Introduction

On September 6, at 03:08 AM, Japan time, an intense earthquake struck the Iburi-Tobu area of Hokkakido, Japan. Its JMA magnitude of 6.7 was registered with its epicenter located at 42.72° North, and 142.0° East. The maximum intensity of 7 was registered at Atsuma Town (Fig. 1). The affected area has been hit by a series of aftershocks, and they are still continuing (Fig. 2: National Institute of Earth Science and Disaster Prevention, NIED). This earthquake reportedly killed 41 persons, including 36 dead by landslides as mentioned herein.

Fig. 1
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The Iburi-Tobu area of Hokkakido, Japan

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Fig. 2
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The affected area was hit by a series of aftershocks, and they are still continuing

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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.

Geology

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

The Geographical Survey Institute, Japan (GSI, hereafter), quickly took aerial photographs (ortho-photographs) for the whole area affected by the earthquake, and has not only been providing them to many public offices relevant to the disaster but also posted them up on the Web together with analyzed satellite images for the public. Also, many aerial surveying companies have taken aerial photographs such as Fig. 3 and posted them up on the Web.

Fig. 3
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Aerial photograph of the area (taken by Asia Air Survey and Aero Asahi Corporation)

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Given many high-resolution ortho-photographs from GSI, Mr. Kouichi Kita of Hokkaido Government has extracted landslide scarps and debris deposits during the photo interpretation using GIS as shown in Fig. 4. He recognized more than 6000 landslide scarps and 1000 associated debris deposits. The reason why the number of the landslide scarps was much larger than that of the debris deposits is that some debris flowing from different sources must have been mixed up into a single larger mass.

Fig. 4
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Photo interpretation of the area using GIS (interpreted by Mr. Kouichi Kita)

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Most landslides are shallow, several meters deep-seated (“hokai” in Japanese), and are mostly classified as planar types (left of Fig. 5) and spoon types (right of Fig. 5), both of which are the same as the rainfall-induced landslides (Yamagishi and Moncada 2017). Some deep-seated landslides of dip-slipping type are also found in the southeast part of the mountains.

Fig. 5
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Most landslides are shallow, several meters deep-seated, and are mostly classified as planar types (left) and spoon types (right)

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Most landslides slid down over the air-fall pumice and ash layers. We visited one landslide at Tomisato to fly a drone for extracting its 3D image by using SFM (structure from motion) (Fig. 6). We have extracted three-dimensional coordinates of surface points making up the terrain as shown in Fig. 6. The landslide masses were detached from both valley walls as well as the head of the valley. The detached debris mass was a mixture of pumice/ash materials and uprooted pine trees, and its surface spreading over the depositional area was rugged and hummocky along its margin. The debris mass that has stopped and spread over rice fields can be divided into the lower and upper parts. The lower part includes many uprooted trees, while most of the upper part is composed of fine sand indicating that the mass flowed like a viscous slurry. The volume of this debris mass is estimated from SFM to be about 65,000 m3.

Fig. 6
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Extracted its 3D image by using SFM (structure from motion) and extracted three-dimensional coordinates of surface points making up the terrain a) is vertical view image, and b) is oblique view

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Conclusion

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.