Tohoku inland area (e.g., Sibson, 1990; Okada et al., 2007; Okada et al., 2012).
For the earthquakes around the Oshika Peninsula and Kinkazan Island, both nodal planes had almost the same dip angle (40–50°) and calculated ST value. Since this area was located in the east of the rift zone during the formation of the Japan Sea, it is possible that high-dip faults were not formed and that an earthquake occurred in the optimal newly formed fault plane for the present stress field.
In the EMJS area, the southern fault plane of the 1993 Hokkaido Nansei-oki earthquake exhibited low ST values. The southern plane was distributed on the western part of Okushiri Island. Mount Katsumayama is a Quaternary volcano located on Okushiri Island; therefore, high pore fluid pressure may possibly have affected fault activity.
In contrast, the 1939 Oga earthquake occurred adjacent to the Quaternary volcanoes (Toga, Megata, and Kanpuzan). The Oga earthquake is presumed to have caused the activity of an east-dipping reverse fault extending N-S. Previous studies (e.g., Sato, 1993) suggested that an east-dipping plane with a low dip angle slipped during the Oga earthquake. Therefore, the ST values obtained in this study were high for both planes.
Some studies have confirmed a low seismic wave velocity area, suggesting the existence of fluid in the Tohoku inland and EMJS areas (e.g., Zhao et al., 2011). However, the distribution of volcanoes suggested that the influence of the pore fluid pressure was greater in the Tohoku inland area than in the EMJS area. Therefore, it is considered that the fault plane, which is unfavorable for slipping in the stress field, gets easily activated in the Tohoku inland area.
However, in the case of the Oga earthquake, there were cases when west-dipping planes with a high dip angle were not active, even if volcanoes existed. This could be because of the following three reasons: 1) there is no high-dip-angle fault plane; 2) the high-dip-angle fault plane has not been active for a long time and the cohesive force is recovering; and 3) the differential stress is small (Sibson, 2012; Sibson and Ghisetti, 2018, etc.). Therefore, comparing past seismic activity with future fault activity is necessary. Furthermore, examining the longer-term activity is necessary to monitor activity on old high-dip-angle fault planes in both the EMJS and Tohoku inland areas.
In this study, the stress fields in Tohoku District, Japan, including the EMJS area, were estimated. Then, the likelihood of fault slip was evaluated using an ST analysis. The reliability of the stress fields was examined by comparing the ST values with actual activities. The following five conclusions were drawn:
1. The stress field in the EMJS area was approximately a reverse fault type, which was also dominant in the Tohoku inland area. Prior to the 2011 Tohoku-Oki earthquake,\({\text{S}}_{\text{H}\text{m}\text{a}\text{x}}\) direction was from E-W to NW-SE; however, after the 2011 Tohoku-Oki earthquake, it changed from ENE-WSW to NE-SW. From the analysis of stress change, the stress disturbance from the 2011 Tohoku-Oki earthquake may have caused changes in the stress fields.
2. From the stress fields estimated using Michael’s method, the results of the 2003 northern Miyagi earthquake and 2016 Oshika Peninsula earthquake had wide uncertainty. Therefore, the stress fields were re-estimated using the slip instability method. The results from the slip instability method were more stable for the 2003 northern Miyagi earthquake and changed for the 2016 Oshika Peninsula earthquake when compared to Michael’s method. This is because the number of focal mechanism solutions was insufficient during the 2016 Oshika Peninsula earthquake. Therefore, more focal mechanisms are required to obtain more reliable results. Also, for the 2003 northern Miyagi earthquake, the magnitude correlation of ST values was almost the same between Michael’s and Vavryčuk’s methods. Therefore, if the stress differences are small, similar ST values can be obtained. 3. The rake differences were larger in the Tohoku inland area than in the EMJS area. The rake difference became larger after the 2011 Tohoku-Oki earthquake than before it. The results of this study and those of previous studies confirmed that the stress field trend changed before and after the 2011 Tohoku-Oki earthquake, although it is unclear whether this temporal change was actual or apparent in a heterogeneous stress field, such as in the aftershock area (e.g., Smith and Heaton, 2015); however, the large rake may have influenced this change of direction. Therefore, when a large earthquake causes a change in the stress field over a wide area, its effect should be excluded.
4. In the EMJS area, eastward-dipping fault planes with low dip angles exhibited large ST values, which is consistent with the actual fault activity. In the EMJS, the fault might slip because it is favorable for slipping with the current stress field. In the Tohoku inland area, the eastward-dipping fault plane showed a large ST value. However, the actual fault plane estimated in previous studies was a westward-dipping fault plane with a high dip angle. The actual fault plane is unlikely to slip with the current stress field in the Tohoku inland area.
