Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, JapanMECHANISM OF THE 2011 TOHOKU-OKI
EARTHQUAKE: INSIGHT FROM SEISMICTOMOGRAPHY
Dapeng ZHAO1
1

Professor, Department of Geophysics, Tohoku University,
Sendai, Japan, zhao@aob.gp.tohoku.ac.jp

ABSTRACT: We relocated the great 2011 Tohoku-oki earthquake (Mw 9.0) and its
foreshocks and aftershocks using a three-dimensional seismic velocity model andhigh-quality P and S wave arrival times recorded by the dense seismic network on theJapan Islands. Then we compared the distribution of the relocated hypocenters with ahigh-resolution tomographic image of the Northeast Japan forearc. The comparison
indicates that the rupture nucleation of the largest events in the Tohoku-oki sequence,
including the mainshock, was controlled by structural heterogeneities in the interplate
megathrust zone.
Key Words: Great Tohoku-oki earthquake, seismic tomography, earthquake mechanism,
structural heterogeneity, interplate megathrust zone

INTRODUCTION
Four lithospheric plates exist in and around the Japan Islands. The Pacific plate is moving toward thenorthwest and is subducting beneath Hokkaido and northern Honshu from the Kuril and Japan
trenches, whereas the Philippine Sea (PHS) plate is moving northwestward and is descending beneathsouthwest Japan from the Nankai trough. Hokkaido and northern Honshu belong to the Okhotsk plate,whereas southwest Japan belongs to the Eurasian (or Amur) plate [Bird, 2003; DeMets et al., 2010;
Zhao et al., 2011a]. The strong interactions among the four plates cause intense seismic and volcanic
activities in and around the Japan Islands. A striking recent example of the intense seismicity in theregion is the great 2011 Tohoku-oki earthquake (Mw 9.0) that occurred at 14:46 local time (05:46
UTC) on 11 March 2011 in the forearc region of the Northeast (NE) Japan subduction zone. This giant
earthquake was caused by subduction of the Pacific plate beneath the Okhotsk plate. A large foreshockof this earthquake took place at 11:45 local time on 9 March 2011, with a magnitude (MJMA) of 7.3, as
determined by the Japan Meteorological Agency [www.jma.go.jp; Shao et al., 2011]. Following the
Tohoku-oki mainshock and on the same day, three aftershocks occurred with MJMA ≥ 7.4, and manysmaller aftershocks were recorded and located by the dense seismic network installed on the Japan
Islands.
Soon after the occurrence of these earthquakes, JMA, United States Geological Survey (USGS)[earthquake.usgs.gov] and several other research agencies published hypocentral parameters for theseearthquakes. The locations were similar, but significant differences were apparent. For example, with275

respect to the Tohoku-oki mainshock, the JMA location is: (38.103N, 142.861E, 24 km), whereas the
USGS location is: (38.322N, 142.369E, 32.0 km). The difference between them is over 50 km and wascaused by several factors, such as the differences in the arrival-time data sets and the velocity models
used for the earthquake location. The JMA hypocenters are determined using the seismic stations on
the Japan Islands and the JMA one-dimensional (1-D) velocity model [Ueno et al., 2002], whereas the
USGS hypocenters are determined with the globally distributed seismic stations and a global 1-D
velocity model [Kennett and Engdahl, 1991].
Precise hypocenters are of fundamental importance in many disciplines of seismology, being
necessary to estimate the rupture process on the fault plane, strong ground motions and crustal andupper-mantle structure in the source area, and so on. The hypocentral distribution of earthquakes in the
NE Japan forearc under the Pacific Ocean has been investigated by deploying
ocean-bottom-seismometer (OBS) stations [Hino et al., 2000; Miura et al., 2003], and using sP depth
phases detected on the seismograms from the dense seismic network on the Japan Islands [e.g., Umino
et al., 1995; Mishra et al., 2003; Wang and Zhao, 2005; Gamage et al., 2009; Zhao et al., 2009]. In thiswork we used a three-dimensional (3-D) seismic velocity model [Huang et al., 2011] to relocate thegreat Tohoku-oki earthquake and its foreshocks and aftershocks. Then we compare the distribution of
the relocated hypocenters with the tomographic image in the NE Japan forearc, which shed light on
the generating mechanism of the 2011 Tohoku-oki earthquake sequence.

