Chapter 3
The Science of Tsunami History
Field of expertise: Geology

Daisuke Sugawara
SummaryTsunami deposit research deciphers the history of past tsunamis from sedimentary records andbenefits to improve our understanding on the recurrence intervals of tsunamis for regions or
periods without sufficient instrumental and written records. What has been learned from the 2011
Great East Japan Earthquake is that if the findings from tsunami deposit research had been
applied to the practices for disaster prevention activities, the damages might have been much
reduced. During the last 10 years, researchers have been dedicating a lot of effort to ensure they
do not overlook unexpected tsunami hazards again.
Keywords: geological record, tsunami deposit, Miyagi-oki earthquake, Jogan earthquake,supercycle model

Introduction
What is the key to predicting and preparing for future tsunami disasters? Given that theearth’s structure, dynamics and principles for physics and chemistry are invariable throughout
space and time, we can acquire useful implications for the future from past records, including
historical documents, archaeological sites and relics, sedimentary deposits and fossils. Such
records are sparsely scattered in space and time, and information tends to be more vague in the
deeper past. Nevertheless, we have to prepare for future disasters. How far back will we need tolook for records to help with this?

1: Problems Revealed by the Great East Japan Earthquake
What happened?
The Pacific coast of Tohoku, which is facing the Japan Trench, has repeatedly beendamaged by past tsunamis. The oldest historical record dates back to the Heian Period (ca. 794 ~1192 C.E.), but the quality and quantity of the records before the Edo Period (1603 ~ 1868 C.E.)
are quite limited. Before the 2011 Great East Japan Earthquake, the Sanriku coast was well known
for its tsunamis. The 1896 Meiji Sanriku earthquake (Mw 8.1) triggered a huge tsunami thatreached a height of 38.2 meters at Ryori Bay in Ofunato City, Iwate Prefecture. The earthquakeand tsunami brought the worst tsunami disaster in Japan and claimed 21,753 lives. The 19331

Showa Sanriku earthquake (Mw. 8.5) also triggered a huge tsunami that recorded the maximum
runup of 28.7 meters at Ryori Bay. The earthquake and tsunami caused 3,064 deaths and missing
persons across the devastated areas. Since then, the Sanriku Coast has been considered to beprone to tsunamis and this promoted the construction of protection walls. The tsunami triggered bythe 1960 Chilean earthquake (Mw 9.5) traveled across the Pacific for 23 hours and struck theSanriku and other coasts in Japan. A maximum runup of 8.1 meters was recorded at Noda Village
in Iwate Prefecture. The tsunami again caused considerable damages with 116 deaths andmissing in Iwate and Miyagi prefectures. Total number of victims was 139 in the country.
During the period from 1897 to 2005, four major earthquakes occurred off the coast of
Miyagi Prefecture (Miyagi-oki earthquakes). According to research on historical and instrumental
records, the Miyagi-oki earthquakes have been considered as a characteristic earthquake in thisregion, with a magnitude ranging from 7.1 to 7.4 and an averaged recurrence interval of 38 years.
In general, tsunamis from the Miyagi-oki earthquakes were minor. For example, the tsunami height
due to the 1978 Miyagi-oki earthquake was around 1 meter within and to the south of the Sendai
Bay. In 1793, a large-scale earthquake occurred off the coast of Miyagi and triggered a tsunami
with heights ranging from 2 to 5 meters that struck the coasts of Iwate, Miyagi, and Fukushima.
The rupture area of the 1793 earthquake has been considered to have extended from off the coastof Miyagi to near the Japan Trench, and the magnitude was estimated at M 8.0. Nevertheless,damages from the tsunami were not so extensive. Before 2011, a Miyagi-oki earthquake wasconsidered to have been quite imminent, and the 10-year probability of occurrence was estimatedat 70% based on the historical and instrumental records. On March 9th in 2011, an earthquakewith Mw 7.3 occurred offshore of Miyagi Prefecture. The focal region and magnitude of theearthquake were within the scope of the supposition. People might have deemed that the threats
from the Miyagi-oki earthquake had passed. Two days later, however, people were stricken withthe unexpected disaster due to the Mw 9.0 off the Pacific coast of Tohoku earthquake. The officialname of the earthquake determined by the Japan Meteorological Agency is long, so it is often
abbreviated as ‘Tohoku earthquake’ or ‘Tohoku-oki earthquake’. In this article, we use the latter
abbreviation.
The reality of the damage
The size of a Mw 9.0 earthquake is 180 times greater than that of a Mw 7.5 earthquake, if
we compare by using the seismic moment. The tsunami triggered by the Tohoku-oki earthquake
reached 40.1 meters at Ryori Bay, which exceeded the record set at the time of the 1896 Meiji
Sanriku earthquake. The generation mechanism of the gigantic tsunami has been explained by ahuge slip in the shallow portion of the plate boundary superimposed on a large fault slip in thedeeper portion. The Sanriku Coast again suffered from devastating damages due to the Tohokuoki tsunami, although many countermeasures had been taken during the last decades to prevent
tsunami disasters. The size of the Tohoku-oki tsunami was unexpectedly large throughout thePacific coast of East Japan. The gigantic tsunami brought both direct and indirect damages to theextensive areas with all possible processes, which resulted in a complex disaster.

