Seismic response of cracking features in Jubao Mountain during the aftershocks of Jiuzhaigou Ms7.0 earthquake

Abstract

Jiuzhaigou is a world-heritage site located in the plateau area of Northwest Sichuan Province, China. Serious slope failures in the epicentral area were triggered by the Ms7.0 Jiuzhaigou earthquake occurred on August 8, 2017. The source areas of the hazards are usually concentrated near ridge crests, revealing the possible occurrence of ground motion amplification phenomena. To explore the role of the amplification of ground motions in the formation of earthquake-triggered slope failures, two seismometers were installed, on the next day after the main shock, at the bottom of the slope of Jubao Mountain near the seismogenic fault. The two monitoring sites are located at elevations of 1414 m (J1) and 1551 m (J2, the top of the mountain). Five aftershocks were recorded by the monitoring instruments. We compared the mean levels of the peak ground acceleration (PGA) observed at different locations, and investigated the directional variations in the shaking energy by analyzing the polar diagrams of the Arias intensity (Ia). Then, in order to identify the directional resonance phenomenon and their frequencies and amplification coefficients, we examined the horizontal-to-vertical spectral ratio (HVSR) and the standard spectral ratio (SSR). Polar diagrams of the Arias intensity (Ia) indicated that the site response of Jubao Mountain showed a pronounced directivity (in the EW direction) with shaking maxima near the hill top oriented orthogonally to the elongation of the relief. We observed an obvious resonance phenomenon at site J2 at relatively low frequencies (2.5–9 Hz) and very weak spectral amplifications at site J1 at high frequencies (5–15 Hz), which suggested that the predominant frequency of monitoring site J2 was obviously attenuated and that the difference in the spectra was related to the influences of the local-scale site conditions of the whole mountain. The results of spectral ratio analyses (HVSR and SSR) showed that the direction of resonance was concentrated around an EW orientation, and the amplification factors near the hill top were larger than 2. It suggests that geologic factors also play a significant role in the anisotropic amplifications affecting the tops of slopes besides the topographic effects.