The 11th March 2011 great mega-thrust Japan Earthquake (Mw 9.0)
A tsunamigenic mega-thrust earthquake of magnitude Mw 8.9 rocked the east
coast of Honshu (38.32N; 142.37E), Japan on 11th March, 2011 at 05:46:23.7 UTC at a
depth of about 24.5 km beneath the ocean (USGS). The earthquake was located about
130 km east of the Sendai and about 430 km northeast of Tokyo (USGS, IRIS). The
earthquake was so powerful that a killer tsunamis wave generated at a height of about
30 – 35 ft, which caused a huge devastation in the entire coastal belts of Miyagi,
Fukusima and Sendai city of Northeast Japan. This is the greatest earthquake so far
recorded in the history of Japan seismology. Maximum damage was reported by several
agencies of Japan and overseas to show the damage in the coastal belt of NE Japan was
mainly due to tsunamis and the subsequent fire that broke out due to strong shaking and
tsunamis. There is a report on nuclear emission and leakage after the mega-thrust
earthquake. This earthquake was associated with tremendously high seismic moment
that was capable to displace the original location of the northeast part of the Honshu
Island.
Seismotectonically, the northeast Japan Forearc has very complex tectonic
settings, which has been surrounded by the ‘Ring of Fire’. The forearc region of
northeast Japan is a region between Japan magmatic front and the Japan Trench, a site
of several great interplate past damaging earthquakes (Nagai et al., 2001) due to
continuous subduction of the oldest, the coldest, and the biggest Pacific Sea Plate
beneath the Island. Statistical analyses of earthquakes for the forearc region revealed
that about 80% of earthquakes occur in the forearc region (Mishra, 2004).
Additionally, Japan Island is highly vulnerable to all kinds of earthquakes of varying
strengths because of existence of four major and mobile tectonic plates (Pacific Sea
Plate, Philippines Sea Plate; North American Sea Plate, and Eurasian Sea Plate)
beneath the Island. The Island is associated with a series of transform faults and the
spreading ridge.
It is worth to mention that the occurrence of such great devastating earthquakes
is confined to the Forearc region of Northeast (NE) Japan for which a detailed 3-D
seismic tomography outside the seismic network, for the first time, has already been
assimilated from the Pacific coast to the Japan Trench using a large number of highquality arrival times from sub-oceanic earthquakes that were well relocated with sPdepth phase data, a novel technique by a scientist of Geological Survey of India under
his research endeavour in Japan that led to an ward of Doctor of Science (D.Sc.) to Dr.
O. P. Mishra from the Geodynamics Research Centre, Ehime University, Japan, which
is evident from the outstanding publications by Mishra et al. (2003) [see the attached
PDF] and Mishra and Zhao (2004) [see the attached PDF]. This detailed study on
NE Japan forearc region demonstrated why and how the great mega-thrust
tsunamigenic, and other past non-tsunamigenic earthquakes occur beneath the ocean.
The study demarcated several high velocity zones and zones between low and highvelocity anomalies in the NE Japan Forearc, suggesting probable locations for great
megathrust earthquakes of magnitude greater than 7.0 beneath the forearc (Mishra et
al., 2003). The region has already been associated with several past damaging interplate
earthquakes having heterogeneous rupture dimensions that overlap one after another
(Nagai et al., 2001; Mishra et al., 2003). The study revealed that varying degree of
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interplate seismic coupling; strong heterogeneities; dehydration embrittlement, and
continuous weakening of the subducting Pacific Sea slab due to its serpentinization are
the plausible cause of occurrence of such great devastating mega-thrust earthquakes in
past (Mishra et al., 2003; Mishra and Zhao, 2004) as well the cause of the greatest
mega-thrust earthquake (Mw 8.9) that rocked the NE Japan forearc on the 11th March
2011.
