The 2017 Jiuzhaigou Earthquake: A Complicated Event Occurred in a Young Fault System

The Minshan Uplift Zone (MUZ) is located at the eastern margin of the Tibetan Plateau, which is the junction of three tectonic terranes. The observed discrepancy between a high uplifting and low shortening rate over the MUZ is attributed to the intrusion of a viscous lower crust. In the last 50 years, several significant earthquakes occurred at the boundaries of the MUZ, that is, the Huya and Mingjiang faults. On 8 August 2017, the Jiuzhaigou earthquake (Mw 6.5) occurred on the northern extension of the Huya fault. We adopt a joint inversion of the interferometric synthetic aperture radar and teleseismic body wave data to investigate the rupture process of this event. The obtained slip model is dominated by left‐lateral strike slips on a subvertical fault presenting significant shallow slip deficit. The rupture initiation is composed of both thrust and strike‐slip mechanisms producing a non‐double‐couple solution. We also resolve a secondary fault branch forming an obtuse angle with the main fault plane at its northern end. These phenomena indicate that the northern Huya fault is a young (less mature) fault system. Focal mechanisms of the regional earthquakes demonstrate that the northern and southern Huya faults present different combinations of strike‐slip and reversed motion. We attribute such discrepancy to the lateral extension of the viscous lower crust, which appears to extrude to the east beyond the northern Huya fault, in comparison with that confined under the MUZ near the southern Huya fault. This conceptual model is also supported by geomorphological and magnetotelluric observations. Plain Language Summary Earthquake ruptures are controlled by tectonic loading; thus, researching on the earthquake rupture mechanism reflects the tectonic loading mechanism. The Minshan Uplift Zone is located at the eastern margin of the Tibetan Plateau. It is the junction of three tectonic blocks, which presents fast uplift rate and slow shortening rate. Thus, injection of a soft or viscous lower crust may explain this deformation phenomenon. By investigating the 2017 Jiuzhaigou earthquake and the regional earthquakes near the Minshan Uplift Zone, we found that the eastern margin of the Minshan Uplift Zone, so‐called the Huya fault, is a young fault system, which presents a high complexity of rupture processes. The sense of motion also varies between the northern and southern Huya faults. These phenomena appear to be related to the difference of the lower crust in