Evidence of hillslope directional amplification from accelerometer recordings at Qiaozhuang (Sichuan — China). Engineering Geology

This work reports the results of an accelerometer monitoring aimed at revealing the seismic response of hillslopes in the area of Qiaozhuan town (Qingchuan county), located near the north-eastern end of the fault ruptured during the 7.9 Mw 2008 Wenchuan earthquake (Sichuan Province). Serious damage and slope failures were induced by this earthquake in the town center and on the hills in the peri-urban zone. This suggested the possible occurrence of amplification phenomena. Five recording stations were emplaced at two topographic reliefs to investigate their response to ground motion during the last part of the Wenchuan seismic sequence. About 50 aftershocks were recorded, whose magnitude (ML) varied between 1.2~5.5 and epicentral distance ranged from a few to 90 km. The recordings provided evidence of directional amplification. This phenomenon was investigated by analyzing polar diagrams of normalized Arias intensity (Ia) and horizontal to vertical spectral ratios (HVSR) to find, respectively, polarization azimuth and resonance frequencies. Evidences of anisotropic dynamic response were found at both investigated topographic reliefs, but with ground shaking maxima differently oriented with respect to the relief elongation: in one case transversal and in the other case sub-parallel. No clear preferential direction of maximum site response was observed at a site in the valley. Furthermore evidence of resonance was derived from the calculation of spectral ratio between the sites on the slope and at the foot of the hills. The resonance was more pronounced at higher elevations, which suggested a role of topographic amplification. Resonance frequencies were lower (3-5 Hz) on the hill consisting of sub-vertically layered phyllites, and higher (up to 7 Hz) on the other, larger hill made mainly of limestones, while an opposite relation between resonance frequency and hill size could be expected from a purely topographic effect. This and the amplification factor larger than 2 suggest that in addition to topographic effects also local geology played a significant role in differentiating the site response.