Source model for generating strong ground motions during the 2011 off the Pacific coast of Tohoku Earthquake
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A source model for generating strong ground motions during the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) is estimated from a comparison of the observed records of the mainshock and synthetic motions, based on the characterized source model using the empirical Green’s function method. The characterized source model consists of several strong-motion-generation-areas (SMGAs) with high slip velocity superimposed upon the larger rupture area of the earthquake. The final source model has five SMGAs with different sizes, using a trial and error approach. SMGA 1 is located west of the hypocenter in the source region of Southern Sanriku-oki and SMGA 2 in that of the Middle Sanriku-oki, north of the hypocenter. SMGA 3, SMGA 4, and SMGA 5 are located near the down-dip edge of the mainshock source fault, ranging from offshore Miyagi to offshore Ibaraki prefectures. The delay times from the origin time are 15.6 sec at SMGA 1, 66.4 sec at SMGA 2, 68.4 sec at SMGA 3, 109.7 sec at SMGA 4 and 118.2 sec at SMGA 5. The probabilities of earthquake occurrence in these source regions including the five SMGAs have been estimated by the Earthquake Research Committee at less than 7% to 99% with magnitude about 6.9 to 7.5, individually, except the source region of the Middle Sanriku-oki.
Key wordsThe 2011 off the Pacific coast of Tohoku Earthquake strong-motion-generation-areas the empirical Green’s function method
The 11 March 2011 giant earthquake with Mw 9.0 occurred off the Pacific coast of Tohoku and is the largest historical earthquake to strike in or near Japan. This event had a maximum seismic intensity of 7 on the Japanese scale (corresponding to greater than 10 on the Modified Mercalli scale) at a site in Kurihara about 50 km north of Sendai. The huge tsunami generated by this earthquake struck the east-coast along Tohoku, resulting in more than 23,000 people being killed or missing. The tsunami waves were also responsible for severe accidents at the nuclear reactors in the Fukushima Daiichi Nuclear Power Plant, which further intensified the scope of the disaster.
Slip distribution models have been inverted using tele-seismic, geodetic, and tsunami data by many researchers (e.g., Fujii and Satake, 2011; Ide et al., 2011; Iinuma et al., 2011; Yagi and Nishimura, 2011; Yamanaka, 2011). However, these initial modeling results exhibit significant variability, with notably dissimilar features. For example, some of the models have the main slip distribution west of the hypocenter, while others locate it east of the hypocenter towards the Japan Trench zone. The slip distributions from the tsunami data show large slip near the Trench zone.
Strong motion data in the near-source region provide more detailed information about the shorter period rupture process during the earthquakes. In our study, we estimate a source model for generating strong ground motions from this earthquake by comparing the observed records from the mainshock with synthesized motions based on a characterized source model and the empirical Green’s function method. The characterized source model consists in several strong-motion-generation-areas (SMGAs) with large slip velocity or high stress drop, distributed across the entire rupture area of the earthquake. The goal of this study is to determine those areas in the regions from off-shore Iwate to off-shore Ibaraki prefectures that contributed most significantly to the generation of strong ground motion during this earthquake.
2. Ground Motion Data
Strong ground motions at more than 1,000 stations of K-NET and KiK-net belonging to National Research Institute for Earth Science and Disaster Prevention (NIED) and other organizations were observed throughout Japan, including near-source areas along the Pacific coast of Tohoku. Inland areas from Miyagi to Tochigi experienced very large high-frequency ground motions with peak accelerations of more than 1000 gals occurring at 20 stations (NIED, 2011). The strongest peak accelerations of 2933 and 2019 gals (composite of three components) were measured in Miyagi prefecture at the K-NET stations MYG004 and MYG012, re spectively, at the shortest distances of about 70 to 80 km from the assumed source fault.
3. Source Model for Generating Strong Ground Motions
3.1 Locations of strong motion generation areas
Based on the aftershock distribution in the first 24 hours and the CMT solution by the Japan Meteorological Agency (JMA), we define the source fault as roughly 450 km long and 200 km wide with strike of 193° and dip 10°. The hypocenter is at 38.1033 N, 142.86 E, and 23.7 km deep by JMA in Fig. 1.
