A team of experts said Wednesday it has detected geological layers that indicate the Kii Peninsula has been hit by huge tsunami every 400 to 600 years since around 8,000 years ago.
The tsunami are believed to have been on a scale comparable with those caused by a huge earthquake in 1707, according to the researchers at the National Institute of Advanced Industrial Science and Technology and other organizations.
The study indicates the arrival of the next huge tsunami may not be very far away.
The team dug six holes 4 to 9 meters deep in the schoolyard of a high school in Kushimoto, Wakayama Prefecture, located on the peninsula, last year to examine the geological formations, which were found to have been laid down 1,600 to 8,000 years ago.
The formations included up to 14 layers of sand believed to have been brought by tsunami.
These findings enabled the researchers to estimate the intervals of the occurrences of tsunami, they said.
It is feared the Kii Peninsula and surrounding areas may be hit by a devastating earthquake centered on the Nankai Trough off central and western Japan.
While quakes with their epicenter in the Nankai Trough are believed to occur on a 100- to 150-year cycle, they vary in scale.
A government panel has estimated that a 9.0-magnitude quake with its epicenter in the Nankai Trough could cause about ¥220 trillion worth of damage, more than twice the entire state budget for fiscal 2013.
Stress tests mandated
The Diet enacted a bill Wednesday that requires large facilities including hospitals and department stores to undergo seismic diagnoses equivalent to those mandated for structures built before enhanced quake-resistance standards were introduced in 1981.
These facilities must complete the diagnoses by the end of 2015. Their operators will be slapped with penalties up to ¥1 million if they refuse to follow the legislation, which revises the promotion of renovation for earthquake-resistant structures law.