Abstract
On November 18, 2017, the Mainling Mw 6.5 earthquake occurred on the northern Namche Barwa syntaxis and was the largest earthquake in the syntaxis and surrounding areas since the Zayu Mw 8.4 earthquake in 1950. Due to inconvenient access and the severe environment in the Namche Barwa syntaxis area, the motions and tectonic structures of most faults remain unclear. Sparsely distributed seismic observation stations make the seismogenic fault and focal mechanism of the Mainling earthquake controversial. We adopt interferometric synthetic aperture radar (InSAR) to invert the slip distribution of this event by defining the fault geometry with relocated aftershocks. Our preferred model suggests that the 2017 Mainling earthquake ruptured two blind faults beneath the Namche Barwa syntaxis. The ruptures were dominated by thrusts with slight right-lateral strike-slip components. The slips on the two faults are equivalent to moment magnitudes of Mw 6.12 and Mw 6.34, with maximum dislocation magnitudes of 0.36 m and 0.43 m, respectively. The model fits well with the InSAR observations and the distribution of aftershocks. The results from the Coulomb stress simulation indicate that the stress loading caused by strong historical events promoted the occurrence of the 2017 Mainling earthquake. Compared with the seismogenic faults of the Mainling earthquake, the larger thrust faults in the southern Namche Barwa syntaxis can generate larger earthquakes. Therefore, we assume that Mw > 6.5 earthquakes may occur beneath the Namche Barwa syntaxis and that the seismic risk has been further promoted by historical events.
Highlights
In the Himalayan region, the Indian Plate is sliding beneath the Eurasian Plate at a rate of 40–50 mm/a, making the 2500-km-long Himalayan arc the most spectacular topographic manifestation on Earth in ~ 50 Ma (Yin and Harrison 2000)
The results show that the 2017 Mainling earthquake occurred in the positive stress zone of the preceding earthquakes
The results of various models show that the Coulomb stress change exceeds 0.217 MPa, which is much higher than 0.01 MPa, a proposed threshold value suggested for earthquake triggering (King et al 1994; Heidbach and Ben-Avraham 2007)
Summary
In the Himalayan region, the Indian Plate is sliding beneath the Eurasian Plate at a rate of 40–50 mm/a, making the 2500-km-long Himalayan arc the most spectacular topographic manifestation on Earth in ~ 50 Ma (Yin and Harrison 2000). The viscosity coefficients of the lower crust and upper mantle are set at 1 × 1019 Pa s and 1 × 1020 Pa s, respectively (model 1; Shan et al 2013); the viscosity coefficients of the lower crust selected by model 2 and model 3 (Yin et al 2018a) are used in stability analysis on the layered viscoelastic half-space crust model (Table 3), which considers gravity and can effectively simulate the coseismic and post-seismic deformation and stress changes caused by earthquakes. Historical earthquakes Li et al (2014) collected the global observational phase recordings of the Mw 7.5 Lang earthquake in 1947 and redetermined the source parameters of the earthquake using the location method provided by the China Earthquake Networks Center (CENC) On this basis, the focal mechanism solution was recalculated as strike = 195°, dip = 84°, and rake = 172°, which is consistent with the slip mechanism of the Lilong fault (Table 4)
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