5. The fault planes, which are difficult to slip into stress fields, such as the fault plane in the Tohoku inland area and the southern part of the 1993 Hokkaido Nansei-oki earthquake, are favorable in the neighborhood of the volcano. Therefore, factors related to volcanic activity (e.g., pore fluid pressure) may cause fault activity.
Since the ST value does not change significantly when the estimated stress field is stable, in the Tohoku inland area, where fault planes are unfavorable for slipping into the stress field, certain factors and other stress fields caused fault slip.
To identify the factors that cause fault slip, increasing the number of cases is necessary to conduct the same evaluation in other regions. Furthermore, to include values such as the pore fluid pressure and friction coefficient in the ST, it is necessary to confirm the factors that cause slip.
Ethics approval and consent to participate
Not applicable
Consent for publication
Not applicable
Availability of data and materials
All the data and materials are available from the corresponding author upon request.
Competing interests
No competing interests.
Funding
This work was supported by “The International Joint Graduate Program in Earth and Environmental Sciences, Tohoku University (GP-EES)” and JST's “Pioneering Research Program for Next Generation Researchers” JPMJSP2114 (JST SPRING, Grant Number JPMJSP2114).
Authors' contributions
Ayaka Tagami, Conceptualization, Data curation, formal analysis, investigation, methodology, visualization, and writing the original draft.
Miu Matsuno, Conceptualization, Writing - review & editing.
Tomomi Okada, Supervision, Writing - review & editing.
Shin’ichi Saka, Data curation, Writing - review & editing.
Mako Ohzono, Data curation, Writing - review & editing.
Kei Katsumata, Data curation, Writing - review & editing.
Masahiro Kosuga, Data curation, Writing - review & editing.
Yoshiko Yamanaka, Data curation, Writing - review & editing.
Hiroshi Katao, Data curation, Writing - review & editing.
Takeshi Matsushima, Data curation, Writing - review & editing.
Hiroshi Yakiwara, Data curation, Writing - review & editing.
Satoshi Hirahara, Data curation, Writing - review & editing.
Toshio Kono, Data curation, Writing - review & editing.
Shu'ichiro Hori, Data curation, Writing - review & editing.
Toru Matsuzawa, Supervision, Writing - review & editing.
Shuutoku Kimura, review & editing.
Takashi Nakayama, review & editing.
Group for the aftershock observations of the 2011 off the Pacific coast of Tohoku Earthquake, data provision.
Acknowledgements
This study used data provided by the Group for the aftershock observations of the 2011 off the Pacific coast of Tohoku Earthquake.
The group members were the authors and coauthors of Okada et al. (2015). We would like to thank Editage (www.editage.com) for English language editing.
Authors' information
A. T.: Graduate students in Graduate School of Science, Tohoku University.
M. M.: Graduate students in Graduate School of Science, Tohoku University.
T. O.: Associate Professor in Research Center for Prediction of Earthquakes and Volcanic Eruptions, in Graduate School of Science, Tohoku University.
S. S.: Professor in Earthquake Research Institute, University of Tokyo.
M. O.: Associate Professor in Institute of Seismology and Volcanology, Faculty of Science, Hokkaido University.
K. K.: Associate Professor in Institute of Seismology and Volcanology, Faculty of Science, Hokkaido University.
M. K.: Professor Emeritus in Graduate School of Science and Technology, Hirosaki University.
Y. Y.: Associate professor in Graduate School of Environmental Studies, Nagoya University.
H. K.: Associate Professor in Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University.
T. M.: Professor in Institute of Seismology and Volcanology, Faculty of Science, Kyushu University
H. Y.: Associate Professor in Nansei-toko Observatory for Earthquakes and Volcanoes, Kagoshima University.
S. H.: Technical Staff in Research Center for Prediction of Earthquakes and Volcanic Eruptions, in Graduate School of Science, Tohoku University.
T. K.: Technical Staff in Research Center for Prediction of Earthquakes and Volcanic Eruptions, in Graduate School of Science, Tohoku University.
S. H.: Technical Staff in Research Center for Prediction of Earthquakes and Volcanic Eruptions, in Graduate School of Science, Tohoku University.
T. M.: Professor in Research Center for Prediction of Earthquakes and Volcanic Eruptions, in Graduate School of Science, Tohoku University.
S. K.: Technical Staff in Research Center for Prediction of Earthquakes and Volcanic Eruptions, in Graduate School of Science, Tohoku University.
T. N.: Technical Staff in Facilities Division, School of Engineering, Tohoku University.
Endnotes
No endnotes.
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