Forward
What was the Great East Japan Earthquake?
Director, International Research Institute of Disaster Science (IRIDeS)

Fumihiko Imamura
Translated by Catherine Sachi Kikuchi

Summary
Ten years have passed since the Great East Japan Earthquake, which caused
unprecedented damage. It was a complex disaster never before experienced by humankind,and there are reports that the impact still exists today. The reality of the disaster, therefore,
is still undetermined, yet collaborative research on the disaster and its impact is moving
forward. At the International Research Institute for Disaster Science (IRIDeS), we are not
only focusing on the traditional natural sciences, but also on the humanities and socialsciences, disaster medicine, and health sciences, in order to uncover other aspects of thesituation that we do not know, and to explore the essential aspects for disaster prevention
and mitigation in the future. The 51 approaches in this book are diverse and we hope they
will serve as a basis for disaster science in the future.

Keywords: interdisciplinary research, multi-stage occurrence, complex disaster, tsunami
response level, advance recovery

1: Introduction
The earthquake that occurred off the Pacific coast of the Tohoku region at around2:46 p.m. on March 11, 2011, was named The 2011 Tohoku-Pacific Ocean Earthquake by
the Japan Meteorological Agency (JMA), and the disaster caused by this earthquake wasnamed the Great East Japan Earthquake. It was the largest earthquake ever recorded inJapan (moment magnitude (Mw) = 9.0), and the subsequent tsunami spread over a widearea, causing tremendous damage in coastal regions. The images of damage in each place
were diverse and complicated, and images of the disaster changed from one moment to thenext. It was a complex disaster never before experienced by humankind. It included strong
earthquakes, tsunamis, liquefaction, landslides, and fires, as well as the nuclear power plantaccident. In the immediate aftermath of the disaster, there were a variety of names for it.
Depending on how it was perceived, it was called the Tohoku-Kanto Earthquake, the Great
East Japan Earthquake, and the 3.11 Great Earthquake. Today, the most commonly usedname is the Great East Japan Earthquake.

12: The Shape of Earthquakes and Tsunamis
Ten years have passed, and images taken at the time of the earthquake and tsunami
are becoming clearer. The huge 500 km × 200 km earthquake is estimated to have linked
with other characteristic earthquakes in areas of repeating activity and the strain energy that
had accumulated for hundreds to thousands of years were released at once. Furthermore,
the earthquakes occurred in a multi-stage rupture process from the epicenter (Hasegawa,2015). The huge tsunami that accompanied the earthquake was also complex. The locationof the fault rupture, and thus the tsunami generation, changed with the passage of time. In
the northern part of Sanriku, some tsunami waves still cannot be explained by theearthquake alone. In addition to the mechanism of huge and destructive tsunamis, thepropagation process lasted for more than two days and included black tsunamis called urban
tsunamis, river tsunamis in which the water flows backwards, shrinking and merging of thewaves within the cities, tsunami fires, etc. We need to reexamine hazard mapping,evacuation, recovery, and reconstruction based on the reality of these phenomena, now
gradually being revealed, which have not been experienced in the past (Imamura, 2015).

3: Process of Damage Occurrence
It is necessary to classify the various aspects of damage caused by the Mw 9.0
mega-earthquake and tsunami into triggers and predisposing factors. Triggers are thenatural forces (hazards) that cause disasters (which affect, damage), and predisposing
factors are the natural factors related to the nature of the earth's surface, such as
topography and ground conditions, as well as the social factors related to humans andsociety such as population, buildings, and facilities. In the case of tsunamis, triggers areclassified into three categories: inundation/crowning, currents, and wave forces.
Predisposing factors include coastal topography such as land and sea floors, land use
patterns, and protective facilities.
In addition, there are disasters caused by multiple phenomena occurring almost
simultaneously or over a period of time. A complex disaster is defined as a disaster in which
one disaster event triggers the occurrence of another. This can result in the expansion of theprimary disaster, the simultaneous occurrence of multiple disaster events, or thedevelopment of a secondary or tertiary fire damage. The images of the disaster seen in theGreat East Japan Earthquake is that of a wide-area, complex disaster.
It is also necessary to organize the time scale of disasters. Even now, aftershocks
continue to occur, and there is a risk of earthquakes, including induced ones. Although wehave yet to return to full normalcy, in addition to the continuing aftershocks, induced
earthquakes have occurred, causing damage. Moreover, crustal movement caused by faults
and changes (recovery) in coastal topography are affecting reconstruction.

