A complex rupture image of the 2011 off the Pacific coast of Tohoku Earthquake revealed by the MeSO-net
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Strong ground motions from the 2011 off the Pacific coast of Tohoku Earthquake, the most powerful earthquake to have occurred in and around Japan after the installation of a modern seismic network, were recorded for more than 300 seconds by a dense and wide-span seismic network, the Metropolitan Seismic Observation Network (MeSO-net), installed around the Tokyo metropolitan area about 200 km away from the epicenter. We investigate the rupture process of the earthquake in space and time by performing semblance-enhanced stacking analysis of the waveforms in a frequency range of 0.05 to 0.5 Hz. By projecting the power of the stacked waveforms to an assumed fault plane, the rupture propagation image of the large and complex earthquake has been successfully obtained. The seismic energy was mainly generated from the off-shore areas of about 100 km away from the coast in Miyagi and Fukushima Prefectures. The shallow and eastern part of the fault along the Japan trench off Miyagi Prefecture released strong seismic energy which might have been related to the excitation of gigantic tsunami. In contrast, the southern shallow part of the fault plane, off Ibaraki Prefecture, released only minor seismic energy. Our analysis suggests that the focal areas combining both the officially-forecasted Miyagi-oki earthquake and those of historical earthquakes that occurred off the coast of Fukushima Prefecture in 1938 were broken, resulting in the 2011 great M 9 earthquake.
Key wordsThe 2011 Tohoku Earthquake rupture images back projection semblance MeSO-net
The 2011 Tohoku Earthquake, which caused a large number of deaths and missing people and brought about serious damage to nuclear power plants, occurred on 11th March, 2011, at 14:46 (JST). While the magnitude was first announced as 7.9 (Mjma) by the Japan Meteorological Agency (JMA), it was corrected to 9.0 (Mw) after two days. Aftershocks in the first three days occurred east off the Pacific coast in a wide region of about 400 km length and 200 km width. Strong ground motion data obtained by the NIED K-NET stations along the coastline indicated that multiple asperities ruptured during the main shock (Noguchi and Furumura, 2011).
Coastal areas have repeatedly suffered severe damage by large earthquakes in the subduction zone. For example, M 7.5, M 7.8 and M 7.4 earthquakes hit the coast of Miyagi Prefecture in 1936, 1978 and 2005. The Headquarters for Earthquake Research Promotion published a long-term earthquake hazard map where the concept of a characteristic earthquake played a substantial role. The occurrence probability of the next one during the coming 30 years has been evaluated at as high as 90% and the asperity of the 1978 event estimated by Yamanaka and Kikuchi (2003) was considered to be typical (the Headquarters for Earthquake Research Promotion, 2011). In the region of off-Fukushima, to the south of off-Miyagi, a series of M 7+ earthquakes occurred in May and November, 1938. This series of earthquakes is known as the 1938 Shioya-oki cluster event. No record of large earthquakes exists in the off-Fukushima region for at least 400 years before the 1938 event. Abe (1977) suggested that a large part of accumulated strains in the region is being released by aseismic slips. In the long-term hazard map, the occurrence probability of a large earthquake (M > 7) in this region was evaluated to be very low. On the other hand, M 7 class earthquakes have occurred repeatedly in the off-Ibaraki region at an interval of 20–30 years. The last one occurred in May 2008 so that it was considered that a large earthquake would not occur in this region in the near future. Recently, Kanamori et al. (2006) pointed out that the seismic slip rate estimated from historical earthquakes accounts only for 25% of the plate convergence rate in the off-Miyagi region. They also pointed out that the seismic slip rate in the 200 km segment to the south of the region is similarly very low. They suggested two possibilities to explain the apparent missing of the plate coupling: (1) a larger part of the relative plate motion is consumed by aseismic slips; (2) the accumulated strain will be eventually released by huge earthquakes which might cause severe damage to the Tohoku region.
The purpose of this study is to elucidate the asperity distribution of the 2011 giant earthquake. We expect that clarification of the asperities will provide valuable information to understand the scheme of plate coupling in this region as well as relations between past large earthquakes, the great 2011 event and possible future ruptures. A recently proposed promising method to investigate the earthquake rupture process utilizes array data from a dense seismic network. Ishii et al. (2005) successfully imaged rupture propagation of the 2004 Sumatra-Andaman earthquake by stacking seismic waveforms on the fault plane. Honda et al. (2008) studied the location of a minor asperity of the 2003 Tokachi-oki earthquake by applying semblance analysis. Further, applying the back-projection method to the 2007 Niigataken Chuetsu-oki earthquake, Honda and Aoi (2009) obtained the distribution of asperities and the rupture propagation velocity. These studies showed that the back-projection method is very useful in analyzing large and/or complex fault processes. We analyze the fault motion of the 2011 gigantic earthquake by applying the back-projection method to seismic data from a dense array.
2. Fault Model
In this study, we adopt the hypocenter determined by the JMA (142.861°E, 38.103°N, 23.7 km), and assume that the strike and dip angle of the fault plane are N200°E and 12°, respectively. The fault plane with a length of 600 km and a width of 270 km that covers the aftershock area is divided into subfaults with a dimension of 30 km × 30 km. In order to diminish the calculation time and memory size, we presuppose that the rupture velocity is smaller than 4 km/s and that each subfault ruptures within 80 s. In principle, these constraints are not necessary. We confirmed in a preliminary analysis that the result did not change significantly if the rupture velocity is taken to be 4.5 km/s and the duration time 140 s respectively, except that the noise level of the stacked waveforms rises somewhat.
