Abstract
Abstract. We investigated 184 landslides that occurred in unwelded pyroclastic flow deposits (Shirasu) on southern Kyushu Island, Japan, that included detailed data on the rainfall characteristics and the timing of slope failure. Localized rainfall intensity, antecedent precipitation index (API), and topography affected the hydrologic processes that triggered landslides. API (adjusted for evapotranspiration losses) for large ( > 200 mm) storms that triggered landslides was much lower than for smaller ( ≤ 200 mm) storms. Mean storm intensity and 7-day API (API7) thresholds of > 5 mmh−1 and ≤ 30 mm (or API30 ≤ 60 mm), respectively, were useful to identify landslides triggered by rapid pore water pressure response, especially for shorter ( < 20 h) duration events. During smaller storms with lower intensity, landslides are likely affected by a combined increase in soil weight and loss of suction when API30 ≥ 150 mm; simulations indicated that these weight and suction changes due to rainfall accumulation decreased the factor of safety in steep Shirasu slopes, but did not necessarily trigger the landslides. Most of the landslides that were plotted below a general rainfall intensity–duration threshold for landslide initiation occurred during smaller storms with API30 values > 200 mm, indicating that they were highly influenced by the combined effects of the accumulated weight of rainfall and loss of suction. Our findings show that both event rainfall characteristics and API affect the hydrogeomorphic processes that trigger different types of landslides in Shirasu. This knowledge and the thresholds we have identified are useful for predicting the occurrence of different types of landslides in unwelded Shirasu deposits and improving sediment disaster prevention practices, including real-time warning systems.
Highlights
Volcanic deposits are highly susceptible to mass wasting and many related sediment disasters have been reported (e.g., Shimokawa et al, 1989; Crosta and Dal Negro, 2003; Ngecu et al, 2004; Bernard et al, 2009; Navarro et al, 2009; Cimarelli and de Rita, 2010; De Falco et al, 2012)
The objectives of this study are to (1) investigate the range of landslide types that occur in Shirasu based on archived data and field inspections, and (2) develop relationships between rainfall characteristics and the timing of landslides
Triggering mechanisms of different types of landslides that we explored include analyses of rainfall amount, intensity, duration, and antecedent precipitation index (API)
Summary
Volcanic deposits are highly susceptible to mass wasting and many related sediment disasters have been reported (e.g., Shimokawa et al, 1989; Crosta and Dal Negro, 2003; Ngecu et al, 2004; Bernard et al, 2009; Navarro et al, 2009; Cimarelli and de Rita, 2010; De Falco et al, 2012). Various types of landslides occur in these volcanic materials, including rock falls, flank collapses, rotational slumps, earthflows, debris slides, lahars, and debris flows, depending on the characteristics of the deposits and triggering mechanisms. In the western Oregon Cascade Range, competent andesite and basalt lava flows overlie weathered volcaniclastic rocks that are highly altered due to water infiltration through cap rock fissures (Swanson and Swanston, 1977). These areas are susceptible to deep-seated mass movements and, where exposed in gorges, promote large rotational slumps (Palmer, 1977).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