DATA AND METHOD
Several permanent short-period seismic networks have been deployed on the Japan Islands, including
the JMA Seismic Network, the Japan University Seismic Network, and the High-Sensitivity Seismic
Network [Okada et al., 2004]. These seismic networks have been combined in order to constitute anationwide, high-quality network, known as the Kiban network that includes over 1,800 seismic
stations densely and uniformly covering the Japan Islands [Okada et al., 2004; Hasegawa et al., 2009].
Since October 1997 waveform data recorded by the Kiban network have been transmitted to andprocessed by JMA to monitor the seismic activity in and around Japan [Okada et al., 2004].
High-quality P and S wave arrival times are measured from the three-component seismograms with anautomatic processing system and monitored by the network staff daily. The accuracy of the arrival
times is estimated to be 0.05-0.10 s for P-waves and 0.1 s for S-waves. The arrival-time data are usedwith the JMA 1-D velocity model [Ueno et al., 2002] to determine the hypocentral parameters for each
earthquake. The resulting data base containing the P and S wave arrival times and the hypocentral
parameters is known as the JMA Unified Catalogue [Okada et al., 2004].
We used the P and S wave arrival-time data recorded by the Kiban network stations in NE Japan torelocate the Tohoku-oki earthquake sequence. Data recorded by the six OBS stations located in the
Pacific Ocean were also used, which provided valuable constraints on the locations of the suboceanic
events, particularly for those events that occurred close to the OBS stations. We relocated 5,949 eventsof the 2011 Tohoku-oki earthquake sequence from 9 March 2011, when the MJMA 7.3 foreshock
occurred, to 27 March 2011 using the high-quality P and S wave arrival-time data from the JMA
Unified Catalogue.
In this work the hypocentral parameters (i.e., the origin time, latitude, longitude, and focal depth)
were determined for each earthquake by inverting the P and S wave arrival-time data iteratively using
a least-squares method [Zhao et al., 1992, 2009]. A 3-D ray-tracing technique [Zhao et al., 1992] wasused to calculate accurate travel times and ray paths in the 3-D velocity model [Huang et al., 2011].
The changes in the hypocenters before and after relocation are smaller (< 5 km) for the events beneathland, whereas the changes become larger for events under the Pacific Ocean, being 5-30 km, inparticular, for events near the Japan Trench. After relocation most suboceanic events move toward theeast, as compared with their JMA locations [Zhao et al., 2011b]. The relocated hypocenters of the 5biggest events (MJMA ≥ 7.3) are all located at or very close to the upper boundary of the subducting
Pacific slab, which is consistent with their thrust focal mechanism [JMA, www.jma.go.jp; Lay et al.,2011; Koper et al., 2011]. For the 5 biggest events, we examined their seismograms recorded by the276

High-Sensitivity Seismic Network and searched for sP depth phases, but did not succeed because theseismograms are very complicated, possibly due to the complex rupture processes.

RESULTSThe 3-D velocity model [Huang et al., 2011] was determined by inverting 310,749 P and 150,563 S
wave arrival times from 4,655 local earthquakes that occurred in the crust and the subducting Pacificslab from the Japan Trench to the Japan Sea. Huang et al. [2011] relocated the suboceanic events
precisely using P and S arrival times as well as sP depth-phase data that were measured from theseismograms recorded by the seismic stations on the NE Japan land area, similar to the previous
studies [Umino et al., 1995; Mishra et al., 2003; Wang and Zhao, 2005; Gamage et al., 2009; Zhao etal., 2009]. The 3-D velocity structure under the Pacific Ocean and the Japan Sea was determinedreliably with a resolution of 30-40 km [Huang et al., 2011]. As compared with the previous
tomographic studies [e.g., Zhao et al., 2009], Huang et al. [2011] used many more suboceanic events
that were relocated with sP depth phase and the events are distributed more densely and uniformly inthe forearc, so they could determine the 3-D velocity structure under the Pacific Ocean more reliably.
Significant velocity variations are noticeable in the megathrust zone under the NE Japan forearc.
Three low-velocity (low-V) anomalies exist off Sanriku, off Fukushima and off Ibaraki [Zhao et al.,2011b]. There is a correlation between the velocity variation and the distribution of large earthquakes(MJMA ≥ 6.0) that occurred from 1900 to 2008, most of which are considered to be interplatethrust-type earthquakes [Umino et al., 1990; Usami, 2003; Yamanaka and Kikuchi, 2004; Zhao et al.,2009, 2011b]. These large earthquakes were located using the seismic network on the Japan Islands
and their epicentral locations are accurate to 10 km [Umino et al., 1990; Usami, 2003; Yamanaka andKikuchi, 2004]. Most of the large earthquakes are located in the high-velocity (high-V) patches or at
the boundary between the low-V and high-V zones, with only a few situated in the low-V patches[Zhao et al., 2011b].
The 2011 Tohoku-oki mainshock and its foreshock (MJMA 7.3) on 9 March 2011 are located in asignificant high-V zone off Miyagi. The northern aftershock (MJMA 7.4) that occurred at 15:08, 11
March 2011 is located at the boundary between the off-Sanriku low-V zone and a high-V zone in thenorth. The southern aftershock (MJMA 7.7) that took place at 15:15, 11 March 2011 is located at thenorthern edge of the off-Ibaraki low-V zone. Such a pattern of the hypocenter distribution for the 2011
Tohoku-oki earthquakes is quite consistent with that of the large earthquakes from 1900 to 2008. The
aftershock (MJMA 7.5) that took place at 15:25, 11 March 2011 is located east of the Japan Trench andis considered to be an outer-rise earthquake [JMA, www.jma.go.jp; Lay et al., 2011; Kanamori, 1971]
beyond the range of the 3-D velocity model.
The 3-D S-wave velocity model [Huang et al., 2011] has a lower resolution in the offshore region,but it shows the same pattern as the P-wave velocity model. In general, the low-V zones exhibit higher
Poisson’s ratio, whereas the high-V zones show lower Poisson’s ratio [Zhao et al., 2009, 2011b; Huang
et al., 2011].