2: Paradigms Destroyed by the Earthquake
Conventional wisdom and necessary responses
Before 2011, attempts to predict future major earthquakes had been based on ‘reliable’
information, such as historical documents since the Edo period and modern instrumental data from2seismology and geodetics. Few researchers have pointed out that the strain rate of the Tohoku
region estimated by geodetic observations (a time span of 100 years) is one order greater than thatby geologic observations (several thousands of years). This discrepancy was explained by a strain
release due to a giant earthquake; unfortunately, the pre-disaster earthquake predictions did nottake this into account. The occurrence of the Mw 9.0 earthquake demonstrated the validity of theview of the discrepancy in the strain rate. After the disaster, the supercycle model was introduced
to explain the occurrence of a giant earthquake. According to the model, a major earthquake does
not release all of the strain caused by the motion of subducting oceanic plates. Residual strains areaccumulating during a period of repetition of several major earthquakes which may have arecurrence interval of 10’s to 100’s of years. After this time frame, a large amount of strain
accumulated between the plate interfaces is finally released by a giant earthquake. Records with amuch longer time span are needed to understand the possibility of the giant earthquake. Althoughsuch records were available at that time, they were almost overlooked.

3: A New Approach
The Great East Japan Earthquake has been considered to be a recurrence of a once-in-amillennium disaster, because of the similarity to the 869 CE Jogan earthquake and tsunami thatstruck the ancient Tohoku region. A historical document named Nihon Sandai Jitsuroku (theVeritable Records of Three Reigns of Japan) tells of damages due to the Jogan earthquake andtsunami at the Tagajo castle in the Sendai Plain. Despite its older age, the descriptions are clear;
however, it does not allow us to read further details about the earthquake and tsunami, such as thewave height, inundation area and earthquake magnitude. Thus, the size and spatial extent of theearthquake and tsunami deduced from the historical document and other relevant legends andfolklore have long been a subject of debate among researchers. Surprisingly, much of our current
understanding on the earthquake and tsunami are based on the findings from tsunami deposits,
which are the geological traces of past tsunami inundations. Geological studies of the Jogan
tsunami deposit were initiated in the late 1980s in the Sendai Plain. Until 2010, researchers havediscovered the Jogan tsunami deposit from the coastal plains of Sendai Bay and Joban Coast withan estimated inundation distance of more than 4 kilometers from the paleo-shoreline. An intraplate
earthquake greater than Mw 8.4 was assumed for the tsunami source, based on the comparison of
spatial extent of the tsunami deposit and inundation area estimated from the numerical simulations.
In the Sendai Plain, 4-5 layers of tsunami deposits, including from the Jogan event, have beenfound within the coastal sedimentary sequences during the last 3000 years. It means therecurrence interval of the tsunamigenic earthquake can be 500 to 800 years. If such findings fromthe tsunami deposits were considered for the attempts of earthquake predictions, damages fromthe Great East Japan Earthquake might have been reduced. Immediately after the earthquake, thelessons learned from the disaster have been implemented to the government’s objective fordisaster management. During the last 10 years, investigations of tsunami deposits have beenpromoted in many coastal areas of the country. Findings from tsunami deposits are going to beconsidered in countermeasures for future earthquakes and tsunamis.

