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General Perspectives

The Highest Magnitude Ever Recorded

The 2011 off the Pacific Coast of Tohoku Earthquake (hereafter, the “2011 Tohoku-Pacific Earthquake”) that occurred on March 11, 2011 (at 14:46) was an inter-plate earthquake, occurring on the boundary between the Pacific plate and the Continental plate.

The magnitude of the 2011 Tohoku-Pacific Earthquake was reported as being 9.0, the highest magnitude ever recorded in Japan. The scale of this event ranks fourth in the world. The fault plane extends to about 500 km in a North-South direction (length) and about 200 km in an East-West direction (width). The scale of the 2011 Tohoku-Pacific Earthquake stands out in comparison even with some of the most destructive past earthquakes.


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Comparison of the Size of Fault Planes of Past Destructive Earthquakes
MJ: Magnitude by Japan Meteorological Agency**
Mw: Moment Magnitude
** M means MJ (JMA magnitude), hereafter.


Prior Estimation of Earthquake Occurrence

The Headquarters for Earthquake Research Promotion had estimated the occurrence probability of large events based on the seismic activity along the Japan Trench, from off-shore of Sanriku to off-shore of the Boso peninsula.

All of the areas of (1) – (6) shown in the figure are considered to have ruptured together, to generate the 2011 Tohoku-Pacific Earthquake. The area (1) that ruptured first among the four areas was estimated to have a high occurrence possibility of a large earthquake over the next 30 years, as was its neighboring area (2).


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Estimation of the Occurrence Probability of Earthquakes in each Area
(By the Headquarters for Earthquake Research Promotion)


Aftershock Sequence

Before and after the main shock (on March 11, 14:46), lots of earthquakes occurred in a broad area, from off-shore of Sanriku to off-shore of the Boso peninsula shown in the figure. Of the earthquakes that had occurred by April 11, the magnitudes of five earthquakes were larger than 7. A large earthquake of M7.3, considered now to have been a foreshock, occurred off-shore of Sanriku on March 9, two days before the main shock. Two large inland earthquakes occurred after the main shock. One was the M6.7 earthquake on March 12 in the northern part of Nagano Prefecture, and the other was the M6.4 earthquake on March 15 in the eastern part of Shizuoka Prefecture. Both recorded 6+ in seismic intensity.

In the past, some destructive inland earthquakes have occurred before and after large events on plate boundaries, as occurred with this earthquake, indicating that attention has to be paid to aftershocks surrounding the fault area of main shock, as well as to inland earthquakes.


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Distributions of Hypocenters (M5) between March 9 through March 20, with the Hypocenters of the Two Large Aftershocks (M7.1, April 7 and M7.0, April 11)


Strong Shaking across a Broad Area

The huge scale of the 2011 Tohoku-Pacific Earthquake produced strong shaking across a broad area. The area with seismic intensity larger than 6- extended to 450 km, and the area with seismic intensity larger than 6+ extended to 300 km. The area with high seismic intensity due to this earthquake was much broader than those in recent inland destructive earthquakes, as is shown in the figure.


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The Spatial Distribution of the Areas with Seismic Intensity larger than 5+
(In comparison with recent destructive inland earthquakes)


Long Duration of Ground Motion

The observed ground motions had a definite feature of a long duration because of the large scale of the fault plane, compared to those of some previous destructive earthquakes, as is shown in the figure.

Seismic hazards are sometimes investigated in terms of peak values of ground motions, such as Peak Ground Acceleration (PGA), Peak Ground Velocity (PGV) etc. The length of ground motion duration might be another key factor in considering the seismic hazards.


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A Comparison of the Observed Records at the K-NET Sendai (NS) and the K-NET Shiogama (EW) of the 2011 Tohoku-Pacific Earthquake with those from Previous Destructive Earthquakes.


Period-dependency of Ground motions

The response spectra** of the observed records at Shiogama City (K-NET Shiogama) and Sendai City (K-NET Sendai) of the 2011 Tohoku-Pacific Earthquake were compared with those from previous hazardous earthquakes, as shown in the figure.

The level of response spectrum of the observed record of the K-NET Shiogama of this earthquake was similar for the 0.6 to 2.0 sec period and larger for other periods compared to the record of Shiogama site of the 1978 Miyagi-Oki earthquake. The level of the K-NET Sendai was even larger for longer than the 0.5 sec period than that of the K-NET Shiogama. However, the level of the K-NET Sendai was smaller than that for Fukiai, Kobe city in the 1995 Hyogo-ken Nanbu earthquake for longer than the 1 sec period.

** The response spectrum represents the maximum responses of a building subjected to ground motion with varied fundamental natural periods.


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A Comparison of the Response Spectra of the K-NET Shiogama (EW component) and K-NET Sendai (NS component) with those from some Previous Hazardous Earthquakes.


Relationship between Ground Motion Levels and Soil Conditions

A comparison of observed seismic intensity levels with the soil conditions on each site revealed that sites with soft soil were likely to have a higher intensity level because soft soil is easily shaken. This indicates that knowledge of the soil conditions of a site is useful in predicting the level of ground motion there.


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Comparison of the Distributions of the Observed Intensity of the K-NET and KiK-net sites by the National Research Institute for Earth Science and Disaster Prevention (NIED) with the Averaged Shear-wave Velocities of Surface Layers (upper 30 m )


Soil Liquefaction

Liquefaction Observed

Soil liquefaction describes a phenomenon whereby saturated sandy soil substantially loses strength and stiffness in response to applied stress, usually shaking due to an earthquake, causing it to deform and behave like a liquid. The phenomenon is most often observed in sandy grounds. The area affected by the 2011 Tohoku-Pacific Earthquake was a broad area, from the eastern part of Miyagi Prefecture (relatively close to the epicenter and its surroundings) to the southern part of Kanto region including the Tokyo Bay area, as shown in the figure. The soil effects of liquefaction were observed in reclaimed areas around bays, former wetlands including old lakes or swamps and rice paddy fields, former river courses developed for housing lots and the areas near rivers. Liquefaction resulted in much of the destruction associated with this earthquake, such as sand/mud boiling, ground cracking, depressions of pavement, ground settlement, leaning buildings and the exposure of underground pipes.


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Distribution Map of Points where Liquefaction was reported


Causes of Liquefaction with Lower Levels of Ground Acceleration

One of the ground-motion characteristics of the 2011 Tohoku-Pacific Earthquake was the long duration of seismic motions, or the large number of waveforms observed, which was higher than massive earthquakes in the past. For the 1995 Hyogo-ken Nanbu Earthquake, the duration was about 20 seconds, compared to about 120 seconds in the Tokyo Bay area affected by this earthquake. This phenomenon caused severe liquefaction effects for the ground in the Tokyo Bay area, supposedly with relatively low levels of ground acceleration (the value of ground surface acceleration = approximately 120-150cm/s2). Furthermore, the 7.7 moment magnitude (Mw) aftershock which occurred about 30 minutes after the main earthquake caused more liquefaction (especially in the eastern Kanto region).


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Acceleration Waveforms of the 2011 Tohoku-Pacific Earthquake and Past Earthquakes


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