Rupture process of the largest aftershock of the M 9 Tohoku-oki earthquake obtained from a back-projection approach using the MeSO-net data
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The largest aftershock (Mw 7.8) of the giant M 9.0 Tohoku-oki earthquake occurred near the coast of Ibaraki Prefecture about thirty minutes after the main shock. We have imaged the rupture process of the Mw 7.8 earthquake by back-projection of waveform data from the Metropolitan Seismic Observation network (MeSO-net). Original acceleration seismograms were integrated. They were then band-pass filtered in the frequency range of 0.1–1.0 Hz. We assumed a fault plane on the plate boundary with a dimension of 115 km ×175 km, and this was divided into 112 subfaults. Travel times from each of the subfaults to observation sites were calculated by using a 3-D velocity structure model. Applying the restrictions that the rupture velocity is smaller than 4 km/s and the rupture duration on each subfault is less than 25 s, we obtained a rupture propagation image by projecting the power of the stacked waveforms. Propagation of the rupture toward north and east was suppressed by the existence of those areas that had radiated a large seismic energy at the main shock occurrence, or at the occurrence of the M 7.0 earthquake in 2008. The westward propagation of the rupture stopped at the area where the Philippine Sea plate lies over the Pacific plate.
Key wordsRupture process back projection 2011 Ibaraki-oki MeSO-net
There is no historical record of a great (M 8 class or larger) earthquake around the hypocenter of the largest aftershock (Utsu, 1999). The seismic catalogue of the Japan Meteorological Agency (JMA) shows that large earthquakes (M > 6) are relatively few in the vicinity of the largest aftershock after 1923, though M 7-class earthquakes occurred recurrently to the east of the area (Fig. 1(b)).
The seismic gap of large earthquakes may reflect the mode of the plate coupling in the region. In the Kanto region, the Philippine Sea plate subducts northwestward from the Sagami Trough and the Pacific plate subducts westward beneath the Philippine Sea plate from the Japan Trench. Several studies have estimated the plate coupling rate around the source region of the largest aftershock (the 2011 Ibaraki-oki event in the following). In the region off Ibaraki Prefecture, to the south of the source area of the main shock (Honda et al., 2011), the plate contacting with the Pacific plate changes from the North American plate to the Philippine Sea plate. The hypocenter of the 2011 Ibaraki-oki event was on the plate boundary between the North American plate and the Pacific plate. From a study of repeating earthquakes on the plate boundaries, Uchida et al. (2009) considered that the coupling coefficient between the Pacific plate and the North American plate is relatively large compared with that between the Pacific plate and the Philippine Sea plate. However, the coupling strength in the focal region of the 2011 Ibaraki-oki event cannot be inferred from such an analysis, for there is no repeating earthquake there. Nishimura et al. (2007) estimated the plate coupling rate in and around the Kanto region using GPS data. According to their result, the 2011 Ibaraki-oki event is located at the border of the areas of low and high coupling rates.
In this study, we investigate the rupture process of the 2011 Ibaraki-oki event by using the back-projection approach, and we discuss tectonic situations that might have caused the rupture to stop.
2. Data and Method
To suppress noises that cause the appearance of pseudo rupture images, we put restrictions on the rupture velocity to be smaller than 4 km/s and on the rupture duration at each subfault to be less than 25 s. Even if a longer duration time (e.g., 90 s at each subfault) is assumed, the result is not changed substantially.
In this study, the hypocenter given by the JMA (141.265E, 36.108N, 35 km) was adopted for the 2011 Ibaraki-oki event. Around the hypocenter a fault plane with a length of 115 km and a width of 175 km was taken and this was divided into subfaults with a dimension of 0.15 × 0.15 degree. Each subfault was clipped at the depth of the upper boundary of the Pacific plate which was modeled by Nakajima and Hasegawa (2009). The total number of subfaults is 112. Figure 2(a) shows the configuration of the fault model.
