At two o'clock in the afternoon on December 7, 1833 -- the twenty-sixth day of the tenth month of Tenpo 4 by the Japanese calendar -- the Sea of Japan heaved. The seafloor off the coast of Yamagata Prefecture ruptured along the eastern margin of the basin, sending shockwaves through Akita, Yamagata, and Niigata prefectures. Within minutes, the ocean withdrew from the shoreline. Then it returned. Tsunami waves reached eight meters at Kamomoya, roughly the height of a three-story building. Along the Shonai coast, fishing boats were swept inland, houses dissolved into debris, and 150 people lost their lives in a disaster that would reshape how the Tokugawa shogunate understood the seismic dangers of Japan's western shore.
Japan sits at the collision of four tectonic plates: the Pacific, Philippine Sea, Okhotsk, and Amurian. Most attention goes to the eastern coast, where the Pacific Plate dives beneath Japan at the Japan Trench -- the same subduction zone responsible for the devastating 2011 Tohoku earthquake and tsunami. But the western coast holds its own dangers. The boundary between the Amurian and Okhotsk plates runs north-south along the Sea of Japan side of Honshu, forming what geologists describe as an incipient subduction zone -- a boundary still in the early stages of development, composed of eastward-dipping thrust faults. These faults have been active since the end of the Pliocene, producing earthquakes in the magnitude 6.8 to 7.9 range. Major seismic events struck this boundary in 1741, 1833, 1940, 1964, 1983, and 1993.
The 1833 earthquake ruptured two segments of the convergent boundary faults along the eastern margin of the Sea of Japan. The rupture zone partially overlapped that of the 1964 Niigata earthquake, though parts extended farther north. Seismologists have estimated the earthquake's magnitude at 7.5 to 7.7 on the JMA scale, while the tsunami magnitude was calculated at 8.1 -- reflecting the outsized wave energy generated by the submarine fault displacement. Both fault planes dipped approximately 60 degrees to the east. The moment magnitude has been modeled at 7.98. Shaking reached JMA seismic intensity 6 across a broad area stretching from Niigata to Yamagata prefectures, and intensity 5 extended even farther, following the Mogami River valley from Niigata through Akita to the Yamagata basin.
The tsunami struck the Shonai coast with devastating force. At Kamomoya in Yamagata Prefecture, waves reached seven to eight meters. Large waves battered a 30-to-50-kilometer stretch of coastline. Farther afield, the tsunami measured five to six meters at Wajima on the Noto Peninsula, where an additional 47 people died. Even distant Sakaiminato in Tottori Prefecture recorded significant wave heights. In the Shonai region, 158 houses and 322 boats and ships were destroyed. Across the affected area, between 360 and 600 homes were washed away or collapsed from shaking, with another 1,790 sustaining partial damage. The towns of Tsuruoka, Oyama, Makisone, Yosida, Okushinden, and Hironoshinden all suffered partial destruction. Liquefaction was observed at Matsugasaki on Sado Island. The greatest damage concentrated along the coast, where communities had the least warning.
The 1833 Shonai earthquake was neither the first nor the last major event along this boundary. It belongs to a sequence of powerful earthquakes and tsunamis that have periodically ruptured the eastern margin of the Sea of Japan over the past three centuries. The 1964 Niigata earthquake struck a partially overlapping rupture zone, causing widespread liquefaction and the famous tilting of apartment buildings in Niigata City. The 1983 Sea of Japan earthquake generated a devastating tsunami that killed 104 people. The 1993 Okushiri earthquake sent waves crashing into Hokkaido. Together, these events reveal a coastline that is seismically active on a timescale measured in decades -- a pattern that modern tsunami hazard assessments now incorporate into regional planning for the communities along Japan's western shore.
The earthquake epicenter was located in the Sea of Japan at approximately 38.90°N, 139.25°E, off the coast of Yamagata Prefecture. From altitude, the affected coastline runs roughly north-south along the Shonai Plain, with the port city of Sakata and the town of Tsuruoka visible as primary coastal settlements. The Mogami River delta is a prominent visual landmark. Shonai Airport (RJSY) sits on the coastal plain approximately 10 nautical miles from the epicenter's nearest onshore point. Yamagata Airport (RJSC) lies approximately 50 nautical miles inland to the southeast. The Sea of Japan coastline and the flat Shonai Plain contrast sharply with the mountainous interior of Yamagata Prefecture.