Dynamic background of the 2017 Ms7.0 Jiuzhaigou (China) earthquake

Abstract

A Ms7.0 earthquake occurred in Jiuzhaigou in the Sichuan Province of China on August 8, 2017. The few published reports on this earthquake have focused on the seismogenic structure and the focal mechanism, but there is currently no detailed account of the dynamic process of this earthquake. We gathered continuous vertical seismic data recorded in 2017 at 18 seismic stations located near the earthquake’s epicenter. After analyzing this data with wavelet filtering algorithms, we obtained seismic data in 6 frequency bands (with central periods of 1\xa0s, 5\xa0s, 10\xa0s, 20\xa0s, 30\xa0s, and 40\xa0s). We determined the maximum amplitude value for every 6-min interval of data, and then calculated the amplitude ratio by dividing the maximum amplitude for each window by the annual amplitude mode value. Our results reveal that the amplitude ratios increased at certain stations from May to July of 2017. In our analysis of the observational gravity data collected from two stations over the same period of time, we found gravity anomalies in the Bayan Har block; these changes corroborate the observed amplitude ratio changes in our study area. In the different frequency bands of the ground vertical motion, the highest amplitude ratio values appear in the frequency band with a central period of 20–40\xa0s. Spatially, these high amplitude ratios coincide with two areas: the southern part of the Minjiang fault, and the northern end of the Longmenshan and east side of the Huya fault. The largest amplitude ratio values on the south side of the Minjiang fault have a small period; these anomalies represent the rapid movement of the shallow crust as the Qinghai Tibet block migrates to the east. We attribute the second region with high amplitude ratios, which is located between the northern end of Longmenshan and eastern side of the Huya faults, to seismic activity accommodated by the Longmenshan fault system. Based on our observations and those of published works, we conclude that these abnormal amplitude ratio values provide insight into the formation and manifestation of the Jiuzhaigou earthquake. The energy accumulation and triggering of this earthquake are closely related to the movement of a high-conductivity layer located in our study area. The stress accumulation between the Minjiang fault and the Huya fault created the conditions that facilitated the earthquake. The Longmenshan fault system obstructed the eastward movement of the Qinghai Tibet Plateau block, and the lateral extrusion of deep crustal material either accelerated or triggered the earthquake. The spatial evolution of the high amplitude ratios indicates that the main seismogenic fault in this earthquake is the northern extension of the Huya fault.