Geological Survey of India attempted to see the record of the Japan mega-thrust
earthquake (Mw 8.9) and its aftershocks that rocked the NE Japan. A team of
seismologists from Geo-Seismology Division, GSI (CHQ), Kolkata rushed to all three
seismological observatories of Geological Survey of India that located at Adampool,
Sikkim (27.3065N; 88.5828E); Agartala, Tripura (23.865N; 91.2975E); and Itanagar,
Arunachal Pradesh (27.0858N; 93.5961E) to collect recorded seismograms of these
events. It is found that seismological observatories of GSI successfully recorded the
event, whilst the estimate of earthquake location by the team suggests that the recorded
event corresponds to the first great Aftershock (Mw 7.1) that occurred about an hour
(06:25:51.4 UTC) after the Mainshock earthquake (Mw 8.9) [USGD, IRIS]. These
seismograph stations of GSI, however, missed recording of the mainshock earthquake
because of our instrumental limitation required for recording the events of magnitude
≥8.0, which might have attained saturation beyond the amplitude of the seismic wave
that corresponds to the earthquake of magnitude (Mw ≥8.0). Up gradation of the
existing seismological observatories with 240 sec broadband in near future will enable
to record such great earthquakes from all over the globe. It is however, all three
observatories of GSI recorded the aftershock (Mw 7.1) (USGS, IRIS). The location by
multi-station method using the recorded seismograms by all of three above mentioned
seismograph stations ascribed to GSI gives acceptable parameters of location of the
aftershock with poor constraints on its depth because of its outside occurrence from the
seismic network used in our location (Figure 1). The 1-D velocity model of Mishra
(2004) used in the location program.
Figure 1: Recorded seismograms by all three seismograph stations of GSI and multi-station
method of locating the first great aftershock of the Japan mega-thrust earthquake (Mw 8.9).
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Seismological Research team of Geological Survey of India estimated the
location, magnitude and origin time of the first big aftershock of the great mega-thrust
mainshock earthquake and they found the location was very much closer to that
estimated by other agencies (e.g., USGS, IRIS) (see Table 1).
Table 1: Earthquake Location Parameters by different Agencies
Date of
occurrence
Latitude
Longitude
Depth
(km)
Magnitude
Agencies
11.03.2011
Origin
time
(UTC)
05:46:23.7
38.32N
142.37E
24.5
Mw 8.9
11.03.2011
06:25:51.4
38.07N
144.56E
26.5
Mw 7.1
11.03.2011
05:46:23
38.36N
142.54E
05.00
Mw 8.6
11.03.2011
05:46:19.82
38.088N
143.580E
4.50 ??
Ms 7.4
USGS /
IRIS
USGS /
IRIS
INCOIS,
India
GSI,
India
The interpreted results of earthquake location are shown in Table 2,
demonstrating a slight difference in epicentre co-ordinates, origin time, and magnitude
estimate by different agencies, which may be due to several reasons, such as difference
in the instrument response function, attenuation factor, 1-D velocity model and picking
accuracy of the direct phase arrivals (P & S-wave) used during location of the event.
Precise location of earthquakes in terms of its depth and epicentre co-ordinates may be
achieved by deploying sP-depth relocation technique as used by Zhao et al. (2002) and
Mishra et al. (2003) for the outside seismic network technique in different tectonic
environ (Mishra et al., 2010).
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Figure 2: A map showing the locations of the great mega-thrust earthquake (Mw 8.9) as shown by
the big golden star and its first great aftershock (Mw 7.1) as shown by the small red star.
Table 2: The interpreted results of Location parameters by the GSI team
References:
IRIS, http: //www.iris.edu
Nagai, R., M. Kikuchi, and Y. Yamanaka, Comparative study on the source process of
recurrent large earthquakes in Sanriku-oki Region: The 1968 Tokachi-oki earthquake
and the 1994 Sanriku-oki earthquake (in Japanese with English abstract), Zisin, 54,
267– 280, 2001.
Mishra, O. P., D. Zhao, N. Umino, and A. Hasegawa, Tomography of northeast Japan
forearc and its implications for interplate seismic coupling, Geophysical Research
Letters, 30 (16), 1850, doi:10.1029/2003GL017736, 2003.
Mishra, O. P., Lithospheric heterogeneities and seismotectonics of NE Japan forearc
and Indian regions, D.Sc. thesis, Ehime University, 223p.
Mishra, O. P., and D. Zhao, Seismic evidence for dehydration embrittlement of the
subducting Pacific slab, Geophysical Research Letters, 31, L09610,
doi:10.1029/2004GL019489, 2004.
Mishra, O. P., D. Zhao, C. Ghosh, Z. Wang, O.P. Singh, B. Ghosh, K. K. Mukherjee,
D. K. Saha, G.K. Chakrabortty, and S. G. Gaonkar, Role of crustal heterogeneity
beneath Andaman–Nicobar Islands and its implications for coastal hazard, Natural
Hazards, DOI 10.1007/s11069-010-9678-3, 2010.
USGS: www.usgs.gov
Zhao, D., O. P. Mishra, and R. Sanda, Influence of fluids and magma on earthquakes:
seismological evidence, Physics Earth Planetary Interiors, 132, 249 – 267, 2002.
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