LETTER

Earth Planets Space, 63, 663–667, 2011

The 2011 off the Pacific coast of Tohoku Earthquake related toa strong velocity gradient with the Pacific plate
Makoto Matsubara1 and Kazushige Obara2
1 National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan
2 Earthquake Research Institute, the University of Tokyo, Tokyo, Japan(Received April 10, 2011; Revised May 15, 2011; Accepted May 18, 2011; Online published September 27, 2011)

We conduct seismic tomography using arrival time data picked by NIED Hi-net, including earthquakes off thecoast, outside the seismic network. For these offshore events, we use the NIED F-net focal depth. We detect
two low-V zones in the uppermost subducting oceanic crust. The landward low-V zone with a large anomaly
corresponds to the western edge of the coseismic slip zone of the 2011 off the Pacific coast of Tohoku Earthquake.
The asperities of the previously known Off-Miyagi and Off-Fukushima earthquakes with magnitudes around 7.0
are also located at the boundary of the low-V and the eastern high-V zones. The initial break point (hypocenter)
is associated with the edge of a slightly low-V and low-V p /Vs zone. The trenchward low-V and low-V p /Vs zone
extending southwestward from the hypocenter may indicate the existence of a subducted seamount. The high-Vzone and low-V p /Vs zone might have accumulated the strain and resulted in the huge coseismic slip zone of the2011 Tohoku Earthquake. The low-V and low-V p /Vs zone is a slight fluctuation within the high-V zone andmight have acted as the initial break point of the 2011 Tohoku Earthquake.
Key words: Seismic tomography, the 2011 off the Pacific coast of Tohoku Earthquake, high-V coseismic slip
zone, seamount, NIED Hi-net, NIED F-net, low-V oceanic crust, asperity.

1.

Introductionarc.

The 2011 off the Pacific coast of Tohoku Earthquake occurred on March 11, 2011. There are few studies (e.g.
Zhao et al., 2007) of the velocity structure near the Japan
trench using data from inland seismic stations. It is difficult to investigate the velocity structure beneath the oceanwith only the inland seismic stations because of the large
uncertainty in earthquake focal depth outside of the seismic
network. Zhao et al. (2007) estimated the velocity structure near the Japan trench using the focal depths constrainedby sP converted waves. The broadband seismograph network (F-net) operated by National Research Institute for
Earth Science and Disaster Prevention (NIED) determines
the focal depths of events with magnitudes larger than 3.5
using a Green’s function approach (Okada et al., 2004).
For the events distant from the seismic network, the depth
determined by NIED F-net is more reliable than that determined by the high sensitivity seismograph network (Hinet) operated by NIED, since there are no stations above
the hypocenter. However, NIED determines reliable arrival
times from those events at onshore stations. To use thosedata effectively, we combined the arrival time data picked
by NIED Hi-net and the focal depth determined by NIED
F-net. By using the distant events off the coast, we canobtain the velocity structure of the lower crust, the upper
mantle, and the plate boundary from the trench to the islandc The Society of Geomagnetism and Earth, Planetary and Space SciCopyright ences (SGEPSS); The Seismological Society of Japan; The Volcanological Societyof Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB.

doi:10.5047/eps.2011.05.018

2.

Data, Method, Resolution, and Comparison
with the Results of OBS

The target region, 20–48◦ N and 120–148◦ E, covers theJapanese Islands from Hokkaido to Okinawa. Matsubara
et al. (2008) used only the earthquakes at depths of 0–
200 km within 50 km of Hi-net stations and some eventsat depths of 200–425 km outside of the network. In thepresent study, we use the events off the coast whose focal
depths are determined by NIED F-net. The depths of theevents off the coast are not well determined with only thestations in land. NIED F-net determines moment tensor solutions using long-period (20–50 s) waves by comparing theGreen’s function database calculated with intervals of 3 indepth and 5 km in the horizontal direction (Fukuyama etal., 1998) using a layered 1-D velocity structure. Sometimes the NIED Hi-net system determines the depths of offshore earthquakes deeper than the true depths because of
the lack of stations above the hypocenters and traveltime
data of P- and S-waves observed at stations distributed only
on the west side of the hypocenters. Those events are located deeper as they get close to the trench in spite of theplate boundary becoming shallower. Earthquakes with lowangle thrust focal mechanism determined by NIED F-net
tend to get shallower close to the trench along the subducting Pacific plate. We fix the depth of those events as determined by NIED F-net. We conduct seismic tomography
with those data and relocate only the horizontal location in
the inversion. A total of manually picked 4,622,346 P-wave
and 3,062,846 S-wave arrival times for 100,733 earthquakes