We identified five wave-packets in the observed seismograms, which originate from five SMGAs on the source fault. The onsets of the wave-packets are found to propagate with a certain velocity, as shown in Fig. 3. We can estimate the locations of the SMGAs based on the onset times of the wave-packets at many stations. The origin of each wave-packet, which is the starting point of each SMGA, is theoretically determined on the source fault from the arrival times at more than three stations by the back-propagation method, taking the coordinates (x, y, and z) and rupture velocity Vr as variables and assuming unbounded media with constant S wave velocity Vs (Kurahashi and Irikura, 2010). However, for the Tohoku earthquake, strong motion data exist only on the west side of the source fault. Therefore, the initiation of each SMGA cannot be isolated at a point, but rather it is blurred along an area about 20 km in length, due to the poor spatial coverage of the observation sites.
After estimating the approximate location of each SMGA by the back-propagation method, we calculated ground motions from the SMGA assuming its area (length × width) and initiation point. We then obtained the final solutions for the area and initiation point by comparing the observed seismograms of each wave-packet and the synthetic ones at many stations using a trial and error approach. The fitting function is adopted to minimize the sum of residuals for acceleration envelopes and velocity waveforms between the observed and synthetic ones.
3.2 Characterized source model and ground motion simulation
To calculate ground motions from each SMGA, the area of the SMGA is divided into equally-sized square subfaults, the area of which is set to be the same as the small event area. The rise time and rupture velocity inside each SMGA are given to be Ws/4Vr (Ws: width of the SMGA and Vr: rupture velocity), following empirical relations by Kataoka et al. (2003). The average S wave velocity is given to be 3.5 km/s from Ts-p time versus arrival time at observed stations. The rupture velocities outside the SMGAs are estimated by the back-propagation method. The total ground motion response at each station is calculated by summing the contributions from the SMGAs with the arrival times from the initiation points of the SMGAs to the station.
The source parameters of each SMGA.
Mo (N m)
Stress drop (MPa)
Delay time from origin time(sec)
4. Discussion and Conclusions
We find that acceleration as well as velocity motions during the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) are well simulated using a characterized source model consisting of five SMGAs. SMGA 1 is located in the source region of Southern Sanriku-oki west of the hypocenter and SMGA 2 in that of the Middle Sanriku-oki north of the hypocenter. SMGA3 is located in the source region of the Miyagi-oki, SMGA 4 is located in that of Fukushima-oki and SMGA 5 is located in that of Ibaraki-oki. SMGA 1, SMGA 2, SMGA 3, SMGA 4, and SMGA 5 initiated to rupture 15.6 sec, 66.4 sec, 68.4 sec, 109.7 sec, and 118.2 sec after the rupture originated at the hypocenter. The probabilities of earthquake occurrence in these source regions including the five SMGAs have been estimated by the ERC at less than 7% to 99% with a magnitude of about 6.9 to 7.5, separately, except the source region of the Middle Sanriku-oki.
For moderate to large magnitude earthquakes (Mw < 8), we usually associate the SMGAs with large slip asperities in the characterized source model. This is based on analyses such as the source characterization by Somerville et al. (1999), which was limited to earthquakes less than about Mw 7.2. However, for great earthquakes of Mw 9.0 such as Tohoku, we recognize that the asperities defined by large slip might not always coincide with the higher frequency strong motion generation areas with high slip velocity or high stress drop.
The locations of the SMGAs coincide with large peak moment-rate areas inverted from strong motions data in the low-frequency range (0.002 Hz–0.05 Hz) by Yoshida et al. (2011), particularly along the down-dip edge of the source fault, as shown in Fig. 4. Ground motion simulation in our method shows large amplitude in the forward rupture direction because of the directivity effect but less amplitude in the backward rupture direction. This may explain why the SMGAs have only source areas west to the starting point of each SMGA, which corresponds to the forward rupture direction for the strong motion sites located in Japan. It is certainly possible that the areas of the SMGAs in our analysis might need to be extended more to the east (up-dip direction), if strong motion data were available in the offshore east of the source fault.
We used the wave data provided by the KiKnet of the National Research Institute for Earth Science and Disaster Prevention (NIED). We also used the hypocentral information from the Japan Meteorological Agency and the moment tensor solution from the F-net (NIED). We are very grateful to Dr. Robert Graves and the anonymous reviewer for reviewing the manuscript and providing valuable comments. Some figures were made using the GMT plotting package (Wessel and Smith, 1998).
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