Alteration of stress field brought about by the occurrence of the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0)
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A giant earthquake of Mw 9.0 took place off the Pacific coast of Tohoku on March 11, 2011. It caused a large tsunami of 10–20 m and devastated the area along the Pacific coast in northeast Japan. The earthquake altered the stress field in the surrounding region immensely. We have calculated the change in Coulomb Failure Function (ΔCFF) due to this earthquake to evaluate the effect on aftershocks and future earthquake probabilities. The results suggest that the increased activity of normal-fault earthquakes after the main shock is explained by a large positive ΔCFF of 1–5 MPa prevailing over a vast area in and around the main-shock fault zone. The areas adjacent to the northern and southern borders of the fault zone where other large interplate earthquakes might occur are occupied by a positive ΔCFF of approximately 0.1 MPa. Based on the ΔCFF result, the future probability of reverse fault earthquakes in the shallow crust is estimated to be decreased in the land area of Tohoku.
Key words2011 Tohoku Earthquake Japan Trench great earthquake ΔCFF Coulomb Failure Function
A giant earthquake of Mw 9.0 occurred at 02:46 p.m. (JST) on 11 March 2011 off the Pacific coast of Tohoku, northeast Japan. A large tsunami following the earthquake devastated the cities and towns facing the rupture zone on the Pacific coast. An interplate earthquake of Mw 7.5–8.0 had been anticipated off the Pacific coast of the Miyagi prefecture in the near future, but an earthquake as large as Mw 9.0 had not been regarded as impending. This earthquake is so huge that the stress field in and around the fault zone has been disturbed immensely. In this paper, we look at the change in stress field brought about by this earthquake through the change in the Coulomb Failure Function (ΔCFF), and discuss its effect on aftershocks and future earthquake probabilities in the surrounding region.
2. Fault Model
3. Distribution of ΔCFF for Selected Target Earthquakes
For the interplate target events, the border areas of the main-shock fault zone are occupied by a large positive ΔCFF in all directions (Fig. 5(left)). At the northern border, the fault adjoins the rupture zone of the 1994 Sanriku-Oki earthquake of Mw 7.7. Probably the Mw 9.0 earthquake stopped there because the stress had already been released by the 1994 event. A positive ΔCFF of approximately 0.1 MPa is distributed over the area off the Pacific coast of the Aomori prefecture. The area corresponds to the northern half of the rupture zone of the 1968 Tokachi-oki earthquake (Mw 8.2) that was not fractured during the 1994 earthquake (Sato et al., 1996). At the southern border, a positive ΔCFF of similar amplitude prevails over the area off the Pacific coast of the Chiba prefecture, where no large event has occurred since the 1677 Enpo earthquake (~M 8). The observation of many interplate aftershocks in the depth range of 40–60 km (Fig. 2) is consistent with the strong positive ΔCFF near the down-dip end of the fault zone. Beyond the depth of 60 km, the still large positive ΔCFF does not directly indicate a high probability for future interplate earthquakes because the slip over the area is considered to be aseismic. The area near the up-dip end of the fault zone, i.e. the area beyond the 10 km depth contour of the plate interface towards the trench, appears to be aseismic because no aftershocks with reverse-fault mechanisms have been observed near the trench so far (Fig. 2) despite the fact that the area is occupied by a strong positive ΔCFF.
Finally, we show the result for reverse faults in the shallow crust in Fig. 5(right). Focal depths are placed at 10 km. The average of ΔCFF obtained for the two fault dips, one dipping at 30° eastwards and the other dipping at 30° westwards, is shown. Expectedly, the spatial pattern of ΔCFF is almost opposite to the result for normal faults shown in Fig. 3(left). A negative ΔCFF of 0.1–1.0 MPa is spread widely over the land area just west of the fault, with a decreasing amplitude towards the back arc side. The areas adjacent to the northern and southern borders of the fault are occupied by a positive ΔCFF in the range 0.01–0.1 MPa. These results suggest that earthquakes having reverse-fault mechanisms in the shallow crust will be suppressed over the land area of Tohoku, although we recall that the Rikuu earthquake (M 7.2) took place near the border of Iwate and Akita prefectures two and half months after the occurrence of the 1896 Meiji Sanriku earthquake (~M 8.5) that may have resulted in a similar stress change to the land area of Tohoku. Based on the fault model proposed by Tanioka and Satake (1996), we have assessed the ΔCFF due to the 1896 Meiji Sanriku earthquake for the same reverse-fault targets and found that, although the ΔCFF are similarly negative over the epicentral area of the Rikuu earthquake, their amplitudes are several ten times smaller than those due to the 2011 event. Probably, some other factor dominated the effect of negative ΔCFF, causing the Rikuu earthquake in 1896. This time it may be difficult for any unknown factors to trigger shallow reverse-fault earthquakes by superseding and negating the effect of a large negativeΔCFF of 0.1–1.0 MPa.
4. Concluding Remarks
The ΔCFF due to the 2011 off the Pacific coast of To-hoku Earthquake (Mw 9.0) has been investigated assuming several target faults in northeastern Japan. The increased activity of normal-fault earthquakes occurring after the main shock is explained by a large positive ΔCFF of 1–5 MPa prevailing over a vast area in and around the main-shock fault zone. As of May 9, 2011, the largest normal-fault aftershock near the Japan Trench was Mw 7.6. It occurred 40 minutes after the main shock. In the case of the great earthquake doublet along the central Kuril trench in 2006, an interplate earthquake of Mw 8.3 was followed by a normal-fault earthquake of Mw 8.1 about 2 months later (Ammon et al., 2008). Because of the magnitude of positive ΔCFF, we cannot avoid suspecting that a great normal-fault earthquake of Mw > 8 may occur close to the trench in the near future. Also, we should be aware of the fact that the areas adjacent to the northern and southern borders of the fault, where other large interplate earthquakes might occur, are occupied by a positive ΔCFF of approximately 0.1 MPa. The ΔCFF for the reverse faults in the shallow crust suggests that the future probability of reverse-fault earthquakes will be decreased in the land area of Tohoku.
We thank the Geospatial Information Authority of Japan (GSI) and the National Research Institute for Earth Science and Disaster Prevention (NIED) for promptly providing information on this earthquake. The authors are grateful to Dr. Okada for providing the source code for calculating the elastic deformation due to dislocation in a half space. The figures are prepared using GMT (Wessel and Smith, 1991). We thank two anonymous reviewers for critical reading of the manuscript and suggestions for its improvement. This study is partly supported by the urgent research for the 2011 off the Pacific coast of Tohoku Earthquake—inland seismic observation—, under the Research Program for Prediction of Earthquakes and Volcanic Eruptions, promoted by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.
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