DISCUSSION AND CONCLUSIONS
We suggest that the low-V patches in the megathrust zone may contain subducted sediments and fluids
associated with slab dehydration [Mishra et al., 2003; Hyndman and Peacock, 2003; van Keken, 2003;
Huang et al., 2011; Zhao et al., 2011b]. Thus the subducting Pacific plate and the overriding
continental plate may become weakly coupled or even decoupled in the low-V areas. Large-amplitudereflected waves from the slab boundary were detected in a low-seismicity area under the forearc
region off Sanriku [Fujie et al., 2002], as were some slow and ultra-slow thrust earthquakes [Heki etal., 1997; Kawasaki et al., 2001]. Both the seismic reflectors and slow thrust earthquakes are thought
to be caused by fluids at the slab boundary [Fujie et al., 2002; Kawasaki et al., 2001], and they are all
located in the off-Sanriku low-V zone. Recent studies have shown that the nucleation of large crustal

277

earthquakes in Japan was also affected by structural heterogeneities including crustal fluids [e.g., Xia
et al., 2008; Cheng et al., 2011; Padhy et al., 2011; Tong et al., 2011].
In contrast, the high-V patches in the megathrust zone may result from subducted oceanic ridges,
seamounts and other topographic highs on the seafloor of the Pacific plate that become asperities
where the subducting Pacific plate and the overriding continental plate are strongly coupled[Kanamori, 1986; Yamanaka and Kikuchi, 2004]. Thus tectonic stress tends to accumulate in thesehigh-V areas for a relatively long time during subduction, leading to the nucleation of large and greatearthquakes in those areas. The off-Miyagi high-V zone where the Tohoku-oki mainshock and its
largest foreshock occurred corresponds to the area with large coseismic slip (> 25 m) during the
Tohoku-oki mainshock [Lay et al., 2011; Iinuma et al., 2011]. This indicates that the off-Miyagi
high-V zone is a large asperity (or a cluster of asperities) in the megathrust zone that ruptured during
the 2011 Tohoku-oki mainshock.
The distribution of structural heterogeneities in the megathrust zone and its correlation with the
distribution of large thrust earthquakes suggest varying degrees of interplate seismic coupling from
north to south in the NE Japan forearc, possibly controlling the nucleation of the large interplate
earthquakes. The great 2011 Tohoku-oki earthquake sequence may be related to such a process.
Differences in interplate seismic coupling could result from variations in the frictional behavior ofmaterials [Pacheco et al., 1993; Heki et al., 1997; Kato and Hirasawa, 1997; Miura et al., 2003]. The
velocity variations in the tomographic image of the megathrust zone may be a manifestation of suchvariations in the frictional behavior [Zhao et al., 2011b].

ACKNOWLEDGMENTS
We thank the data center of the JMA Unified Catalogue for providing the high-quality arrival-time andwaveform data used in this study. This work was partially supported by the Global-COE program ofEarth and Planetary Sciences, Tohoku University, and a research grant from Japan Society for the
Promotion of Science.

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