Source process of the 2011 off the Pacific coast of Tohoku Earthquake with the combination of teleseismic and strong motion data
We have derived preliminary results for the source process of the March 11, 2011 off the Pacific coast of Tohoku Earthquake (the 2011 Tohoku Earthquake; Mw = 9.0) from two types of seismic waveform data: teleseismic P waves and regional strong motion data. The common features of these two analyses are as follows: (a) The main rupture is located to the east of the initial break point (the shallower side of the hypocenter), and maximum slip amounts were more than 25 m. (b) The size of the main fault was about 450 km in length and 200 km in width; the duration of rupture was more than 150 s; and Mw was 9.0. (c) The initial rupture gradually expanded near the hypocenter (0–40 s) and subsequently propagated both southwards and northwards.
Key wordsSource process teleseismic wave strong motion the 2011 off the Pacific coast of Tohoku Earthquake
A gigantic interplate earthquake occurred on March 11, 2011 at 14:46 (JST) to the northeast of mainland Japan. The hypocenter determined by the Japan Meteorological Agency (JMA) was at 38.10N, 142.86E, depth 23.7 km. A very large tsunami hit the Pacific coastline of the Tohoku and Kanto area with maximum heights greater than 10 m. JMA estimated Mw to be 9.0 from centroid moment tensor (CMT) analysis using long-period global data. It was the greatest earthquake ever recorded by seismometers around Japan. Tens of thousands of fatalities occurred, mainly from the tsunami.
In this region, the Pacific plate is subducting beneath northeastern Japan at the Japan Trench. The convergence rate is about 8 cm/yr in the WNW direction (Wei and Seno, 1998). Many M 7 class earthquakes have occurred in this region (Yamanaka and Kikuchi, 2004), but few events greater than M 8 have occurred in the last several hundred years. The Meiji-Sanriku earthquake of 1896, which triggered an abnormally large tsunami, is one such event. The average slip during this event was 5.9–6.7 m (Tanioka and Seno, 2001), yet the total slip released by seismic waves was far less than that accumulated from plate coupling over the last several hundred years. This suggests a high coupling rate as pointed out by GPS analysis (Nishimura et al., 2004). The mechanism of releasing the slip deficit in this region is a topic of much research. One candidate for this kind of event is the Jogan earthquake of AD 869, for which Mw has been estimated at more than 8.4 by comparing the distribution of tsunami deposits with numerical tsunami simulations (Satake et al., 2008; Namegaya et al., 2010), but the size and mechanism of this event are uncertain.
In this study, we have estimated the source process of the 2011 Tohoku Earthquake by using both teleseismic P waves and regional strong motion data. We then considered the relation between the large slip area and interplate coupling in the source region.
2. Teleseismic Waveform Analysis
2.1 Data and method
2.2 Inversion results
3. Regional Strong Motion Waveform Analysis
3.1 Data and method
The Green’s function for each subfault was calculated by the discrete wavenumber method (Bouchon, 1981) using reflection-transmission matrices (Kennett and Kerry, 1979). The anelasticity effect was included by the use of complex velocity (Takeo, 1985). A stratified layered structure (Wu et al., 2008) was assumed in calculating the Green’s functions. The moment rate function for each subfault was expressed by 20 basic triangle functions with 8 s duration overlapping by 4 s, which can cover an 84-s rupture duration at each subfault. The maximum rupture velocity was set at 2.5 km/s to minimize variance. We used the linear multiple time window inversion method with constraints on the smoothness of the spatiotemporal slip distribution (e.g., Ide et al., 1996; Nakayama and Takeo, 1997). The smoothness parameters (hyperparameters) were selected to minimize Akaike’s Bayesian information criterion (ABIC) (Akaike, 1980; Fukahata et al., 2003). Waveforms were aligned by onset time and weights on waveforms were equal for all stations.
3.2 Inversion results
4. Discussion and Conclusions
A large slip area near, and offshore of, the hypocentral region with maximum slip exceeding 25 m is obtained by both teleseismic and regional source process analyses. The shape of the large slip area is similar in both analyses, but the points at the maximum slipped area near the trench differ somewhat (about 50 km). This might be due to the location error of these analyses. The maximum slip is larger in the regional analysis because of a spatially finer resolution. This region also coincides with the area of large coseismic slip obtained by GPS analysis (GIJ, 2011) except near the trench, where a small coseismic slip was estimated by GPS. The difference partly reflects the poor resolution of GPS data near the trench. In the tsunami waveform inversion (Fujii and Satake, 2011), the area of large slip also can be seen near the trench just east of the hypocenter. The region where tsunami back-propagation curves of initial crests are concentrated (Hayashi et al., 2011), shown in Fig. 3, is almost coincident with the area of large slip obtained in this study. These results strongly suggest the existence of a strong asperity near the trench. However, the inferred slip distribution in the southern part of the rupture area is not identical in the teleseismic and regional analyses. This suggests that the location error is greater than that in the northern part, but the slip amount we inferred is necessary to explain the strong waveform peaks at stations in the Kanto area (e.g., station HITACH in Fig. 4).
Interplate coupling in northeastern Japan has been investigated by many researchers using the GPS network. Nishimura et al. (2004) found that interplate coupling was strong during 1995–2002 in the epicentral region, where a very large slip was estimated by this study. A low ratio of the number of small repeating earthquakes to the total number of earthquakes was observed in the area of large slip (Uchida et al., 2002), which is also suggestive of strong interplate coupling in this region. These observations suggested the potential for the occurrence of earthquakes.
Aoki et al. (2011) carried out a rough estimation of the sources of high-frequency energy using the Source-Scanning Algorithm developed by Kao and Shan (2007). The short period (4–8 Hz) RMS velocity envelopes of K-NET and KiK-net stations were used. Five high-frequency sources (HFS) were imaged during this event (Fig. 5). The first HFS was in the first rupture stage, the second and third HFSs in the second stage, and the fourth and fifth HFSs in the third stage. The rupture progress of HFSs in a NS direction was almost the same as that estimated by the strong motion data analysis. The HFSs are generally located on the rim of the large slip patch obtained by the strong motion data analysis. This result is similar to that of the 1994 Sanriku-Haruka-Oki earthquake (Nakayama and Takeo, 1997).
In source process analyses with the combination of tele-seismic and regional strong motion data, we have found the following features: The main rupture is located to the shallower side of the hypocenter, and maximum slip amounts were more than 25 m. The size of the main fault was about 450 km in length and 200 km in width; the duration of rupture was more than 150 s; and Mw was 9.0. The initial rupture gradually expanded near the hypocenter (0–40 s) and subsequently propagated both southwards and northwards. But there are some differences between the two approaches. Constructing a source process model by joint inversion of teleseismic and regional strong motion data such as Yagi et al. (2004) is an important next step.
The authors thank M. Hoshiba, K. Kuge and an anonymous reviewer for helpful comments. Figures were prepared using Generic Mapping Tools (Wessel and Smith, 1995). Teleseismic seismograms were distributed by IRIS DMC. Strong motion seismograms from NIED K-NET, NIED KiK-net and the JMA network were used in this study. We used a teleseismic body-wave inversion program developed by Kikuchi and Kanamori (2003).
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