LANDSLIDES TRIGGERED BY THE APRIL 2016 KUMAMOTO EARTHQUAKE

In addition to the widespread damage caused by the direct effects of the April 2016 Kumamoto Earthquakes, many landslides were triggered in forest and grassland hillslopes near Mount Aso. The seismic activity began with a M 6.5 foreshock on 14 April followed 28 hours later by the major (M 7.3) earthquake (01:25 JST, 16 April 16). Between and just after these major events, five other major shocks occurred, ranging from M 5.4-6.4. All of these earthquakes had very shallow focal depths (»10 km), which caused the highest shaking intensity (7) on the Japanese scale. Because soils were relatively dry during these earthquakes, occurrence of debris flows was limited. Instead, most landslides travelled limited distances and consisted of ruptured blocks of cohesive volcanic soil. Landslides initiated along ridgelines or on convex and planer slopes due to amplifications of seismic waves in these areas. Slope gradients were relatively gentle (14-25°) in grasslands. The shape of these landslides was unique - most were rectangular with the downslope portion of the slide nearly as wide as upslope, and the regolith broke into a series of cohesive, grass-cover blocks with the surface ash soil reinforced by dense root matts. Landslides in forests occurred on steeper slopes and their shape was more elongated due to the gradient; however, large displaced soil blocks were common in the landslide and were thrust to the surface on ridgelines during the intense shaking. Large, parallel fissures appeared along the ridgelines providing opportunities for further mass wasting during heavy rainfall. These unusual blocky landslides have contributed substantial sediment to channels that can be easily mobilized into damaging debris flows during future storms.