4: Tsunami Response Level 1 and Level 2
In order to prevent a repeat of this tragedy, various efforts have been made to date.
These efforts can be organized into risk assessment of low-frequency mega-disasters,
enhancement of proactive measures, post-event risk avoidance systems, and efforts toimprove resilience. A typical response to tsunamis is the establishment of two tsunami

2

levels, and I believe that the roles of each level have been organized in the comprehensivetsunami countermeasures (hardware measures such as seawalls, software measures suchas evacuation and regional planning). Level 1 is the structural measures to protect humanlife and communities from tsunamis that occur once every tens to hundreds of years, while
Level 2 focuses on non-structural measures and community planning to protect lives fromtsunamis of a larger scale.
At the time, in the recovery areas, there were still issues to be addressed in thedesign of facilities (placement and height of seawalls, etc.), such as how to harmonize safetywith the environment and landscape, and how to promote consensus building in thecommunity, but I believe that these issues were the driving force behind the rapid project
implementation. However, in the future, it is necessary to discuss not only Level 1 design,
but also when and at what stage to implement maintenance (facility protection). The reason
for this is that there are many constraints and restrictions within a given project period, andthere have been changes in public sentiment and awareness of disaster prevention andsafety in the immediate aftermath of this devastating disaster and in the following years.
Therefore, it is necessary to discuss reconstruction in advance in unstricken areas, and at
this point, I would like to arrange what recovery should look like.
In order to cope with the possibility of a larger-than-expected tsunami (equivalent toLevel 2), we need to introduce probabilistic assessments that are not limited to past events,
monitor tsunamis in real time, and improve high-precision predictions and evacuation
systems. All of these measures are designed to support flexible and resourceful decisionmaking and action in response to natural disasters, which are always uncertain. This willreinforce self-help and mutual-help in disaster prevention, but it goes without saying thatindividuals and communities are the main actors and that voluntary efforts are essential.

Conclusion
In the Great East Japan Earthquake, strong earthquakes, liquefaction, landslides,
occurred, followed by tsunami inundation and flooding. This resulted in destruction of coastalstructures, tidal forests, houses, buildings, and infrastructure, along with topographicalchanges due to erosion and sedimentation. It destroyed and displaced debris, which led toaquaculture rafts and ships drifting offshore. Furthermore, spills and fires of combustible
materials, damage to transportation networks such as roads and railways (includingvehicles), and impact on facilities such as nuclear and thermal power plants also occurred. Itis thought that the majority of the complex tsunami damage patterns currently presumed toexist took place. Based on the information from natural science, disasters should beconsidered as an issue within the humanities, social sciences, disaster medicine, and healthscience. Why did the damage occur? What could have been done at the time? What isneeded in the future? While asking these questions, I’d like to summarize and propose futuredisaster prevention and mitigation. We believe that disasters will continue to evolve, and thatthe process of their occurrence and spread will become more complex through a chain ofevents.
The research and publication of this book were supported by the world’s top-levelresearch center for disaster science at a designated national university. I would like toexpress my gratitude to them.

3References
Hasegawa, A. (2015). What happened in the source area of the 2011 Tohoku-Oki
Earthquake?—The mechanism of the Tohoku-Oki Earthquake. Earthquake Journal, 60, 215. (in Japanese).
Imamura, F. (2015). Mechanism and damage prediction of giant tsunamis caused by the2011 Tohoku-Oki Earthquake, Earthquake Journal, 60, 16-23. (In Japanese).

4