## 2 Study Area  
The Japanese archipelago lies at the interaction between the Pacific, North American, Eurasia, and Philippine plates, where there exists the deepest trench on the Earth's surface. The North American and Eurasia plates here can be further subdivided into the Okhotsk Sea and the Amur plates. On 5 September 2018 (UTC Time), an Mw 6.6 earthquake occurred in the Oshima Belt region, east of Tomakomai on the island of Hokkaido, Japan (Figure 1). The strata of the whole tectonic area include from Holocene to Jurassic. The detailed description of lithology of different ages can be seen in Table S1. The study area extends on a surface of about 600 km\({}^{2}\), spanning from 42.6\({}^{\circ}\)N to 42.9\({}^{\circ}\)N of latitude and from 141.8\({}^{\circ}\) to 142.2\({}^{\circ}\). The elevation of the area ranges between 0 and 624 m, with an average value of 166 m. The study area is made up of Miocene sedimentary rocks as its basement, and covered mostly up with air-fall lapilli-sized pumice layers. Surface soil layers covering low to middle mountain ranges are inter-bedded with the pumice and ash. Total thickness of the surface layer is about 4-5 m [3].  
At the location of this earthquake, the Pacific plate moves northwestward at a velocity of about 87 mm/yr relative to the North America plate and collides with the North American Eurasia and Philippine plates along the West Pacific subduction zone, causing frequent earthquakes. The focal mechanism solutions of the Tomakomai earthquake (Table 1) indicate that faulting occurred on either a moderately dipping reverse fault striking Northwest, or on a shallow-to-moderately dipping fault striking southeast. In view of these focal mechanism solutions and focal depth (35 km), the earthquake likely represents rupture of a fault within the upper North America plate or Okhotsk microplate, instead of the subduction zone plate boundary interface between the Pacific and North America plates (which is 100 km deep at the location of this epicenter) ([[https://earthquake.usgs.gov](https://earthquake.usgs.gov)]([https://earthquake.usgs.gov](https://earthquake.usgs.gov))).  
Figures 2 and 3 are photos and satellite images of the landslides triggered by the 2018 Tomakomai earthquake. As shown in Figure 2, the density of landslides triggered by this earthquake is high, most of them are shallow landslides, and distributed continuously. Substantial landslides have destroyed the ridges and caused the landform changes (Figure 2a,c). Figure 3a,b show the landslides damaged many houses and farmland, and blocked roads and valleys. Figure 3c shows that the debris area of the landslide was a mixture of the uprooted trees and the pumice and ash precipitate, and spreads over croplands and villages.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Latitude (\({}^{\circ}\))** & **Longitude (\({}^{\circ}\))** & **Dip (\({}^{\circ}\))** & **Depth (km)** & **Rake (\({}^{\circ}\))** & **Mo (Nm)** & **Var. Red.** & **Strike (\({}^{\circ}\))** \\ \hline
42.6908 & 142.0067 & 30; 65 & 35 & 59; 107 & 1e + 19 & 89.47 & 134; 349 \\ \hline \end{tabular}
\end{table}
Table 1: Parameters of the 2018 Mw6.6 Tomakomai, Japan earthquake.  
Figure 1: Map showing topography and tectonic setting of 2018 Tomakomai earthquake (Active faults are modified from Active Fault Research Group [28] and Nakata [29]; The topographic background comes from National Geophysical Data Center, NOAA [30].