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Focal mechanisms and stress field in the Nobi fault area, central Japan
© Katsumata et al. 2015
- Received: 4 March 2015
- Accepted: 16 June 2015
- Published: 25 June 2015
In this study, we obtained 728 focal mechanisms of small earthquakes with depths shallower than 20 km that occurred from May 2009 to May 2013 in the Nobi fault area in central Japan. The averages of the azimuths of the P- and T-axes were N97° ± 23° E and N6° ± 32° E, and the averages of the dips of the P- and T-axes were 11° ± 10° and 32° ± 25°, respectively. These variations in the P- and T-axes come from variation of the focal mechanisms; both strike-slip and reverse fault earthquakes were observed in the study area. A stress tensor inversion method was applied to the focal mechanisms, and we obtained and characterized the spatial pattern of the tectonic stress. We found that the maximum principal stress (σ 1) is oriented E–W over almost the entire study area. The stress ratio R, which is defined as R = (σ 1 – σ 2)/(σ 1 – σ 3), ranges from 0.65 to 0.98, and the average R over the entire study area is 0.82. The average stress ratio is close to unity, indicating σ 2 ≈ σ 3, and thus the dominant stress in this region is a uniaxial compression in the direction of σ 1. The direction of the σ 1-axis fluctuates locally at the southeastern end of the seismic fault ruptured by the 1891 Nobi earthquake. This fluctuation is limited to within a very narrow zone across the seismic fault in the upper crust shallower than approximately 10 km, suggesting that most of the deviatoric stress at the southeastern end of the seismic fault ruptured by the 1891 Nobi earthquake was not released.
- Nobi fault system
- Nobi earthquake
- Focal mechanism
- Stress tensor inversion
- Intraplate earthquake
- Active fault
The region of the Eurasian plate containing central Japan is compressed by both the Philippine Sea (PH) plate from the south and the Pacific (PA) plate from the east (Fig. 1). The PH plate and PA plate are subducting beneath the Nankai Trough and Japan Trench, respectively. Although it is clear that the tectonic stress due to the subduction of the two plates is loaded on intraplate faults in central Japan, the detailed mechanism of the stress accumulation is still debated. Two models for the occurrence of the intraplate earthquakes have been proposed recently. Kawanishi et al. (2009) named the two models the regional stress model and the local stress model. The difference between these models is whether or not the tectonic stress due to the subduction of the plates is locally concentrated on the intraplate earthquake faults. The regional stress model assumes that the stress builds up uniformly in a broad area that includes the intraplate earthquake faults (e.g., Sykes 1978; Hinze et al. 1988; Johnston and Kanter 1990; Zoback 1992). Conversely, the local stress model assumes that the stress is locally concentrated on the intraplate earthquake faults (e.g., Campbell 1978; Liu and Zoback 1997; Stuart et al. 1997). To investigate which model is plausible, the spatial pattern of the stress field is of importance.
The first purpose of this study was to obtain precise focal mechanisms in the Nobi fault area using P-wave first-motion data collected by temporary seismic observation. The second purpose was to obtain and characterize the spatial pattern of the stress field based on the focal mechanisms.
To obtain a comprehensive understanding of the mechanism of the stress accumulation, a temporary observation was conducted in and around the Nobi fault system in central Japan from 2009 to 2013 by the Research Group for the Joint Seismic Observations at the Nobi Area, which consists of Hokkaido University, Hirosaki University, Tohoku University, the University of Tokyo, Nagoya University, Kyoto University, Kyushu University, Kagoshima University, and the National Research Institute for Earth Science and Disaster Prevention. A total of 70 temporary seismographic stations were deployed in an area of 100 × 100 km that includes the Nobi fault system (Fig. 1). The temporary seismographic stations consisted of 45 telemetry online systems and 25 portable offline systems. A three-component seismograph with a natural frequency of 1 Hz was installed at each station. Fifty-three permanent online seismographic stations in and around the temporary seismic network were also used in this study.
Results of stress inversiona
N of F.M.
N of nodes
0.7 ± 0.15
0.8 ± 0.12
0.9 ± 0.11
0.8 ± 0.15
Some earthquakes were triggered by the M9.0 Tohoku-Oki earthquake in March 2011, and some of these were even located outside of the focal area. Although the study area is located far from the focal area, a temporal change in the stress field may have occurred due to the Tohoku-Oki earthquake as well as a subsequent large afterslip. However, we found no clear temporal change in the azimuth and dip of P- and T-axes. Note that the higher number of focal mechanisms beginning around the beginning of 2011 was an apparent change. This change occurred because we detected the polarity of the first P-wave motion for all earthquakes larger than M = 1.5 before 2011, whereas after 2011, we detected the polarity for all earthquakes larger than M = 1.0.
