The 869 Jōgan earthquake (貞観地震, Jōgan jishin) and its associated tsunami struck the area around Sendai in the northern part of Honshu on 9 July 869 (the 26th day of the 5th month in the 11th year of Jōgan; or 13 July 869). The earthquake had an estimated magnitude of at least 8.6 on the moment magnitude scale, but may have been as high as 9.0, similar to the 2011 Tōhoku earthquake and tsunami. The tsunami caused widespread flooding of the Sendai plain. In 2001, researchers identified sand deposits in a trench more than 4.5 kilometres (2.8 mi) from the coast as coming from this tsunami.
Why this event still matters
Historical incidents shape the rules that protect people today — building codes, evacuation policy, warning systems and inspection regimes almost always trace back to a specific disaster. Studying 869 Jōgan earthquake alongside current events is how preparedness improves.
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