Knee-High Tsunami Can Immobilize Adults, Expert Warns

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On the evening of April 20, a magnitude 7.7 earthquake with a maximum seismic intensity of upper 5 was recorded off Japan's northeast coast. The Japan Meteorological Agency issued tsunami warnings for areas including the Pacific coasts of Hokkaido and Aomori Prefecture.

In light of a relatively increased likelihood of a major earthquake in the near future, the agency issued a "subsequent earthquake advisory" warning of the possibility of another large quake. It stressed that continued "special preparedness" for tsunamis and other hazards associated with strong tremors remains essential.

After the earthquake occurred at 4:52 PM, the JMA issued the advisory for 182 municipalities across seven prefectures, stretching from Hokkaido to Chiba. Authorities estimate that the probability of another magnitude 8 or higher earthquake occurring within the next week has risen to around 1%, roughly 10 times the usual level.

Authorities are urging people in the affected municipalities to stay ready to evacuate at a moment's notice over the coming week, including during the night, while keeping daily life and economic activity as normal as possible.

Even Small Tsunamis Are Dangerous

Tsunami waves were observed in multiple locations on April 20. In Ishinomaki, Miyagi Prefecture, a 20-centimeter tsunami was recorded at 6:02 PM, about 70 minutes after the earthquake. This is below knee level for an adult. However, the agency and earthquake experts continue to caution that even tsunamis of this size can be dangerous.

*Tsunami heights and their effects, according to government documents and other sources.*

JMA explains that as tsunami height increases, the force of the moving seawater also becomes stronger. "A tsunami of 20–30 centimeters (around 8–12 in) can easily knock a person off their feet," a representative notes.

"Even at 20 centimeters, the force can be enough to immobilize an adult. Regardless of the tsunami's height, we ask people [in affected regions] to evacuate immediately when they feel an earthquake."

Check Evacuation Routes

Associate Professor Daisuke Sugawara of the International Research Institute of Disaster Science, who specializes in tsunami geology, emphasized the importance of constantly being aware of whether one's surroundings are safe.

He explains that tsunami danger goes beyond simple flooding. "It is not the same as simply being submerged in 20 centimeters of water," he notes. The speed of the flow, wavelength, and wave period all combine to make tsunamis far more deadly than their height alone might suggest.

In Japan, television and radio broadcasts provide immediate estimates of tsunami heights and issue evacuation instructions shortly after an earthquake. However, Sugawara notes that these forecasts include margins of error and inherent uncertainty.

"People should review their local government's hazard maps to locate evacuation sites, and think carefully in advance about the safety of their surroundings and the routes they would take to get there," he said.