Temporal and Spatial Evolution of Load/Unload Response Ratio Before the M7.0 Jiuzhaigou Earthquake of Aug. 8, 2017 in Sichuan Province

Abstract

The load/unload response ratio (LURR) method has been proposed to evaluate earthquake potential. This method can be used to investigate the regional stress–strain field change and fault instability associated with the preparation of a large earthquake. In this study, we explore the temporal and spatial evolution of the LURR anomalies before the M7.0 Jiuzhaigou earthquake that occurred on Aug. 8, 2017 in Sichuan Province. We calculate the LURR values by using Benioff strain of the small earthquakes within 100, 200, 300 and 500 km from the epicenter and 12 years prior to the main shock. The loading and unloading phases are determined by calculating the change of Coulomb failure stress caused by earth tide in the tectonically preferred slip direction on the fault surface of the major event. Results show that the LURR values remain at a low level for years until 2014 when the LURR values increase to an anomalously high level. Meanwhile, the magnitude of the anomalies decrease with the distance from the epicenter, and can hardly be observed within the 500-km radius. Combining with the spatial distribution of the LURR, we find that the development of anomalies before the mainshock can be divided into two stages: during the periods of 2014–2015, the significant anomalies were observed in the epicenter and its northwest region; and then in 2016, the anomalies were detected in the southeast area of the earthquake, which expanded around the Huya fault and finally reached the epicenter months before the mainshock. The earthquake is located at the intersection area of LURR anomalies of the two time periods, suggesting the generation of the Jiuzhaigou earthquake.