REPORT ON THE TOHOKU AREA PASIFIC OFFSHORE EARTHQUAKE GENERAL PERSPECTIVE 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. 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). 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. 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.
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 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. 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.
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.
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).
TSUNAMI The 2011 Tohoku-Pacific Earthquake caused tsunami across a very wide area, mainly along the Pacific coastline from Hokkaido to the Kanto area. Tsunami, whose wave height were more than 1m, were observed not only in the Hokkaido, Tohoku and Kanto regions, but also in the Tokai, Shikoku and Kyushu regions. Especially in Iwate prefecture and the northern area of Miyagi prefecture, the inundation height and the run-up height of tsunami exceeded 10m. Definitions of Tsunami Height The three different values of wave height, inundation height and run-up height are often used as definitions of tsunami height, all of which usually take their point of reference from the ordinal sea level when the tsunami arrives. The Inundation Area of the Tsunami Disaster of the 2011 Tohoku-Pasific Earthquake One of the features of the tsunami disaster that accompanied the 2011 Tohoku- Pacific Earthquake was that the inundations on land spread out quite widely across the Tohoku region. The table below shows the inundation area in each prefecture of the Tohoku and Kanto regions that were promptly reported by the Geospatial Information Authority of Japan (GSI). Especially in the Sendai plain in Miyagi Prefecture, the inundations reached to about 4 to 5km inland from the coastline. Inundation Area of Tohoku Region (Prompt report by GSI)
Comparisons with Past Tsunami in Northeastern Japan The figure below shows a comparison with the run-up heights of past tsunamis that have attacked northeastern Japan. The tsunami that accompanied the 2011 Tohoku- Pacific Earthquake had relatively higher wave heights and occurred over a wider area compared to tsunamis in the past.
GROUND MOTION SIMULATIONS Ground Motion Simulation for the 2011 Tohoku-Pasific Earthquake The 2011 Tohoku-Pacific Earthquake produced large ground motions including long period motions even in the Tokyo Metropolitan area. We have tried to reproduce the observed records of this earthquake by using the method that we developed (Satoh et al., 2008) for ground motion simulation. Two fault planes are assumed for the simulation of the current earthquake. The results of simulations for two sites agree well with the observed records. Ground Motion Simulation for a Hypotetical Tokai-Tonankai-Nankai Earthquake The same method has been applied to simulate the ground motions of a hypothetical Tokai-Tonankai-Nankai earthquake (Mw 8.7) for the site (TKY016, K-NET Shinonome station) located in the Tokyo Metropolitan area. Even though the magnitude of this hypothetical earthquake is smaller than that of the 2011 Tohoku Earthquake, the simulated ground motion has larger components beyond a 4 sec period than the observed records of the 2011 Tohoku-Pacific Earthquake.
EFFECTS OF SEISMIC ISOLATION Overview of the Four Seismic Isolated Buildings Three Seismic Isolated Buildings at the Institute and the Test Building at Tohoku Univ. Three different types of seismic isolated buildings stand in the grounds of the Shimizu Corporation s Institute of Technology in Tokyo. The effects of applied seismic isolation methods have been verified through the observed earthquake responses of the three buildings subjected to the 2011 Earthquake off the Pacific coast of Tohoku (hereafter, the 2011 Tohoku-Pacific Earthquake ). Shimizu Corporation and Tohoku University jointly built the test building for seismic isolation within the Sendai campus (in Miyagi prefecture), and have carried out seismographic observations since 1986. The effects of the applied seismic isolation method have been confirmed through the observed records of the test buildings that stood near the epicenter.
Observed Earthquake Responses of the Four Seismic Isolated Buildings The Effects of Seismic Isolation in the 2011 Tohoku-Pacific Earthquake In each of the three seismic isolated buildings in the institute (the main building, the safety & security center and the wind tunnel testing laboratory), the observed accelerations on the floors were reduced to about half compared to those on the ground. In the test seismic isolated building in Tohoku University, the observed accelerations on the roof were reduced to about one third compared to those in the adjacent conventional seismically designed building.
Verification of the Effect of Seismic Isolation by Simulation Analysis Response Analysis of the Main Building in the Institute Earthquake response analysis of the main building in the institute was carried out using the observed seismographs of the 2011 Tohoku-Pacific Earthquake. The results of the response analysis agreed well with the observed values, which demonstrate the validity of the analytical model and simulation method.
Estimated Effect of Seismic Isolation against Anticipated Great Earthquake Estimated Effect of Seismic Isolation of the Main Building against a North Tokyo Bay Earthquake or a Kanto Earthquake Earthquake response analysis of the main building in the institute has been carried out for the anticipated great earthquakes in the Tokyo metropolitan area to show the estimated effect of seismic isolation. It is supposed that the main building could be struck by either a North Tokyo Bay Earthquake (a near-fault earthquake) or a Kanto Earthquake (an inter-plate earthquake, whose seismic source is in the Sagami trough). The results of simulation analyses has revealed that the maximum accelerations of the upper building would be reduced from one third to one-fourth compared with those on the ground, showing the great effects of seismic isolation. A Study on the Effect of Seismic Isolation The Effect of Seismic Isolation in Decreasing the Risk of Overturning Furniture In each of the three seismic isolated buildings in the institute (the main building, the safety & security center and the wind tunnel testing laboratory), there was no
overturning of equipment in the rooms nor mess on the tables due to the 2011 Tohoku-Pacific Earthquake. In accordance with the developed evaluation criteria of overturning, the levels of the maximums accelerations of the floors of the three seismic isolated buildings could be evaluated to be within the range of no danger of overturning, which agrees well with the real situation in the buildings.