4. Discussion and Conclusions
From Fig. 3(b), it is seen that the area where greater than 10% or more, of the peak value was radiated extended over a region of about 100 km × 100 km around the hypocenter (Fig. 3(b)). The distribution of the radiation strength during the 2011 Tohoku-oki earthquake obtained by the back projection approach, using MeSO-net, K-NET and KiK-net data from NIED (Honda et al., 2011), is also plotted in Fig. 3(b). Note that the north-western end of the rupture area of the 2011 Ibaraki-oki event is delineated by the area where substantial seismic energy, greater than 50% of the maximum value, was radiated at the main shock. On the other hand, the rupture area overlaps partly with the focal region of an M 7 earthquake that occurred on May 8, 2008. Around the source area of the 2008 Ibaraki-oki event, M 7-class earthquakes are known to have occurred at an interval of about 20 years (Earthquake Research Committee, 2012). The estimated rupture areas of the past two events (1982 and 2008) are plotted in Fig. 1(b). Large parts of them overlap. (Murotani et al., 2003; Nagoya Univ., 2008). Although it is seen that a certain amount of energy was radiated from the rupture area of the 2008 Ibaraki-oki event, the radiation strength diminishes rapidly on the eastern side of the area. This suggests that the rupture that propagated eastward of the 2011 Ibaraki-oki event was suppressed when it entered into the recently ruptured area.
The strongly-radiated area corresponds to a gap of large earthquakes (M > 6) and repeating small earthquakes. From the occurrence rate of repeating small earthquakes, Uchida et al. (2009) estimated that the interplate coupling coefficient between the North American plate and the Pacific plate is 0.7~0.8. Our results suggest that the rupture of the 2011 event which initiated at the strongly-coupled area extended to the region where interplate coupling had been partly released by stable slips. Another notable finding is that the rupture stopped at the northeastern end of the Philippine Sea plate (dashed line in Figs. 3 and 4). Based on the calculation of accumulated slips using repeating earthquakes, Uchida et al. (2009) estimated that the interplate coupling coefficient between the Philippine Sea plate and the Pacific plate is smaller than 0.5. This is smaller than that between the Pacific plate and the North American plate. It is very probable that the weakness of the interplate coupling suppressed the rupture from extending further beyond the border.
Our results indicate that the area around the hypocenter was a strong source of the radiated seismic energy (>0.8 in Fig. 3). However, we cannot comment on the magnitude of the slip there. Performing a nonlinear inversion using GPS data, Nishimura et al. (2011) estimated 3.76 ± 0.5 m as the average amount of the slip on a rectangular fault that almost overlaps with the rupture area. If the radiated energy distribution (Fig. 3(b)) is supposed to correspond to the slip distribution on the fault, the maximum slip could have reached 7~8 m. This is a reasonable amount of slip for an M 8 class earthquake. However, the estimated slip is considerably smaller compared with the slip deficit predicted by the relative plate velocity of 8.3 cm/y (DeMets et al., 1994), or 7.2 cm/y (Sella et al., 2002), if we note that there has been no record of large earthquakes there during the past several hundred years (Utsu, 1999). This may imply that a substantial part of the accumulated strain might have been released in another way, for example, by aseismic slips (e.g., Seno, 2012). We think it may also be possible that the accumulated strain still remained in a large area, including the focal region of the 2011 Ibaraki-oki event. In that case, the southeastern part of the fault plane where seismic energy was not radiated much at the 2011 Ibaraki-oki event is a potential source region of a future large earthquake.
From seismic surveys, Mochizuki et al. (2008) deduced the existence of a seamount in the region near the Japan Trench. The seamount could be a barrier of the rupture during the 2011 Ibaraki-oki event. This idea agrees with strong plate coupling in that region estimated from GPS data (Nishimura et al., 2007). On the other hand, Mochizuki et al. (2008) maintains weak interplate coupling around the seamount from the results of seismic surveys and laboratory experiments. In any case, whether the coupling strength at the region near the Japan Trench is strong or weak, we think much attention should be paid to the seismic activity and/or afterslip there.
The present work was supported by JSPS KAKENHI Grant Number 24740314 and the Special Project for Earthquake Disaster Mitigation in Tokyo Metropolitan Area of the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT). We used digital data of the upper surface of the Pacific plate compiled by Dr. Fuyuki Hirose at the Meteorological Research Institute. The Generic Mapping Tools (Wessel and Smith, 1995) was used to make figures. We are grateful to Dr. Naoki Uchida for his valuable comments and two anonymous reviewers for their constructive comments which were very helpful for improving the manuscript.
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