Site Effect Analysis around the Seismically Induced Ananevo Rockslide, Kyrgyzstan

Abstract

In 1911, the surface-wave magnitude 8.2 Kemin earthquake hit northeastern Tien Shan (Kyrgyzstan), close to the cities of Bishkek and Almaty, the capitals of Kyrgyzstan and Kazakhstan, respectively. Several hundreds of people were killed by the earthquake, some by indirect effects such as landslides and mudflows. A particular but nonfatal landslide triggered by the Kemin event was a rockslide in the vicinity of Ananevo, north of lake Issyk Kul (Kyrgyzstan) rockslide located above the fault zone activated in 1911. In the summer of 1999, a geophysical-seismological field trip was organized to study geology and to record seismic ground motions on and around the Ananevo rockslide. The work was part of project assessing seismogenic landslide hazard in northern Kyrgyzstan, based on various case studies of slope failures in connection with site-specific ground-motion dynamics. The geophysical investigations consisted of seismic refraction tests processed as 2D seismic tomographies and surface-wave inversion, which were combined to build a 3D geophysical model of the landslide site. Ground motions from small earthquakes were analyzed using several techniques to define site effects over the mountain massif. Both H/V and standard spectral ratios indicated lower dominant frequencies with stronger amplification in the crest region with respect to the mountain slope. These effects could be partially simulated by 1D, 2D, and 3D finite-element modeling. By comparing the numerical results with the experimental data, the presence of a surficial low-velocity layer of varying thickness appeared to be the key factor controlling the ground motion around the rockslide.