By TOMOYUKI SUZUKI/ Staff Writer
October 7, 2024 at 17:52 JST
A relatively small amount of rainfall last month apparently triggered mudslides in the Noto Peninsula because debris had accumulated along mountain streams during an earthquake in January, a scientist said.
Mudslides destroyed buildings in a residential area in the Sosogi district of Wajima, Ishikawa Prefecture, on Sept. 21, when heavy rains fell in the northern part of the peninsula.
The cumulative rainfall in the district was 167 millimeters.
“It is extremely rare that a mudslide occurs with such a (small) amount of rainfall (except for some areas),” said Hiroshi Takebayashi, an associate professor of erosion control engineering at Kyoto University’s Disaster Prevention Research Institute.
In the Sosogi district, steep mountain slopes collapsed near the residential area when the magnitude-7.6 temblor struck on Jan. 1, and earth, sand and rocks piled up along streams.
Takebayashi believes that the mudslides started with those debris flows around 9 a.m. on Sept. 21.
“It is necessary to quickly remove unstable earth and sand generated by an earthquake,” he said.
Rocks measuring more than two meters long were found among the debris during an on-site survey on Sept. 30.
According to Takebayashi’s simulation based on topographical and other data, earth and sand from the first mudslide engulfed the residential area in 33 seconds.
He estimates that the mudslide was 1.8 meters deep when it struck at a speed of 25 kph.
“It is extremely difficult to evacuate in time even if a sensor confirms a mudslide when it is generated,” Takebayashi said.
On a different slope of the same mountain, a mudslide occurred around 9:40 a.m. on Sept. 21 and destroyed the home of Shunin Hifumi, priest of Iwakuraji temple.
Hifumi said the slope where the temple stands did not collapse during the earthquake but that cracks appeared in the soil.
Takebayashi said such cracks may have contributed to the creation of the mudslide.
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Earth and sand left at the site of a mudslide in the Sosogi district of Wajima, Ishikawa Prefecture, on Sept. 30 (Tomoyuki Suzuki)
