Stress before and after the 2011 great ... - Wiley Online Library

GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L03302, doi:10.1029/2011GL049729, 2012

Stress before and after the 2011 great Tohoku-oki earthquake and induced earthquakes in inland areas of eastern Japan Keisuke Yoshida,1 Akira Hasegawa,1 Tomomi Okada,1 Takeshi Iinuma,1 Yoshihiro Ito,1 and Youichi Asano2 Received 26 September 2011; revised 20 December 2011; accepted 30 December 2011; published 1 February 2012.

[1] Stress fields in inland areas of eastern Japan before and after the Tohoku-oki earthquake were estimated by inverting focal mechanism data. Before the earthquake, s1 axis was oriented EW in Tohoku but NW-SE in Kanto-Chubu. The stress fields changed after the earthquake in northern Tohoku and in southeastern Tohoku near Iwaki city, where the orientations of the principal stresses became approximately the same as the orientations of the static stress change associated with the earthquake. This indicates that differential stress magnitudes in these areas before the earthquake were smaller than 1 MPa. The stress field did not change in central Tohoku, even though the stresses loaded after the earthquake had nearly reversed orientations, which indicates that the differential stress magnitudes there were significantly larger than 1 MPa. In Kanto-Chubu, stresses having nearly the same orientations as the background stresses were loaded after the earthquake, and the stress fields did not change as expected. This may have caused very high induced seismicities in Kanto-Chubu. Citation: Yoshida, K., A. Hasegawa, T. Okada, T. Iinuma, Y. Ito, and Y. Asano (2012), Stress before and after the 2011 great Tohoku-oki earthquake and induced earthquakes in inland areas of eastern Japan, Geophys. Res. Lett., 39, L03302, doi:10.1029/2011GL049729.

1. Introduction [2] After the 11 March 2011 great Tohoku-oki earthquake (Mw9.0), induced earthquakes actively occurred in the crust of inland areas of eastern Japan, in addition to ordinary aftershocks [e.g., Hirose et al., 2011]. Space-time plot of earthquakes (Figure 1) clearly shows a sharp change in seismicity after the earthquake: seismicities abruptly increased in some areas, but suddenly decreased in other areas. This indicates that the static stress change caused by the fault slip of the earthquake considerably affected seismicity pattern in inland areas. The induced earthquakes include remarkable normal fault type activity in and around Iwaki city, which is located 40 km to the south of the 2nd Fukushima Nuclear Power Plant near the Pacific coast (cf. Figure 1). This activity is particularly unique, since most shallow inland earthquakes in northeastern (NE) Japan are known to have thrust type mechanisms [e.g., Hasegawa et al., 1994]. Except for this anomalous activity, induced earthquakes are especially active

1

Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai, Japan. 2 National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan. Copyright 2012 by the American Geophysical Union. 0094-8276/12/2011GL049729

in Kanto-Chubu, far from the mainshock rupture area, although the induced activities appear to be widespread throughout eastern Japan (Figure 1). [3] In order to understand why such peculiar induced activities occurred, we investigated stress fields before and after the earthquake, and compared with the static stress change which is not negligibly small even in inland areas because of its extraordinary size.

2. Stress Fields Before and After the Tohoku-oki Earthquake [4] We used shallow (4.0, 2) >3.5 and 3) 1.0–5.0, respectively. Focal mechanisms are plotted in Figure 2, which clearly shows an increased tendency for strike-slip or normal mechanisms, as contrasted with thrust ones prior to the earthquake. This indicates that the least compression had changed from vertical to horizontal. [5] We apply the damped stress tensor inversion method of Hardebeck and Michael [2006] to the data shown in Figure 2 in order to compute the stress fields. The region is gridded with 0.5°spacing, and each focal mechanism is assigned to the nearest grid node. For the damped inversions, we chose the spatial damping parameter es = 0.6 on the basis of the trade-off between model length and data variance [Hardebeck and Michael, 2006]. We inverted the pre- and post-mainshock data together using the temporal damping parameter et = 0.6, so that there are no apparent temporal rotations that are not strongly required by the data. Obtained results are shown in Figure 3 and Figure S1 in the auxiliary material, which show that the maximum compression (s1) is nearly horizontal for the whole area before the earthquake.1 s1 is oriented E-W in Tohoku nearly parallel to the plate convergence, whereas that in Kanto-Chubu is 1 Auxiliary materials are available in the HTML. doi:10.1029/ 2011GL049729.

