Landslides by the 2018 Hokkaido Iburi-Tobu Earthquake on September 6
In the early morning of September 6th, 2018, an intense earthquake struck Hokkaido Iburi-Tobu area. By this earthquake, many landslides occurred and claimed 36 lives. The landslide numbers reached 6,000 and mostly they are shallow landslides moving down of the air-fall pumice layer from Tarumai volcano which erupted ca. 9,000 years ago. However, southeast of the area, deep-seated landslides of dip-slipping type are also found.
KeywordsIntensive earthquake Landslides Hokkaido Japan
Climate condition of the day
The earthquake occurred just 1 day after powerful typhoon Jebi (No. 21 in Japan) left torrential rains in the region. The Meteorological Agency of Japan reported that the precipitation had accumulated within the last 3 days from September 3 to 5 reached almost 100 mm.
The huge tremor triggered landslides over about 20 km × 20 km area behind Atsuma Town, wiping out homes at the foot of the slopes there. This area is composed of Neogene sedimentary rocks as its basement, and covered mostly up with air-fall lapilli-sized pumice layers (1.5 m thick; Ta-d: ca. 9000 years ago) from Tarumai Volcano to the east. Surface soil layers covering low to middle mountain ranges are inter-bedded with the pumice and ash. Total thickness of the surface geology is about 4–5 m in and around the epicentral area.
Landslide distribution and characteristics
This earthquake is very similar to the October 23, 2004, Chuetsu Earthquake (M = 6.8), in that it triggered many landslides given the maximum intensity of 7 on the JMA Intensity scale and the JMA magnitude of 6.7 (Yamagishi and Iwahashi 2007). The number of landslides triggered by this earthquake was about 6000 while it was 3370 in the Chuetsu Earthquake according to Niigata Prefecture, though the focal depth of the Hokkaido Iburi-Tobu Earthquake was 37 km, about 4 times as deep than that (10 km) of the Chuetsu Earthquake. It may be a little premature to say the following, but this may be attributed to the volcanic-product-rich surface geology covering Neogene hard rocks of the affected area while the area struck by the Chuetsu was dominated by sedimentary rocks. We are currently planning to scan the terrain using a 2-km-long-range terrestrial LiDAR and then to run debris mass flow simulations in the near future.
We are grateful to the reviewer for useful comments and suggestions.
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