SAKIGAKE管理人 Introduction
On July 9, 869 (the 11th year of the Jogan era), a massive earthquake — estimated at magnitude 8 class (Mw 8.3 to 8.6, or possibly even larger) and centered along the Japan Trench — struck Mutsu Province in the Heian period. This disaster, commonly known as “the Jogan earthquake and Jogan tsunami,” brought a massive tsunami across a wide area centered on the coast of present-day Miyagi Prefecture, and the Nihon Sandai Jitsuroku (the official history of the period) records that approximately 1,000 people drowned as a result of the tsunami.
The reason this earthquake carries special significance today is not only the scale of its damage. Even before the Great East Japan Earthquake (2011), multiple researchers, through surveys of tsunami deposits and numerical analysis, had indicated that the Sendai Plain, the Ishinomaki Plain, and other areas could once again be struck by a massive tsunami comparable to the Jogan tsunami.
However, the Great East Japan Earthquake occurred before this scientific knowledge had been sufficiently shared with society and reflected in disaster prevention measures. This article looks back at the reality of the Jogan earthquake and considers what implications its lessons hold for modern disaster prevention and corporate BCP (Business Continuity Planning).
A Massive Tsunami in the Heian Period — What Actually Happened
The damage from the Jogan earthquake is recorded in the Nihon Sandai Jitsuroku, the official history of the Heian period. It records that, around Taga Castle, numerous fortifications, storehouses, gates, and watchtowers collapsed, ships were swept away, and many people lost their lives to the tsunami.
Furthermore, geological surveys conducted since the 1990s have identified layers of sand carried by the tsunami (tsunami deposits) along the Sendai Plain and other coastal areas. These surveys revealed that the Jogan tsunami reached several kilometers inland from the coastline of that era — meaning the existence of this massive tsunami is corroborated not only by historical documents, but geologically as well.
With historical records and scientific surveys converging on the same conclusion, the Jogan earthquake has come to be widely recognized not as legend, but as a disaster that actually occurred on a massive scale.
Researchers Had Indicated the Possibility of a Massive Tsunami Before the Disaster
Entering the 2000s, research institutions such as the National Institute of Advanced Industrial Science and Technology (AIST) and various universities advanced their analysis of tsunami deposits and numerical simulations. As a result, academic papers and conference presentations began to indicate, one after another, the possibility that a massive tsunami comparable to the 869 Jogan tsunami could occur again in the Sendai Plain, the Ishinomaki Plain, and elsewhere. This was not an analysis presented for the first time after the Great East Japan Earthquake — it was scientific knowledge that had already been published before the disaster, indicating the possibility of a massive tsunami.
At the same time, uncertainty remained in the research of that era, and it did not accurately predict the scale or rupture extent of the Tohoku-oki earthquake (Mw 9.0) that occurred in 2011. Even so, the fact that “extremely large tsunamis have repeatedly occurred throughout history” had already been scientifically demonstrated.
Why Wasn’t This Knowledge Sufficiently Put to Use?
The problem is that the Great East Japan Earthquake occurred before these research findings had been sufficiently reflected in long-term national and municipal assessments, tsunami projections, and evacuation plans. Several factors lie behind this.
First, there is the reality that low-frequency, large-scale disasters occurring on a scale of several hundred to a thousand years tend to receive lower priority in the day-to-day operations of government administration and corporate management.
Second, the process of translating academic research findings into practical application — such as disaster prevention plans and facility design — takes time. Work was underway to reflect the research findings on the Jogan tsunami in the national government’s long-term assessment, but the Great East Japan Earthquake occurred in the midst of that process.
In other words, “the scientific knowledge existed,” but “the disaster struck before its social implementation had sufficiently progressed” — this is the major lesson of this case.
Implications for Corporate Disaster Prevention and BCP
Applying the lessons of the Jogan earthquake to corporate disaster prevention yields three important implications.
First, “low probability of occurrence” and “no need for countermeasures” are entirely different things.
Massive disasters occurring on a scale of several hundred to a thousand years are risks that are difficult to feel as a real possibility in daily life. However, once such a disaster occurs, it can have a devastating impact on corporate activity and social infrastructure. Risk assessment must account not only for frequency of occurrence, but also for the magnitude of impact.
Second, establish a mechanism for translating expert knowledge into practical application.
Simply reading academic papers and research findings as they are does not connect to corporate disaster prevention measures. What matters is building a system that translates outside expert knowledge into risk assessments tailored to your own company’s location and business characteristics.
Third, use historical disasters as an important source of information for risk assessment.
Modern observational data alone cannot adequately assess massive disasters occurring on a scale of several hundred to a thousand years. Historical records — such as old documents and geological surveys — are an important source of information, revealing risks that cannot be seen through modern statistics alone.
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Conclusion
More than 1,150 years have passed since the Jogan earthquake. Yet its record remains today, not only in historical documents, but also in the form of geological strata. And even before the Great East Japan Earthquake occurred, the possibility of a massive tsunami had already been indicated through scientific research based on these records. At the same time, the Tohoku-oki earthquake that struck in 2011 turned out to be a disaster on a scale that further exceeded the assumptions of the time. This fact demonstrates, once again, that while there are limits to predicting natural disasters, it remains crucial to continuously and swiftly reflect the knowledge gained from history and science back into society.
Learning from past disaster records and continuously incorporating expert knowledge into disaster prevention plans and BCPs — that is the first step in preparing for the next large-scale disaster.
Through corporate disaster prevention consulting, BCP (Business Continuity Planning) development support, and disaster risk management, SAKIGAKE JAPAN connects the lessons of history with the latest scientific knowledge to social implementation, contributing to the resilience of companies and communities.
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