Landslide disasters occur very frequently worldwide causing substantial human loss and property damage (Geertsema et al., 2009, Yilmaz, 2009, Petley, 2012; Centre for Research on the Epidemiology of Disasters, 2017). Such events are predisposed by various physical factors inherent to the slope in question, such as geology, geomorphology, steepness, drainage system and others (Guzzetti et al., 1996; Dai et al., 2002), but the major triggering factor in most of the cases is rainwater infiltration (Osanai et al, 2009). Most of the landslide disasters around the world happen during the rainy seasons of the respective region, such as January in South America (Canavesi et al., 2013) or July in Japan (Matsushi et al., 2014). The problem of rain-induced landslides in Japan was exemplified during the July 2018 heavy rain-induced disasters in west Japan including in the city of Kure, in Hiroshima Prefecture (Fig. 1).
Landslides and floods triggered by the heavy rains, officially referred to as "Heavy Rains of July 2018" were the main causes of the disasters. In the course of about 10 days from 28 June until 8 July, the rainfall records reached as much as 1800 mm on the island of Shikoku and 1200 mm in Tokai region. Many cities recorded more than 400 mm of rainfall in the course of 72 hours (Japan Meteorological Agency, 2018).
In Hiroshima Prefecture, one of the most affected areas was Kure, with 10 people deceased due to landslides. Additionally, most transportation lines into the city (except maritime ways) were cut off and 760 houses were damaged (Fig. 2).
Southern Hiroshima Prefecture is occasionally affected by heavy rain-induced landslide and flooding disasters, such as in 1999 (Chigira, 2001; Ushiyama et al., 2001), 2014 (Matsushi et al., 2014) and 2018. A common factor for all of the mentioned landslide disasters in the city is that these occurred during periods of continuous heavy rain between June and July, a fact that points out the substantial role of rainfall in landslide occurrence. Although necessary for the predisposition of a slope to lose its stability and fail, landslides rarely occur only with predisposed factors such as geology, soil condition, slope geometry, etc. A triggering mechanism is considered necessary to spark the final break of stability and consequently, mass movement (Highland & Bobrowsky, 2008). The most common trigger in the majority of landslide-affected areas all around the world is break of slope stability caused by pore water density and saturation increase led by unusually heavy rainfall (Guidicini & Isawa, 1977; Caine, 1980; Chigira, 2001; Guzzetti et al., 2007; Dahal & Hasegawa, 2008; Highland & Bobrowsky, 2008; Dahal, 2012; Wang et al., 2015; and others).
In landslide hazard assessment studies, rainfall data usually comprise of mean annual precipitation data collected via rain gauge stations. Typically, each station is referent to a whole municipality, and is located near city centers. In the case of the study area, Japan Meteorological Agency (JMA) has a measurement station in downtown Kure, with the other nearest stations being at Kurahashi, 16 km southwards, and Hiroshima, 18 km northwest. Slope failure assessment analysis using rainfall data have been widely investigated in the literature, including for the present study area (Ushiyama et al., 2001; Matsushi et al., 2014). However, rain gauge stations gather information in the scale of whole municipalities may not be representative of the actual spatial distribution of precipitation in a degree of detail considered ideal for various methods of slope failure assessment. In reality, rainfall intensity values may vary in the order of less than hundreds of meters, especially in areas with rugged mountainous terrain or coastal regions. However, recent advancements in radar technology such as XRAIN (eXtended RAdar Information Network) data have allowed for instant measurement of rain intensity in much more detailed scales of spatial distribution.
In view of the need of more thorough analysis of rainfall patterns and their relationship and relevance with landslide disasters, this analytical work aims at investigating rainfall data in the study area of Kure City (Southern Hiroshima Prefecture, Southwestern Japan) in the context of the July 2018 landslide disasters, using innovative XRAIN radar-acquired rainfall data. The research plans to correlate landslide occurrence (during the July 2018 disasters) with rainfall volume distribution in varied time windows in search of localization patterns. The identification of such patterns may evidence effectiveness of XRAIN radar-acquired rainfall data in landslide hazard assessment, as well as lead to better understanding of the effects of rainfall in landslide activation and probability which may contribute to better strategies in landslide disaster prevention methods.