The violent ground motion before the Jiuzhaigou earthquake Ms7.0

An Ms7.0 earthquake occurred in Jiuzhaigou, Sichuan Province, China, on August 8, 2017. In this study, we used the vertical component data from 31 seismic stations near the epicenter from May 1 to August 20, 2017, to calculate the amplitude spectrum hourly using the fast Fourier transform method. Furthermore, the spectral area of the low frequency band (0.02-1 Hz), which represents the energy of low frequency motion, was calculated. In this way, the temporal and spatial variations in the vertical ground motion in this region were determined. Four high-value processes occurred in mid-June, mid-July, late July, and early August (the last three are discussed in this article). Based on a comparison with the local meteorological data, the meteorological factors had no influence, local geological factors did not affect the results, and typhoon factors did not show obvious correlation. Combined with the results of previous studies, we believe that the increase in the spectral area reflects the intense movement of deep material in the region related to the Jiuzhaigou earthquake. The spatial distribution of the spectral area (energy) reveals that the deep material moved eastward rapidly, was obstructed by the Sichuan Basin, and expanded to the northwest in a U-shaped channel, which may be the main dynamic factor for the formation or triggering of the earthquake.