IN-04
Potential interactions between prior earthquakes and later landslide initiation –evidence from Central Asia and Japan
1University of the Sunshine Coast, Australia
Tokyo University of Agriculture and Technology, Japan
3
Forest Institute of National Academy of Sciences, Bishkek, Kyrgyzstan
2○Roy C. Sidle1, Takashi Gomi2, and Nurstan Chyngozhoev3
1. INTRODUCTION
Factors affecting earthquake-initiated landslides include earthquake magnitude, focal depth, andseismic wave propagation and attenuation. Landslides triggered by earthquakes often occur on convex
slopes and near ridgelines, while landslide initiated by rainfall or snowmelt usually occur on hillslope
locations where subsurface water accumulates (e.g., geomorphic hollows, locations downslope wheresubsurface water accumulates) (Sidle and Ochiai, 2006). Evidence is presented from Fergana Basinin Kyrgyzstan and the area west of the Mount Aso in Kumamoto Prefecture, Japan, on how fissuresdeveloped during earthquakes may promote subsequent initiation of rainfall-triggered landslides.
Such fissures have been recognized in areas that have experienced large earthquakes, but these havenot been directly associated with later landslide initiation.
2. RESULTS AND DISCUSSION
More than 1800 recent major landslides in hilly terrain and soft sediments of the Fegana Basinhave been largely attributed to accumulation of heavy rainfall and snowmelt (Haberland et al., 2011).
Most of these landslides have occurred near ridgelines and/or on convex slopes – not typical locationsof rainfall-initiated mass failures. While no recent large earthquakes have occurred in the Fergana
Basin, smaller earthquakes have generated fissures near ridgelines and on convex slopes (Figure 1a).
The connection of fissures, developed years or decades before slope failure, with preferential
transport of rainwater and runoff into the soil has not been previously investigated. Fissures havebeen observed to expand with time, particularly during subsequent minor earthquakes, furtherpromoting preferential infiltration. Because the soil mantle is rather homogeneous, the position anddepth of these fissures may control the location and depth of slope failures. Zones in the soil whereFigure1. Earthquake generated fissures on ridgelines near areas of landslide initiation or potential newlandslides: (a) landslides that occurred in 2003 near Uzgen, Kyrgyzstan, with fissures upslope; (b)multiple fissures along a ridgeline near Mt. Aso where a landslide occurred during 2016 earthquake.

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surficial inputs of water are preferentially transported, augment natural subsurface accumulation ofantecedent rainfall and snowmelt. Many landslides in the eastern Fergana Basin occur after several
months of accumulated precipitation, and shallow groundwater has been observed emerging on
critical hillside locations prior to slope failure.
An unusual temporal series of earthquakes struck the area near Kumamoto, Japan in April 2016.
A large (M 6.5) foreshock on April 14th was followed 28 hours later by the major (M 7.3) tremor (e.g.,Kawase et al., 2017). The earthquakes had shallow focal depths (about 10-12 km) and generated
shaking intensities of 7, the highest on the Japanese scale. The most intense ground shaking occurredon the west side of Mount Aso causing numerous landslides. Although a few longer runout
landslides/debris flows occurred, most landslide deposits accumulated on hillslopes or within
headwater channels. Many landslides initiated along ridgelines or on convex and planer slopes due toamplifications of seismic waves. Within the Tokosegawa-Nigorigawa basins (total area 13 km2) that
we examined, forested areas occupied steeper terrain than grasslands; however, grasslands with slope
gradients > 30° had a larger percentage of land disturbed by landslides compared to forests. Largedisplaced soil blocks were common in the landslides and were thrust to the surface on ridgelines
during the intense shaking. Despite the many landslides that occurred during the earthquakes, muchof the damage from mass wasting may be realized later. Future landslides along ridgelines will likely
occur where large fissures (1-2 m deep) appeared during the earthquakes (Figure 1b).
As noted, fissures can preferentially route rain water and runoff into the soil mantle, thus
augmenting pore water pressures that may trigger landslides in future periods of heavy or
accumulated rainfall. Additionally, landslide sediment stored in headwater channels because to thelimited mobility of many of the 2016 earthquake-initiated failures poses a risk for future debris flows
during large storms (Lin et al., 2006). Surface erosion from landslide scars together with newlandslides along ridgelines will exacerbate sediment accumulation in these headwaters. As channels
accumulate sediment, they become increasingly vulnerable to debris flows during large flow events(Sidle and Ochiai, 2006). This scenario represents a cascading sequence of hydrogeomorphic eventsthat generate future environmental and human risks, including impacts on water quality, sedimentdisasters in rural communities, and damage to buildings and infrastructure.
REFERENCES
Haberland, C., Abdybachaev, U., Schurr, B., Wetzel, H.-U., Roessner, S., Sarnagoev, A.,
Orunbaev, S., Janssen, C., 2011. Landslides in southern Kyrgyzstan: understanding tectoniccontrols. Eos Trans. AGU 92, 169.
Kawase H, Matsushima S, Nagashima F, Baoyintu, Nakano K (2017) The cause of heavy damage
concentration in downtown Mashiki inferred from observed data and field survey of the 2016
Kumamoto earthquake. Earth, Planets and Space 69(3), doi 10.1186/ s40623-0160591-1.
Lin Y-P, Chang T-K, Wu C-F, Chiang T-C, Lin S-H (2006) Assessing impacts of typhoons and theChi-Chi earthquake on Chenyulan watershed landscape pattern in central Taiwan using landscapemetrics. Environ Management 38(1): 108-125.
Sidle RC, Ochiai H (2006) Landslides: processes, prediction and land use. Am. Geophys. Union,Water Resources Monograph No. 18, Washington, D.C., 312 p.
Keywords: Earthquake induced landslides, Fissures, hydrological pathway, slope stability- 385 -