By RYO SASAKI/ Staff Writer
October 9, 2022 at 07:00 JST
Boulders lie scattered around the Hashigui-iwa Rocks in Kushimoto, Wakayama Prefecture. (Provided by the National Institute of Advanced Industrial Science and Technology)
Massive boulders strewn around a scenic natural rock formation called the Hashigui-iwa Rocks in the Pacific coastal town of Kushimoto, Wakayama Prefecture, are likely vestiges of gigantic tsunami that occurred in the distant past, according to a study.
Researchers from the National Institute of Advanced Industrial Science and Technology (AIST) and other institutions said the boulders offer strong evidence of catastrophic seismic activity in the Nankai Trough, a roughly 4-kilometer depression on the seabed that extends about 700 kilometers from Suruga Bay off Shizuoka Prefecture on the main island of Honshu to eastern Kyushu.
Hashigui-iwa, which literally means “bridge pillar rocks,” consists of more than 40 rock pillars of varying sizes arranged in a straight line that stretch for nearly a kilometer. The sightseeing site is designated as a national natural monument by the central government.
The researchers studied the location and size of 1,103 boulders scattered at distances of 15 meters or more landward from the rock pillars. The boulders measure between 60 or so centimeters in size and slightly shy of 6 meters. They weigh between 100 kilograms and around 85 tons.
Simulations suggest the boulders were brought ashore by incredibly powerful tsunami in the distant past.
The theory was bolstered by a finding that roughly 4 to 20 percent of the boulders, particularly some of the biggest ones, would have stayed put during the sort of tsunami that was likely spawned by the magnitude-8.6 Hoei Earthquake of 1707.
The Hoei Earthquake was believed to have been the largest among Nankai Trough quakes in history.
The presence of the boulders suggests they were swept there during major tsunami events generated by unknown earthquakes greater in scale than the Hoei disaster, the researchers said.
“We are the first to have demonstrated that the boulders were likely taken there during tsunami events,” said Yuichi Namegaya, an AIST senior researcher and leading member of the research team who is an expert in paleoseismology and paleotsunami.
“We hope, in the future, to study fossil shellfish attached to the boulders and other materials to find out when those tsunami events struck,” Namegaya added. “The data will help us to estimate the occurrence probabilities and recurrence intervals of hugely powerful Nankai Trough earthquakes.”
The research results were published in (https://www.sciencedirect.com/science/article/pii/S0040195122002815), a science journal, on Sept. 6.
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