Crustal velocity structure around the eastern Himalayan syntaxis: Implications for the nucleation mechanism of the 2017 Ms 6.9 Mainling earthquake and regional tectonics

A magnitude (Ms) 6.9 earthquake occurred on November 18, 2017, in Mainling, southeastern Tibet, which is the largest earthquake to occur in or around the eastern Himalayan syntaxis within the last 50 years. To further understand the nucleation mechanism of this earthquake, the local seismicity distribution, and the regional tectonics, the detailed crustal velocity structure is obtained by direct ambient noise tomography using the Rayleigh wave phase velocity data (T=5 ∼ 40 s) in this area. The spatial correlation of the velocity model with the distribution of the seismicity reveals that the nucleation mechanism of the 2017 Ms 6.9 Mainling earthquake is closely related to the local stress condition and velocity structure and to the aqueous fluid and geothermal states. Our velocity model is extensively compared with previous magnetotelluric inversion results and reveals the distribution and connectivity of the crustal low velocity zones (LVZs). The general patterns of the velocity model in the middle crust show LVZs in the northwest and high velocity zones in the southeast that are truncated by major faults. We also find a relatively low-velocity anomaly in the upper crust near the Namche Barwa massif; this can be explained by the conceptional 'tectonic aneurysms' model, which emphasizes the coupling effects of climate, erosion, and tectonics. To reconcile our velocity model with previous observations, we propose a tentatively integrated geodynamic model to explain the regional seismicity and stress condition.