Interpretation of dynamic response of a very complex landslide (Latian-Tehran) based on ambient noise investigation

Single station ambient noise measurements were performed in 122 points on a complex landslide at the left bank of Latian Dam reservoir in the north-east of Tehran (Iran), to investigate the seismic site response of a landslide, especially the nature and causative factor of directional site amplification. Recorded data were analyzed through the horizontal to vertical spectral ratio technique to obtain the resonance frequencies (f0), spectral ratio amplitude (A) and orientation and intensity of directional response over sliding and surrounding area. The results have a great coherence to geotechnical and geological investigation result and are tightly related to the landslide structure in such a way that outside the sliding area there is no or negligible sign of directional response. In general the result revealed two range of frequencies with high amplitude and directivity over the landslide mass; the first one around 1.2 Hz, ubiquitous, that seems to be related to normal mode vibration of the entire unstable mass, named as underlying layer in this paper. The second one at higher frequencies that is spatially variable over the landslide, which seems to be related to the effect of local surficial features (such as local sharp curvatures and ridges or blocks detached by tension fractures). Directional response pattern of both the underlying layer and surficial features is controlled by 2D or 3D features causing higher amplitude in the most kinematically free direction. It seems that the seismic response of the underlying layer at the surface is modified by directional filtering of the overlying superficial layer. This modification effects tightly related to the transfer function of surficial features especially their cross-coupling terms. Based on this study, a general rule is proposed to declare conditions which are essential to induce severe directional response at a specific site.