A five-minute warning
On July 12, 1993, at 10:17 PM local time, a magnitude 7.7 earthquake occurred in the Sea of Japan. The quake, known as the Hokkaido-Nansei-Oki earthquake, triggered a devastating tsunami. For the 4,700 residents of nearby Okushiri Island, there was almost no time to react. The first waves struck the island's southern coast just two to five minutes after the shaking began. In the town of Aonae, the water surged inland, causing immense destruction.
The tsunami's power was not uniform across the island. While waves on the eastern coast reached 2 to 5 meters, the western coast faced a much larger assault. Along a 20-kilometer stretch of the southern part of the island, wave run-up—the maximum vertical height the water reached onshore—was consistently measured between 15 and 30 meters. The highest run-up was recorded in a small, narrow valley near the Monai district, where the topography funneled the water to an astonishing height of 31.7 meters. The force of the water stripped vegetation from hillsides and moved boulders up to a meter in diameter.
The geologic after-image
When the water receded, it left behind a distinct geological signature: a layer of sand and sediment scraped from the seafloor and deposited across the inundated land. This tsunami deposit is a perfect modern analog for paleoseismologists, scientists who study the geologic record for evidence of past earthquakes and tsunamis. By digging trenches and analyzing the soil layers, or stratigraphy, they can read the story of the 1993 event.
A typical tsunami deposit from this event is a sheet of sand, sometimes up to 36 cm thick, overlying the old topsoil. The sediment often shows a "fining upward" sequence, with coarser, heavier grains at the bottom and finer grains toward the top. This pattern reflects the decreasing energy of the water as it slows down and retreats. The deposits are composed of marine sands, gravels, and even the microscopic shells of foraminifera, tiny sea organisms dragged from depths of up to 100 meters. By analyzing the grain size, thickness, and landward extent of this 1993 layer, scientists create a baseline model. They can then search for older, buried sand sheets with similar characteristics. Finding these paleotsunami deposits allows them to reconstruct the history of major tsunamis in the region over thousands of years, showing their frequency and estimating their magnitude long before written records existed.