The angle φ is defined as the angle between the σ 1-axis and the strike of the fault. It is clear that the angle φ of the Umehara fault is different from those of the other two segments at depths of 5–10 km. The strikes of the faults are as follows: N40° W for the Nukumi fault, N35° W for the Neodani fault, and N55° W for the Umehara fault. The direction of the σ 1-axis applied to the Umehara fault is N70° W at (35.5° N, 136.8° E), and thus φ = 15°. On the other hand, the direction of the σ 1-axis applied to the Neodani fault is N83° W at (35.7° N, 136.6° E), and thus φ = 48°. The direction of the σ 1-axis applied to the Nukumi fault is N79° W at (35.8° N, 136.5° E), and thus φ = 39°.
The focal mechanisms also change in the shallow portion (Fig. 7). Whereas the numbers of strike-slip and reverse faults are approximately the same west of 136.5° E, the number of reverse faults decreases between 136.5° E and 136.9° E. The surface trace of the seismic fault ruptured by the 1891 Nobi earthquake is located around 136.7° E–136.8° E. This strongly suggests that the 15° rotation between 136.5° E and 136.9° E is related to a tectonic process of the Nobi fault system. Although we focus on the stress variation at the southern end of the fault in the following discussion, the σ 1-axes in the northern part of the study area above latitude 35.8° N appear to be oriented entirely ESE–WNW.
We find that (1) both strike-slip and reverse fault earthquakes are observed in the study area, (2) the σ 1-axis is oriented E–W or ESE–WNW, and (3) the stress ratio is close to 1.0, indicating σ 2 ≈ σ 3. Townend and Zoback (2006) investigated the spatial pattern of the stress field at a regional scale all over Japan and found that the axis of greatest horizontal compressive stress is oriented ESE–WNW west of approximately 136° E in central Japan, which is consistent with our results. Terakawa and Matsu’ura (2010) found that compressional stress is dominant in central Japan and that its axis is oriented E–W, which is also consistent with our results. Tsutsumi et al. (2012) inverted 169 fault-slip data points from 37 active faults in the eastern part of the Southwest Japan arc, including the Nobi fault area. They found that the direction of the σ 1-axis tends to be ESE–WNW and that the stress ratio is close to 1.0, indicating σ 2 ≈ σ 3, which suggests that the stress field in central Japan has been uniform and stable over the past approximately 105 years. The direction of the σ 1-axis and the stress ratio obtained by Tsutsumi et al. (2012) are consistent with the results in this study. Hiramatsu and Iidaka (2015) obtained a spatial distribution of S-wave splitting parameters and found that the observed polarization orientation ranges from E–W to NW–SE, which is consistent with the direction of the σ 1-axis obtained in this study.
As mentioned above, the stress field obtained in this study is in very good agreement with the results of previous studies except for the local fluctuation obtained in the southwestern part of the Nobi fault system. The direction of the σ 1-axis changes steeply in the narrow zone south of 35.6° N at depths shallower than ~10 km. Kato et al. (2006) observed rotation of the σ 1-axis around the southwestern end of the aftershock area of the 2004 mid-Niigata prefecture (M w 6.6) earthquake, and they suggested that the rotation of the σ 1-axis might be caused by the lateral variation of low-velocity bodies in the hanging wall. In the case of the Nobi fault system, however, the P- and S-wave velocity structures in the local fluctuation area appear to be the same as those of other areas along the Nobi fault system (Nakajima et al. 2014). The spatial distribution of coda Q is also uniform along the entire Nobi fault system (Tsuji et al. 2014). Therefore, the local fluctuation of the σ 1-axis may not be caused by differences in the crustal structure. Fujino and Katao (2009) also reported a sharp change in the stress field across the Hanaori fault, which is located near the western boundary of the study area, and they presented no hypothesis to explain the sharp change in the stress field.
