5:15 PM - 7:15 PM
[U11-P03] Geomorphological and Geological Characteristics of Slope Failure by Compound Factors of the 2024 Noto Peninsula Earthquake and 2024 Heavy Rainfall in Okunoto, Japan.
Keywords:slope failure, Geomorphology, 2024 Noto Peninsula Earthquake, compound disaster
On January 1, 2024, a magnitude 7.6 earthquake occurred at 37°29.7' north latitude and 137°16.2' east longitude, at a depth of 16 km. The earthquake caused many slope failures on the Noto Peninsula, resulting in extensive human casualties and infrastructure damage. In addition, from September 20 to 22 of the same year, Wajima City experienced torrential rainfall exceeding 500 mm, causing further slope failure damage in the affected area. It is generally believed that ridge topography is more prone to slope failure in earthquakes and valley topography in rainfall, but it is not known whether the earthquake caused slope failure more severely in heavy rainfall, and there is no other case like this one.
Therefore, in future slope disaster prevention, it is necessary to estimate and evaluate not only slope failures caused by large earthquakes and heavy rainfall but also combined disasters caused by large earthquakes and heavy rainfall.
In this study, we investigated the geomorphological and geological characteristics of the slope failure hazard areas to estimate the risk of slope failure considering the compound hazard of large earthquakes and heavy rainfall. Slope failures caused by the 2024 Noto Peninsula earthquake and the 2024 Okunoto heavy rainfall event are examined and the effects of earthquake motion on the failures caused by rainfall events are summarized. Aerial photographs taken after the earthquake and heavy rainfall were used to interpret the location of surface slope failures. The area of about 100 km2 between about 37.38°N - 37.51°N, 137.03°E - 137.23°E, where many slope failures occurred during the earthquake and the heavy rainfall, was subjected. The coverage area had 1,825 earthquake-induced and 1,643 rainfall-induced slope failures. Out of the heavy rainfall-induced slope failures, 601 were classified as earthquake-influenced. These occurred after an earthquake, with visible cracks observed on the ground surface. The remaining slope failures were caused by heavy rainfall alone, without earthquake influence. To characterize the slope failure points, 90,500 points were randomly sampled as non-slope failure points. For the topographic analysis, the “Slope” and “brightness to ridge-valley” degrees were calculated using the pre-earthquake 1m DEM.
In terms of the occurrence of slope failure, the Middle Miocene volcaniclastic rocks had the highest rates of earthquake and heavy rainfall failures, while the Middle Miocene siltstones had the highest rates of combined failures.
Earthquake slope failure rates of both siltstone and volcaniclastic rocks are very low in brightness to ridge-valley, but the rate of slope failure increases in the valley areas with slopes of 40° or more. The slope failure severity at the high brightness to ridge-valley is prominent, and at the ridge area, the higher the slope, the greater the collapse severity.
Heavy rainfall slope failures differ from earthquake slope failures in both siltstone and volcaniclastic rocks, and the failure rate is high even in valley areas. In siltstones, the higher the slope, the higher the failure rate in both brightness to ridge-valley. On the other hand, the slope failure rate of volcaniclastic rocks increases with the slope in the valley but decreases with the slope in the ridge with a peak at 35°.
In the case of compound slope failures of volcaniclastic rocks, the slope failure rates by slope are like those of the heavy rainfall events in both valley and ridge areas. Therefore, it is possible that the slope failure was caused by heavy rainfall regardless of the earthquake effect. On the other hand, the compound slope failures of siltstones at the brightness to ridge-valley level showed a similar trend to that of heavy rainfall. Both ridge and valley show similar trends to earthquake failures, with a rapid increase in failure rate from 40° and above. This suggests that slopes steeper than 40° were destabilized by the earthquake and failed due to heavy rainfall.
Therefore, in future slope disaster prevention, it is necessary to estimate and evaluate not only slope failures caused by large earthquakes and heavy rainfall but also combined disasters caused by large earthquakes and heavy rainfall.
In this study, we investigated the geomorphological and geological characteristics of the slope failure hazard areas to estimate the risk of slope failure considering the compound hazard of large earthquakes and heavy rainfall. Slope failures caused by the 2024 Noto Peninsula earthquake and the 2024 Okunoto heavy rainfall event are examined and the effects of earthquake motion on the failures caused by rainfall events are summarized. Aerial photographs taken after the earthquake and heavy rainfall were used to interpret the location of surface slope failures. The area of about 100 km2 between about 37.38°N - 37.51°N, 137.03°E - 137.23°E, where many slope failures occurred during the earthquake and the heavy rainfall, was subjected. The coverage area had 1,825 earthquake-induced and 1,643 rainfall-induced slope failures. Out of the heavy rainfall-induced slope failures, 601 were classified as earthquake-influenced. These occurred after an earthquake, with visible cracks observed on the ground surface. The remaining slope failures were caused by heavy rainfall alone, without earthquake influence. To characterize the slope failure points, 90,500 points were randomly sampled as non-slope failure points. For the topographic analysis, the “Slope” and “brightness to ridge-valley” degrees were calculated using the pre-earthquake 1m DEM.
In terms of the occurrence of slope failure, the Middle Miocene volcaniclastic rocks had the highest rates of earthquake and heavy rainfall failures, while the Middle Miocene siltstones had the highest rates of combined failures.
Earthquake slope failure rates of both siltstone and volcaniclastic rocks are very low in brightness to ridge-valley, but the rate of slope failure increases in the valley areas with slopes of 40° or more. The slope failure severity at the high brightness to ridge-valley is prominent, and at the ridge area, the higher the slope, the greater the collapse severity.
Heavy rainfall slope failures differ from earthquake slope failures in both siltstone and volcaniclastic rocks, and the failure rate is high even in valley areas. In siltstones, the higher the slope, the higher the failure rate in both brightness to ridge-valley. On the other hand, the slope failure rate of volcaniclastic rocks increases with the slope in the valley but decreases with the slope in the ridge with a peak at 35°.
In the case of compound slope failures of volcaniclastic rocks, the slope failure rates by slope are like those of the heavy rainfall events in both valley and ridge areas. Therefore, it is possible that the slope failure was caused by heavy rainfall regardless of the earthquake effect. On the other hand, the compound slope failures of siltstones at the brightness to ridge-valley level showed a similar trend to that of heavy rainfall. Both ridge and valley show similar trends to earthquake failures, with a rapid increase in failure rate from 40° and above. This suggests that slopes steeper than 40° were destabilized by the earthquake and failed due to heavy rainfall.