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YOSHIDA ET AL.: STRESS CHANGE BY 2011 TOHOKU EARTHQUAKE

Figure 1. Shallow (shallower than 20 km) inland earthquake activity before and after the great Tohoku-oki earthquake. (a) Epicenter map showing events that occurred 90 days before (grey dots) and 90 days after (red dots) the Tohoku-oki earthquake. Green contours show the slip model of the Tohoku-oki earthquake by Iinuma et al. [2011]. Centroid moment tensor solution of the Tohoku-oki mainshock by USGS is indicated by a beach ball. (b) Space-time plot of shallow inland events that occurred for 180 days before and after the mainshock.

oriented NW-SE. This clockwise rotation of s1 in KantoChubu is probably caused by the collision between NE Japan and southwestern (SW) Japan and by the collision between the Izu Peninsula and the SW Japan arc [e.g., Townend and Zoback, 2006]. [6] After the earthquake, stress field in northern Tohoku (regions A, B and C) and that in an area near Iwaki of southeastern (SE) Tohoku (region G) changed significantly; s1 in A, B and C rotated counterclockwise and has a NE-SW direction, whereas the least compression (s3) in G approached the plate convergence direction by its clockwise rotation. However, in the other areas from central Tohoku to Kanto-Chubu, there was no significant change in the stress field.

3. Comparison With Static Stress Change by the Tohoku-oki Earthquake [7] The static stress change by the Tohoku-oki earthquake was estimated based on the equations of Okada [1992] and using a slip model by Iinuma et al. [2011]. We assume a Poisson’s ratio of 0.28 and Young’s modulus of 70 GPa. Estimated stress change is shown in Figures 4 and S2. The figures show that s1 in northern Tohoku (A, B and C) and s3 near Iwaki (G) are nearly consistent with the observed directions in those two areas after the earthquake, respectively (Figure 3b). This indicates that the induced earthquakes in those areas are caused by the static stress change whose orientations are different from the background stress before the earthquake, suggesting the deviatoric stress magnitudes before the earthquake are smaller than 1 MPa.

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[8] The static stress change shows s3 in central Tohoku (D, E and F) has an EW direction. It is s1 that has an EW direction both before and after the earthquake. This indicates that the stress change by the earthquake did not significantly affect for this area, showing the deviatoric stress magnitude before the earthquake is significantly larger than the static stress change (larger than 1 MPa). [9] In Kanto-Chubu (I through M), the static stress change show that s1 has a NW-SE direction, which is approximately the same as that observed before the earthquake (Figure 3a). As expected, the stress field after the earthquake is also nearly the same. This indicates that the deviatoric stress in those regions simply increased by the mainshock fault slip. This is thought to be the reason why remarkable induced earthquake activities are observed in those regions of KantoChubu despite being distant from the mainshock rupture. [10] In region H, s1 has a NW-SE direction both before and after the earthquake as in the other regions of KantoChubu (Figure 3), but s3 is nearly EW in the static stress change (Figure 4). Static stress change calculated by adopting a different slip model [Lay et al., 2011] (triangles in Figure S2h) shows that s1 is oriented approximately NW-SE as in the other regions of Kanto-Chubu, indicating that H is located at a place sensitive to the slip model. Therefore, the background deviatoric stress in H may have simply increased by the mainshock fault slip as in the other regions of Kanto-Chubu.

4. Discussion and Conclusions [11] We estimated stress fields in inland areas of eastern Japan before and after the Tohoku-oki earthquake, and showed that the stress fields changed in northern Tohoku and in an area near Iwaki in SE Tohoku after the earthquake. [12] Wesson and Boyd [2007] proposed a method to estimate deviatoric stress magnitude and estimated it in the focal area of the 2002 Denali fault earthquake. If the stress field changes after the earthquake, we can estimate absolute deviatoric stress tensor from the least squares method by equating the difference in the stress tensor before and after the earthquake to the static stress change [Wesson and Boyd, 2007]. We follow their method to estimate deviatoric stress magnitudes. Regions C and G are used for this estimation, because changes in stress field after the Tohoku-oki earthquake were clearly observed. The slip model adopted here are the models of Iinuma et al. [2011] and Lay et al. [2011]. Results are listed in Table 1, which shows estimated differential stress (s1 -s3 ) before and after the earthquake. Differential stress magnitudes are on the order of 0.2–0.9 MPa before the earthquake and 0.8–1.6 MPa after. It is considered that these stresses after the earthquake (0.8–1.6 MPa) caused the induced earthquakes in those regions, which seems to favor a fault that is weak. [13] More precisely, for region G, focal depth ranges of events used in the inversions before and after the earthquake are slightly different from each other. Events in the region occurred at depths of 10–20 km before the earthquake, whereas in a wider depth range of 0–20 km after. In order to compare the stress fields for the same depth range, stress inversions using events with focal depths of 10–20 km were performed, and are shown in Figures 3b and S1. Clockwise rotation of s3 became smaller. Estimated differential stress