3Figure 3-1. Tsunami deposits found from an archeological site in Iwanuma City, Miyagi Prefecture.
Three layers of sandy tsunami deposits, which correspond to the 2011 Tohoku-oki, 1611 Keicho
Oshu and 869 Jogan tsunamis have been identified at this site.

4: Achievements and the Future
A new approach to disaster science
Whether or not tsunami deposits can be formed and preserved as a sedimentary record
depends on environmental factors, such as local topography and geology, in addition to thetsunami size. Field investigation taken after the Great East Japan Earthquake revealed thattsunami-induced sediment erosion may erase earlier tsunami records. This means that
sedimentary records are not necessarily continuous; there may be missing intervals within therecords. To compensate for that and to better determine the timing and spatial extent of thetsunamis, researchers are attempting to discover more tsunami deposits and correlate them on aregional scale. The International Research Institute of Disaster Science (IRIDeS) developed an
open database that combines historical and geological records of past tsunamis in Japan based on
old documents, articles, research reports and papers (Tsunami Trace Database). This will help toscreen potential areas, in which information on tsunami deposits is not yet sufficient and newfindings from the area are indispensable for reconstruction of paleotsunami history.

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Figure 3-2. A screenshot of the Tsunami Trace Database.
Occurrences of giant earthquakes are impending in regions facing the Kuril Trench andNankai Trough. To utilize findings from tsunami deposits to predict future earthquake disasters,
improvements must be needed for the clarity and accuracy of tsunami deposit surveys. To thisend, the IRIDeS is promoting research projects that employ non-destructive data acquisition
technology, such as ground-penetrating radar and X-ray fluorescence scanning. High-precision
radiocarbon (14C) dating for better correlation of paleotsunami events and tsunami sedimenttransport simulations for screening candidate survey area and tsunami source modeling are ofadditional great interest of the research.
We sometimes have a poorer understanding of the details for historic tsunamis, even if they
are recent ones. Identification of the predecessor of the 2011 Tohoku-oki and descendant of the869 Jogan earthquakes is crucial for the long-term assessment of risks from giant earthquakes. Inthe Tohoku region, two historical earthquakes, the 1611 Keicho Oshu and 1454 Kyotoku events,
have been known as candidates for giant earthquakes between 869 and 2011 CE. Regarding theKeicho Oshu earthquake, which is considered to have occurred along either the Kuril or Japan
Trench, paucity of historical records makes their interpretations difficult, and this has left theidentification of the main rupture area of the earthquake as the subject of debate. The majority ofdeposits of the Keicho Oshu tsunami have disappeared due to modern anthropogenic activities,
and they do not provide conclusive evidence. Historical records of the 1454 Kyotoku earthquake
and tsunami are much rarer. Despite tsunami deposits with their corresponding ages have beenfound from some localities around the Sendai Bay, the magnitude of and damages from theearthquake and tsunami are quite uncertain. The main problem for discrimination of the two
historical earthquakes comes from the uncertainty in 14C dating due to the fluctuation of thecalibration curve for calendar ages. To establish a better understanding of the history of giant
earthquakes, IRIDeS promotes collaboration between the humanities and sciences that combine
information from a broad spectrum of fields.

5Conclusion - from the author
After the Great East Japan Earthquake, the importance of geological records such as
tsunami deposits have drastically increased. In order not to repeat the unexpected damages fromfuture earthquakes and tsunamis, further efforts are needed to reconstruct much morecomprehensive paleotsunami history and to utilize them for predictions and countermeasures for
future disasters.

References
Headquarters for Earthquake Research Promotion. (2000). Long-term assessment of the Miyagioki earthquake. https://www.jishin.go.jp/main/chousa/kaikou_pdf/miyagi.pdf (In Japanese)
Headquarters for Earthquake Research Promotion. (2002). Long-term assessment of seismic
activity in the offshore of Sanriku to Boso coasts.
https://www.jishin.go.jp/main/chousa/kaikou_pdf/sanriku_boso.pdf (In Japanese)
Taniguchi, H., Sugawara, D., Ueki, S. (Eds.). (2019). The Great East Japan Earthquake: Learningfrom Disaster Heritages. Kaibundo. ISBN 9784303731359 (In Japanese)6