663

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M. MATSUBARA AND K. OBARA: STRONG VELOCITY GRADIENT WITH THE 2011 TOHOKU EARTHQUAKE

ters at NIED (Ukawa et al., 1984) is used as the initialmodel. We do not impose velocity discontinuities in thisstudy since incorrect configuration of the boundaries would
lead to an incorrect solution.
We solve for the location and origin time of all the earthquakes as well as the P and S-wave slowness at each grid
point with more than 10 associated rays. In the final iteration, we estimate the P-wave slowness at 458,234 nodes
and the S-wave slowness at 347,037 nodes. The inversion reduces the root mean square of the P-wave traveltime
residual from 0.455 s to 0.187 s and that of the S-wave datafrom 0.692 s to 0.228 s after eight iterations.
We conduct a checkerboard resolution test in order toevaluate the reliability of the obtained solution. We assumed a ±5% checkerboard pattern and calculated synthetic traveltimes with random noise of zero mean and standard deviations of 0.05 s and 0.1 s for P- and S-waves,respectively, derived from the estimated uncertainty of thearrival times. Figure 2(a) shows the result of the checkerboard resolution test at a depth of 30 km. We identify thewell-resolved region numerically with consideration of therecovery rate and stability of the solution of the checkerboard test using the same method as Matsubara et al. (2004,
2005, 2008, 2009) and show the results in color where goodFig. 1. Distribution of hypocenters and seismic stations used in this resolution is obtained as Fig. 2(b) hereafter.
study. Black dots and brown crosses denote the hypocenters and seismic
The results of the velocity/perturbation structures within
stations, respectively. Blue lines denote the coastline and prefecture
the Pacific plate and vertical cross sections are represented
boundaries. Red lines show the plate boundaries at the surface.
(Figs. 3 and 4). The velocity perturbation is relative tothe averaged velocity at each depth. We compare the verTable 1. Grid interval and resolution size.
tical cross section of the velocity structure off-Fukushima(Fig. 4(e)) and off-Miyagi (Fig. 4(b)) with those derived
Grid interval
Resolution/Checkerboard pattern
Depth
Horizontal
VerticalHorizontal
Verticalfrom the OBS observations (Miura et al., 2003, 2005). The
0–10
2.5 km5 km
high-V uppermost mantle at the depths of 30–50 km as the
10–40
5 km
10 km
mantle wedge zone is consistent with the results from OBS
40–60
10
km
20
km
analysis (Fig. 4(e)) off Fukushima. This result supports the
0.1◦
0.2◦
60–18015 km
30 km
reliability of the current study.
180–300
300–

20 km
25 km

40 km
50 km

recorded at 1,212 stations (Fig. 1) from October 2000 to August 2009 are available for use in the tomographic method.
These stations are maintained by multiple organizations including NIED, the Japan Meteorological Agency (JMA),
and universities. The details of this method are explainedin Matsubara et al. (2004, 2005). The data set consists ofmanually picked data; NIED picks the arrival time at stations as distant from the epicenter as possible.
We follow the technique of Matsubara et al. (2004,
2005), who introduced spatial velocity correlation and station corrections to the original code of Zhao et al. (1992).
Grid nodes were placed with a separation smaller than thespatial resolution, with smoothing performed in order to stabilize the solution. The inverse problem is then solved with
the LSQR algorithm (Nolet, 1987) since we can assume arbitrary damping matrix with a combination of diagonal andsmoothing matrices.
We employ a 3D grid net to construct the velocity (slowness) structure. The grid spacing is shown in Table 1. The
1D structure used in the routine determination of hypocen-

3.

ResultWe determine the upper boundary of the Pacific platebased on the velocity structure and earthquake hypocentral distribution. The upper boundary of the low-V oceanic
crust corresponds to the plate boundary where thrust earthquakes are expected to occur. Where we don’t observe lowV oceanic crust, we determine the upper boundary of theupper layer of the double seismic zone within the high-V
Pacific plate. We assume the depth at the Japan trench as
7 km.
Figure 3 shows the map view of V P perturbation within
the Pacific plate 10 km beneath the boundary. In the inlandarea, large earthquakes occurred around the region whose
lower crust has low-V , suggesting a role of magma andfluid (Zhao et al., 2010). Then we investigate the velocity
structure within the Pacific plate such as 10 km beneath theplate boundary.
We detected two low-V zones in the oceanic crust at
the uppermost part of the Pacific plate on the west side of
the hypocenter (Fig. 3). We define one of them at longitudes of approximately 142.5E adjacent to the hypocenteras the trenchward low-V zone and another at longitudes of141.5–142.0E as the landward low-V zone. We also call