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Figure 2. Focal mechanisms (a) before and (b) after the Tohoku-oki earthquake used in the present study. Thrust, strikeslip and normal faults are indicated in red, green and blue colors. Fault types are classified according to Frohlich [1992].

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Figure 3. Orientations of the best fit s1 and s3 axes obtained by stress tensor inversions for the periods (a) before and (b) after the Tohoku-oki earthquake. s1 and s3 axes are indicated by red and by blue bars, respectively, at each grid node. Length of bar corresponds to the plunge of the principal stress axes. Red and blue bars in a small frame at the right bottom of Figure 3b show the best fit s1 and s3 axes after the earthquake for region G obtained using events with depths of 10–20 km.

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Figure 4. Orientations of s1 axis (red bar) and s3 axis (blue bar) of the static stress change by the Tohoku-oki earthquake at a depth of 10 km. Slip model adopted is after Iinuma et al. [2011]. Differential stress is shown by contours of 1.0, 0.5 and 0.1 MPa, respectively. magnitudes for this case (Table 1) before the earthquake became slightly larger. [14] As mentioned before, in central Tohoku, direction of s1 did not change although the nearly reversed stresses were loaded after the earthquake. We estimated by trial and error how large the differential stress magnitude before the earthquake is in order that the s1 direction does not change even if the stress tensor associated with the earthquake is added. Result for region E (Table 1) shows that the differential

stress magnitude before the earthquake is at least greater than 2.5 MPa. Comparison with regions C and G indicates that the highest differential stress prior to the earthquake was located in central Tohoku just west of the largest slip area of the mainshock rupture. It also indicates that a significant variation in differential stress is evident within these regions similarly to the case of the Denali fault earthquake [Wesson and Boyd, 2007].

Table 1. Estimated Differential Stress Magnitudes (MPa)a Before Area N Tohoku (region C) C Tohoku (region E) SE Tohoku near Iwaki (region G) (region G with h > 10km) Kanto-Chubu

After

All

Mean

S.D.

All

Mean

S.D.

Slip Model

0.3 0.6 > 2.5 > 2.5 0.6 0.6 0.9 0.8 -

0.3 0.6 > 2.5 > 2.5 0.6 0.2 0.7 0.5 -

0.2 0.2 > 2.5 > 2.5 0.2 0.2 0.3 0.4 -

0.6 0.8 1.0 1.3 1.0 1.7 -

0.6 0.7 0.9 1.2 0.8 1.5 -

0.1 0.2 0.2 0.2 0.4 0.4 -

Iinuma et al. [2011] Lay et al. [2011] Iinuma et al. [2011] Lay et al. [2011] Iinuma et al. [2011] Lay et al. [2011] Iinuma et al. [2011] Lay et al. [2011]

a “All data” indicates solutions obtained using all focal mechanism data. “Mean” and “S.D.” are the mean value and standard deviation obtained from bootstrap analyses of the uncertainty in the orientation of the principal stresses.

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[15] Several studies that estimated deviatoric stress magnitudes have also reported small values of shear stress magnitude on the order of 1–22 Pa [e.g., Hardebeck and Hauksson, 2001; Wesson and Boyd, 2007; Hasegawa et al., 2011], which are slightly larger than the present values. Smaller values in the present study might originate in part from the estimation error. Uncertainty in the principal stress orientations is considered here and is shown as the standard deviation in Table 1. The sources of uncertainties include other factors such as uncertainty resulting from the least squares inversion [Wesson and Boyd, 2007], which were not considered. In addition, the deviatoric stresses in the present case caused only small earthquakes with magnitudes of at most 4.5 (earthquakes with magnitudes larger than this occurred in region G, but at depths shallower than 10 km), whereas those in the previous studies caused larger earthquakes with magnitudes of 7.3–9.0: This might be the reason for smaller values. In any case, these observations suggest that seismogenic faults in those areas are very weak. [16] Acknowledgments. The Editor wishes to thank Jeanne Hardebeck and an anonymous reviewer for their assistance evaluating this paper.

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