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

Abstract

This work reports the results of an accelerometer monitoring aimed at revealing the seismic response of hillslopes in the town of Qiaozhuang, in Qingchuan County, near the north-eastern end of the fault ruptured during the 2008 Mw 7.9 Wenchuan earthquake in 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 accelerometer 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 and 5.5 and epicentral distance ranged from a few to 90km. The accelerometer records provided evidence of directional amplification, which was investigated by analyzing the polar diagrams of normalized Arias intensity (Ia) and the horizontal to vertical spectral ratios (HVSR). Evidence of the anisotropic dynamic response and site specific resonance frequencies was obtained for both topographic reliefs.However, the ground shaking maxima orientations differed depending on the local geological setting: in one case they were transversal and in the other case sub-parallel to the relief elongation.No preferential direction of maximum shaking was observed at the site in the valley. Furthermore, evidence of resonance was derived from the calculation of spectral ratio between the sites on the slope and those at the foot of the hills. The resonance was more pronounced at higher elevations, which suggested a possible occurrence of topographic amplification. Resonance frequencies were lower (3–5Hz) on the smaller hill consisting of sub-vertically layered phyllites and higher (up to 7Hz) on the larger hill made mainly of limestones, whereas an opposite relation between resonance frequency and hill size could be expected from a purely topographic effect. This and the presence of amplification factors larger than 2 suggest that, in addition to topographic effects, local geology also played a significant role in differentiating the site response.