M. MATSUBARA AND K. OBARA: STRONG VELOCITY GRADIENT WITH THE 2011 TOHOKU EARTHQUAKE

665

Fig. 2. Result of checkerboard resolution test and P-wave velocity perturbation at a depth of 30 km. Black broken lines surround the resolved zone.

the high-V zone between them the ‘central high-V zone.’The landward low-V zone extends broadly and has large
anomaly; however, the trenchward low-V zone has slightanomaly and is a small fluctuation within the high-V zone.
The hypocenter of the 2011 Tohoku Earthquake is located at
the northern edge of the resolved zone; however, the structure on the southern side of the hypocenter shows that the
hypocenter is located at the boundary of the trenchwardslightly low-V and eastern high-V zone.
The vertical cross sections along the direction of the
subducting Pacific plate are shown in Fig. 4. The lowV oceanic crust at the uppermost part of the Pacific plateis imaged extending downdip from the hypocentral region
and diminishes beneath the land area (Fig. 4(a)–4(e)). The
trenchward low-V zone near the Japan trench is imaged
off Iwate, Miyagi, and Fukushima and corresponds to theoceanic crust of the Pacific plate (Fig. 4(a)–4(e)); however,there is no clear low-V oceanic crust off the boundary of
Fukushima and Ibaraki near the Japan trench (Fig. 4(f)). We
only observe low-V oceanic crust at the uppermost part of
the Pacific plate at depths of 40–60 km within the landward
low-V zone.

4.
Fig. 3. Map view of P-wave velocity perturbation within the Pacific plate
10 km beneath the upper boundary of the Pacific plate. Red star on
B–B =H–H shows the location of the hypocenter of the 2011 Tohoku
Earthquake. Small red star on G–G denotes the hypocenter of the aftershock off Ibaraki. Large and small ellipses show the location of landward and trenchward low-V zones, respectively. Blue arrows between
the ellipses denote the central high-V zone. Purple broken contours
show the area of strong energy radiation estimated with MeSO-net data
(Honda et al., 2011). Thin black line shows the Japan trench. Blacklines denote the location of cross sections in Fig. 4. Iwa: Iwate, Miy:
Miyagi, Fuk: Fukushima, Iba: Ibaraki prefecture.

Discussion

The high-V region on the east side of the hypocenter andthe central high-V region between the two low-V zones
are covered with the coseismic slip regions estimated using GPS data (Geospatial Information Authority, 2011) or
waveform data (e.g. Yagi, 2011; Shao et al., 2011). Honda
et al. (2011) estimate a rupture of the earthquake with theseismic network in the metropolitan area (MeSO-net). The
area of strong energy radiation is located at the west sideof the hypocenter offshore of Miyagi and extends eastward
and southward. The area of the strongest energy radiation isconsistent with the central high-V zone between the trench-

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M. MATSUBARA AND K. OBARA: STRONG VELOCITY GRADIENT WITH THE 2011 TOHOKU EARTHQUAKE

Fig. 4. WNW-ESE vertical cross sections of V p perturbation and V p /Vs beneath the Tohoku region. Black dots denote the hypocenters used for seismic
tomography within 20 km of the cross section. Blue lines show the upper boundary of the Pacific plate. Large stars in B–B and H–H show the
hypocenter of the 2011 Tohoku Earthquake determined by JMA. Small star in G–G denotes the hypocenter of the aftershock off Ibaraki. Circle in
D–D corresponds to the relatively large slip zone estimated by Honda et al. (2011). Arrows in the upper panels of D–D and E–E denote the locationof convex configuration of seafloor found. Arrows in the lower panels of A–A to E–E show the low-V oceanic crust.

