Geophysical investigation and dynamic modelling of unstable slopes: case-study of Kainama (Kyrgyzstan)

Abstract

S U M M A R YThe presence of massive Quaternary loess units at the eastern border of the Fergana Basin (Kyrgyzstan, Central Asia) makes this area particularly prone to the development of catastrophic loess earthflows, causing damages and injuries almost every year. Efficient disaster management requires a good understanding of the main causes of these mass movements, that is, increased groundwater pressure and seismic shaking. This paper focuses on the Kainama earthflow, mainly composed of loess, which occurred in 2004 April. Its high velocity and the long run-out zone caused the destruction of 12 houses and the death of 33 people. In summer 2005, a field survey consisting of geophysical and seismological measurements was carried out along the adjacent slope. By combination and geostatistical analysis of these data, a reliable 3-D model of the geometry and properties of the subsurface layers, as shown in the first part of the paper, was created. The analysis of the seismological data allowed us to point out a correlation between the thickness of the loess cover and the measured resonance frequencies and associated amplification potential. The second part of this paper is focused on the study of the seismic response of the slope by numerical simulations, using a 2-D finite difference code named FLAC. Modelling of the seismic amplification potential along the slope confirmed the results obtained from the seismological survey-strong amplifications at the crest and bottom of the slope where there is a thick loess cover and almost no amplification in the middle part of the slope. Furthermore, dynamic slope stability analyses were conducted to assess the influence of local amplifications and increased groundwater pressures on the slope failure. The results of the dynamic modelling, although preliminary, show that a combination of seismic and hydrologic origin (pore pressure build-up during the seismic shaking) is the most probable scenario responsible for the 2004 failure.

Tien Shan Geohazards Database: Earthquakes and landslides

“…Notably, in the central Tien Shan, field surveys are carried out by students in the frame of a field school organised by Strom and Abdrakhmatov (2009). Geophysical field surveys have been completed on/near a dozen of mainly earthquake-triggered mass movements; related results were presented in Master's Theses (de Marneffe, 2010;Dupont, 2011;Deprez, 2012); earlier surveys are summarized by Havenith et al (2003Havenith et al ( , 2006a, Danneels et al (2008). For sub-regions, such as the Maily-Say Valley (part of Z4 in Fig.…”

“…On the basis of these investigations and modelling of possible trigger factors, Danneels et al (2008) concluded that this loess flow was most likely due to a combination of long-term slope destabilisation and short-term triggers. The latter include intensified water infiltration in ground fractures that may have appeared after two Ms = 4.4 and Ms = 4.5 events that occurred 2 weeks before the landslide (on April 8 and 9, 2004, at an epicentral distance of 43 and 45 km, respectively).…”

A new classification of earthquake-induced landslide event sizes based on seismotectonic, topographic, climatic and geologic factors

“…These observations highlight the particular susceptibility of loess areas to seismic ground failure, such as it was clearly shown by Derbyshire et al (2000) analysing geological hazards affecting the loess plateau of China. In addition to those notes directly concerning the 1920 Haiyuan-Gansu earthquake, we also refer to the observations by Danneels et al (2008) who proved that a thick cover of loess (especially if present on the top of slopes) can induce strong amplification of the seismic motion. Thus, the maximum value of 4 was attributed to the 'Lithological factor' of the Haiyuan-Gansu event due to both the weak stability of loess-covered slopes and to site amplification.…”

Evaluation of Remote-Sensing-Based Landslide Inventories for Hazard Assessment in Southern Kyrgyzstan

“…These variations have been linked to snow accumulation in the winter months, intensity of snow melting and additional precipitation during the snow melt [18,19]. Earthquakes contribute to the destabilization and can act together with the hydrometeorological factor [20].…”