3. Data and Method
The back-projection method is based on slant stack processing (e.g., Yilmaz, 1987). Band-pass filtered waveforms are stacked taking the travel-time difference into account after rotating the horizontal components into radial and transverse directions with respect to the line connecting stations and subfaults. The travel time for every pair of subfaults and stations was calculated using ray theory. The stacked waveform for a particular subfault is regarded as a time series of energy release at that subfault. The variation of arrival times of waves observed in the central part of the Tokyo area, where a thick sediment layer exists underneath, is large compared to that at other stations. The local velocity structure directly beneath the observation array, such as a sediment layer, could produce significant variations in the travel time which might affect the results of the stacking procedure. In order to avoid an error caused by the local velocity structure when making the back-projection image, we applied a site correction to the arrival times. The site correction for each of the stations was estimated from observed-calculated arrival time residuals in the determination of the hypocenters of earthquakes occurring around the Kanto area.
Although the high density and wide span of the observation network substantially reduce noises which come from areas other than asperities, we have introduced additional data-processing steps to improve the resolution. We have applied Nth root stacking (e.g., Rost and Thomas, 2002) and semblance-enhanced stacking (Matsumoto et al., 1999) to the waveforms to intensify coherent wave energy. First, waveforms are stacked using Nth root stacking at a certain time corresponding to the center of the semblance time window. Then, the stacked amplitude is multiplied by the semblance value calculated for the time window. We selected the 4th root in the stacking procedure and 8 s as the semblance time window. By repeating this procedure for all subfaults and times, we obtain the released seismic energy from each subfault as a function of time. Finally, integrating the absolute values of the time series of the released energy with respect to time, we obtain the average of the integrated values for the horizontal two components that are supposed to represent the total energy released from the subfault. Thus, the amplitudes in the back-projection image are considered to be proportional to the radiated seismic energy. What physical meaning should be assigned to the stacked value is still a challenging problem. Here we are interested in estimating the extent, velocity, duration of the rupture and direction of the propagation in performing the back-projection analysis.
Our results suggest that the rupture near the epicenter shortly after the nucleation of the giant earthquake was complex. The rupture propagated eastwards in the first 40 s, and extended northwards in the next 20 s, and then headed back to the epicenter. On the other hand, the propagation of the rupture during the later stage was relatively smooth. Southern parts of the fault plane ruptured 70–150 s after the initiation, and the rupture terminated in the area off the coast of Ibaraki Prefecture. The total duration time of the rupture is estimated to be about 150 s.
5. Discussion and Conclusions
We have successfully described the rupture process of the 2011 off the Pacific coast of Tohoku Earthquake applying a back-projection approach. A map of the total energy released from each subfault (Fig. 3) shows the extent of the rupture clearly. The fault plane taken in the analysis is wide enough to include the focal region of the largest aftershock. Note that the energy release from the epicenter of the largest aftershock is small. This indicates that the rupture terminated before reaching the source region of the largest aftershock. Because the MeSO-net has enough resolution in the hypocenter determination of earthquakes occurring in the area east-off Ibaraki Prefecture, we believe that the southern border of the rupture of the main shock is well delineated. On the other hand, the northern border of the rupture is somewhat uncertain. Our result indicates that large energy was released offshore of Iwate Prefecture (more than 39.5N). However, the displacement estimated from the analysis of ocean bottom pressure sensors in that area was not so large (Maeda et al., 2011).
The focal area of the expected Miyagi-oki earthquake was broken 40–60 s after the initiation of the rupture, although the contribution of the radiated energy to the total energy release was not so large. The source region of the 1938 Shioya-oki cluster event where it has been considered that most of the accumulated strains are released by aseismic slips (Abe, 1977) also ruptured during this M 9 event about 90–130 s after its initiation. The source region of the preshock seems to have released seismic energy as well, although the spatial resolution inferred from the grid spacing of 30 km may be not sufficient to conclude this convincingly. It should also be added that the source region of the largest aftershock, off-Ibaraki Prefecture, overlaps partly with the focal region of the earthquake which occurred in May 2008. These observations indicate that large or significant seismic energy was released from the regions where accumulated strains are supposed to have been already released by recently-occurred earthquakes or by aseismic slips.
From the point of view of rupture propagation, the complexities in rupture velocity and the directivity are remarkable. The rupture around the hypocenter in the early stage was especially complicated, while the southward propagation of the rupture in the later stage was relatively fast and smooth. The average propagation velocity of the rupture estimated from the rupture time (i.e., the time that energy was mostly released) at each subfault is considerably small.
We feel these observations contribute to understanding the mechanism of earthquake occurrence in the subduction zone. One other aspect worth mentioning is that we do not know, at the present time, whether most of the accumulated strain energy in these source regions has been released by the gigantic event or whether a substantial amount still remains.
We would like to thank Dr. Takuto Maeda for his valuable comments about tsunami source regions around Japan. We are very grateful to Professor Yomogida for his valuable advices, especially about the Shioya-oki event. We thank Dr. M. Mai and an anonymous reviewer for carefully reviewing our manuscript and making many helpful comments. The present work is partly supported by the Special Project for Earthquake Disaster Mitigation in Tokyo Metropolitan Area of the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT)
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