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Torrential rainfall in the summer of 2004 caused a series of natural disasters in the Niigata region of Honshu, the largest island in Japan. The heaviest downfall occurred on 13 July, flooding Japan’s longest river, the Shinano-gawa. A little over three months later, on 23 October, intensive earthquakes shook another hilly area just south of the area hit by the summer rains.

The two incidents triggered close to 8000 landslides. More than 40 people were killed.

A team comprising Hiromitsu Yamagishi from Niigata University, Junko Iwahashi from the Geographical Survey Institute, and Toko Takayama from Asia Air Survey used GIS to analyse the landslides and their causes.

‘Different triggers caused the landslides in the mountains’, the research team writes in its analysis. ‘But the two events were very useful for clarifying these differences. We studied both types in the field just after they occurred, and later analysed air photographs using GIS technology.’

In their analysis, the researchers used ESRI’s ArcGIS software and geological data sources to explore whether the landslides were induced by rainfall or earthquakes.

GIS was used to generate thematic maps, which helped to illustrate patterns that might not be easily identifiable to a researcher looking at data tables. The team also used GIS to investigate if the area’s geology and geomorphology played a part.

By mapping the two landslide triggers against the geological features, they were able to visualise what affected the density and severity of the landslides. This provided valuable information that could be used for future disaster mitigation.

To begin, the researchers looked at rainfall on 13 July in the Izumozki and Tochio regions. The AMEDAS (Automated Meteorological Data Acquisition System) station in Tochio City recorded more than 400 mm of rain in 24 hours, starting on the evening of 12 July

They prepared GIS precipitation and contour maps of the area, depicting the landforms and identifying the flooding and landslides caused by the rainfall. Coloured lines detailed the size and type of the event. They also indicated both deep and shallow landslides, as well as long mudflows up to several hundred metres in length.

All the landsides were in close proximity to the mudflows. Most were shallow and several metres wide and thick.

A GIS slope map was produced with data on landslides from the National Research Institute for Earth Science and Disaster Prevention (NIED). This map showed the number of rainfall-induced shallow landslides in relation to old, deep landslides frozen in the earth. The region’s mountains are shaped by former events. Some old deep-seated landslides had created slopes in the land, as seen in the team’s strata dipping contour maps. These maps revealed that the July 2004 landslides were concentrated in the steeply sloped areas of those old landslides.

Surface materials were also considered in the analysis. A geological map modified with GIS was used to see whether the area’s geology had an affect on the number of rainfall-induced landslides. The affected region has a similar amount of sandstone and mudstone surface material. Where there was sandstone earth, there were more landslides.

The team’s GIS analysis unveiled some important observations in relation to rainfall-induced landslides. Firstly, most of the Izumozki and Tochio landslides were shallow, and all were closely associated with long-run mudflows. Also, the majority of incidents occurred along the ridges and dips caused by old, deep-seated landslides. And lastly, sandstone surfaces experienced more of the rainfall-induced landslides than the mudstone.

The team’s findings have provided a better understanding of rainfall-induced landslides. Importantly, they also show the types of geology and geomorphology that are less safe for building roads and communities.

With the earthquake-induced landslides, the team again used a GIS to compare landslide density in sandstone- versus mudstone-rich zones. But the findings were different this time. These landslides were concentrated along the mudstone surfaces.

Once more, they turned to the contours of the land. A topographic contour line map layered with a geologic stratum dipping contour map showed a higher number of landslides in the areas of old, deep-seated landslides.

They also found that the number of slides was affected by the gradient, and that the earthquake-induced landslides were larger on steeper slopes.

In the case of the October 2004 landslides, it was thought they might have been accelerated as they ran their course. The areas contain numerous paddy fields -- flooded parcels of arable land used for growing rice. The researchers suggest that the flow of the landslide could easily gain momentum by mixing with water from the paddy fields and ponds.

GIS analysis indicated more flow-type landslides in proximity to the fields, and fewer landslides around abandoned fields and ponds, where there would be less water.

Yamagishi’s team found both contrasts and similarities when comparing the earthquake- and rainfall-induced landslides. Mudstone, rather than sandstone, became the unstable surface. Yet, when both trigger events were charted, more landslides were shown to occur in the sandstone-rich areas.

The team’s analysis was able to link more of the events to the areas of old deep-seated slides, and concluded that the earthquake-induced slides were larger in scale. Water standing in surrounding paddy fields was also shown to create unstable terrain.

Regardless of what triggered it, a landslide that mixes with water gains momentum.

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