Aftershock sequence and fault geometry of the 2017 Mw 6.5 Jiuzhaigou earthquake: an application of AsA network, denoising algorithms and fault geometry determination techniques

Micro-seismicity is a powerful tool to investigate seismic activity, fault geometry, seismic and aseismic fault behaviors. To investigate the fault property of the Northern Huya Fault, we deployed an locally refined network near the source region of the 2017 Mw 6.5 Jiuzhaigou earthquake for three months to detect its aftershock activities. We carried out an experimental seismic network using locally refined station distribution (Array of small Arrays, AsA). Meanwhile, the Geometric Mean Envelop (GME) algorithm is proposed for AsA to suppress random and impulsive noises, in terms to enhance weak seismic signals for better detection performances. The performance differences between GME algorithm and other denoising algorithms are compared. We adopted STA/LTA algorithm to the denoised waveforms and further adopt Matched Filter Technique and double difference relocation techniques. A more complete aftershock sequence of the observation period is obtained with 49639 events detected.We adopted K-means method to cluster aftershocks and use principal component analysis to determine fault geometry from each cluster of aftershocks. We identified complex fault geometry from the aftershock catalog. Our results show that: (1)The AsA network is an effective design which is suitable for fast and short term aftershock monitoring purposes over high relief area. (2) The associated signal enhancement algorithms, such as GME algorithm, can effectively suppress impulse noises, and improve the detection performances of traditional detection algorithms. (3)The northern Huya fault is complicated fault system, which shows multiple fault branch and different dipping angles. Its northern segments dips 84.45° to the west which is also accompanied with multiple branches. The southern segment dips 68.02° to the east. The hypocenter of the Jiuzhaigou earthquake locates at the junction of these two segments.