ward and landward low-V zones (Fig. 3). The landward
low-V zone is located at the western boundary of the zoneof strong energy radiation. The asperities of the preknown
Off-Miyagi and Off-Fukushima earthquakes with magnitude around 7.0 (Sato et al., 1989) are also located at theboundary of the landward low-V zone and the central highV zone.
The initial break point (hypocenter) of the 2011 Tohoku Earthquake is located at the boundary of the trenchward slightly low-V and high-V zone (Fig. 3). There
are some seamount chains offshore Fukushima and Ibaraki(Yamasaki and Okamura, 1989). A convex seafloor configuration may indicate the existence of subducted seamounts
beneath the cross sections D–D and E–E (Hudnut, 2011)
corresponding to the trenchward low-V zone. In terms of
E–E , there is a convex undulation at the plate boundary imaged by refraction seismology (Miura et al., 2003). The
trenchward low-V region on the west side of the hypocenter has low-V p /Vs (Fig. 4(h), 4(j)). We propose that fluidwith high aspect ratio pores exists there, as proposed by
Matsubara et al. (2004, 2009) based on the result of Takei(2002). Nakamichi et al. (2007) and Kato et al. (2010)
also considered the high aspect ratio crack with low-V p /Vszone. If a large seamount has already subducted beneaththe plate, a low-V zone would be imaged (Cummins et al.,2002). The low-V and low-V p /Vs zone beneath D–D may
indicate the subducted seamount. This trenchward lowV p /Vs zone extends to the initial break point of the mainshock (B–B ).
The area of strong energy radiation also extends to thehigh-V zone on the east side of the hypocenter (Fig. 3). The
high-V zone on the east side of the landward low-V zone
and low-V p /Vs zone might have accumulated the strain andresulted in the huge coseismic slip zone of the 2011 Tohoku
Earthquake. The trenchward low-V and low-V p /Vs zone
is a slight fluctuation within the high-V zone and might
have acted as the initial break point of the 2011 Tohoku
Earthquake.
Beneath the southern Ibaraki prefecture, the low-V
oceanic crust of the Pacific plate is clearly imaged at depthsof 40–130 km (Fig. 4(g)), which is consistent with previous
studies (e.g. Matsubara et al., 2005; Shelly et al., 2006).
Matsubara et al. (2005) proposed that the subducting Philippine Sea plate distorts the corner flow induced by the Pacificplate and suppresses thermal recovery in the mantle wedge.
There is a large aftershock with magnitude 7.7 off southern
Ibaraki half an hour after the 2011 Tohoku Earthquake. The
hypocenter is also located at the boundary of the easternM. MATSUBARA AND K. OBARA: STRONG VELOCITY GRADIENT WITH THE 2011 TOHOKU EARTHQUAKE

low- and western high-V zones.

5.

Conclusion

We combine the NIED F-net hypocentral and Hi-net arrival time data and apply seismic tomography to estimate
the seismic velocity structure outside of the network. We
obtain the horizontal 20-km and vertical 10-km scale structure near the Japan trench. There are two low-V zones
within the subducting oceanic crust. The landward low-Vzone with a large anomaly is consistent with the westernedge of the coseismic slip zone of the 2011 off the Pacificcoast of Tohoku Earthquake. The asperities of the previously known Off-Miyagi and Off-Fukushima earthquakes
with magnitudes around 7.0 are also located at the boundaryof the landward low-V zone and the central high-V zone.
The zone beneath the initial break point of the 2011 Tohoku Earthquake is associated with the edge of the slightly
low-V and low-V p /Vs zone corresponding to the boundaryof the low- and high-V zone. The trenchward low-V andlow-V p /Vs materials extending to the southwest from the
hypocenter may indicate a subducted seamount. The highV zone on the east side of the landward low-V zone andlow-V p /Vs zone might have accumulated the strain and resulted in the huge coseismic slip zone of the 2011 Tohoku
Earthquake. The low-V and low-V p /Vs zone is a slightfluctuation within the high-V zone and might have acted
as the initial break point of the 2011 Tohoku Earthquake.
Acknowledgments. We used the seismic data provided by the
National Research Institute for Earth Science and Disaster Prevention, the Japan Meteorological Agency, Hokkaido University,
Hirosaki University, Tohoku University, the University of Tokyo,
Nagoya University, Kyoto University, Kochi University, Kyushu
University, Kagoshima University, the National Institute of Advanced Industrial Science and Technology, the Geographical Survey Institute, Tokyo Metropolis, Shizuoka Prefecture, the Hot
Springs Research Institute of Kanagawa Prefecture, Yokohama
City, and the Japan Agency for Marine-Earth Science and Technology. We are very grateful to David Shelly for kindly discussing
and improving our paper. We also thank William Ellsworth forthe kind discussion and suggestions. We appreciate the many suggestions from the two anonymous reviewers. This study was supported by the project on the Operation of Seismograph Networks
for the NIED. Some of the figures were drawn using Generic Mapping Tools software (Wessel and Smith, 1995) and the softwarefor viewing 3D velocity structures beneath whole Japanese Islands(Matsubara, 2010).

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