Hardebeck and Hauksson (2001) calculated Δθ as a function of θ, where θ is the angle between a seismic fault and the direction of the σ 1-axis applied to the seismic fault before an earthquake and Δθ is the rotation angle of the σ 1-axis due to the occurrence of an earthquake. Based on this model, we are able to present a hypothesis that explains the local fluctuation of the σ 1-axis found in this study. Although we stated in the introduction that the stress accumulation process for intraplate regions is still under debate, we assume that local stress accumulation occurs near the source fault. Before the 1891 Nobi earthquake occurred, the σ 1-axis may have been oriented roughly NW–SE, rather than E–W, along the entire Nobi fault ruptured by the 1891 event, indicating that the deviatoric stress accumulated in the narrow zone along the entire Nobi fault. Thus, we assumed that the σ 1-axis was oriented N70° W before the 1891 Nobi earthquake occurred, which is the present orientation of the σ 1-axis at (35.5° N, 136.8° E) obtained by the stress inversion at the depth of 5–10 km. The strikes of the faults are N40° W for the Nukumi fault, N35° W for the Neodani fault, and N55° W for the Umehara fault; therefore, θ for the model by Hardebeck and Hauksson (2001) is equal to 30° for the Nukumi fault, 35° for the Neodani fault, and 15° for the Umehara fault. When the 1891 Nobi earthquake occurred, the σ 1-axis rotated from N70° W to N83° W in the central segment of the Nobi fault, where the stress drop may have been very large. The N83° W direction is the present orientation of the σ 1-axis at (35.7° N, 136.6° E) in the Neodani fault obtained by the stress inversion at the depth of 5–10 km. In this case, Δθ = −13° for the Neodani fault, and thus approximately 70 % of the deviatoric stress was released based on the model by Hardebeck and Hauksson (2001). However, the rotation of the σ 1-axis was very small, that is, Δθ = 0°, and the N70° W direction of the σ 1-axis was maintained at the southern end of the seismic fault due to the small stress drop, indicating that most of the deviatoric stress around the Umehara fault was not released by the 1891 event. Fukuyama and Mikumo (2006) presented evidence supporting the assumption of the stress drop distribution, and they found that the static stress drop was 3.6–12.9 MPa in the central part of the seismic fault ruptured by the 1891 Nobi earthquake and 1.8–3.6 MPa in and around its southern end. Takano and Kimata (2013) reevaluated the fault-slip model based on geodetic survey data and found that the coseismic horizontal displacement was 4.2–7.3 m in the central part and 2.0–3.8 m in and around the southern end, which also supports our assumption. The hypothesis that the deviatoric stress was not released at the southern end of the fault, based on the model by Hardebeck and Hauksson (2001), depends strongly on the direction of the σ 1-axis prior to the mainshock. Therefore, the assumption that the σ 1-axis was oriented N70° W before the Nobi earthquake must be validated based on more compelling evidence to determine whether this hypothesis is correct. The 2011 Tohoku-Oki (M w 9.0) earthquake is another reliable example of the release of almost all deviatoric stress (Hasegawa et al. 2012). In this case, a coseismic rotation of the principal stress axis was clearly observed. However, no coseismic rotation of the principal stress axis was observed as a result of the 2000 western Tottori (M w 6.6) earthquake (Yukutake et al. 2007). In this case, it is likely that only a small amount of the deviatoric stress was released by the mainshock.
The strike of the Umehara fault in the area of the spatial stress rotation is different from the strike of the other two faults in the central part ruptured by the 1891 Nobi earthquake. Additionally, a buried fault, a reverse fault dipping to the east, located to the southwest of the Umehara fault might have been ruptured by the 1891 Nobi earthquake (Kuriyama et al. 2013). Therefore, an alternative hypothesis is that the spatial rotation of the σ 1-axis is related to the change of strike or/and the rupture of the buried fault.
We obtained 728 focal mechanisms of small earthquakes in the Nobi fault area in central Japan. The averages of the azimuths of the P- and T-axes are N97°±23° E and N6°±32° E, respectively. The P-axes are nearly horizontal, and the T-axes have a larger dip than the P-axes. We observed both strike-slip and reverse fault earthquakes in the study area. Few normal fault earthquakes were observed. The focal mechanisms were applied to a stress tensor inversion method to investigate the stress field and its spatial pattern. We found that over the entire study area, the σ 1-axis is oriented E–W. However, the spatial pattern of the σ 1-axis fluctuated locally at the southeastern end of the seismic fault ruptured by the 1891 Nobi earthquake. This local fluctuation suggests that most of the deviatoric stress at the southern end of the seismic fault was not released. However, this hypothesis depends strongly on the direction of σ 1-axis prior to the 1891 Nobi earthquake. Therefore, the assumption that the σ 1-axis was oriented NW–SE before the Nobi earthquake must be validated based on more compelling evidence to determine whether the hypothesis is correct.
This study was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its Earthquake and Volcano Hazards Observation and Research Program. We used data from JMA and Hi-net/NIED. We thank A. Nishizawa, who is an editor of this journal, and two anonymous reviewers for valuable comments. GMT-SYSTEM (Wessel and Smith 1991) was used for data mapping. To determine focal mechanism solutions, we used the program HASH produced by Hardebeck and Shearer (2002). To conduct the stress tensor inversion, we used the program SATSI developed by Hardebeck and Michael (2006).
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