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On January 21, 2016, an MS6.4 earthquake occurred in Menyuan county, Qinghai province. Field investigations suggest that the long axis of the elliptical affected area is N120°-140°E. Comparative interpretations of high resolution remote sensing images before and after the shock show more than 23 sites of landslides and collapses, which are more distributed in the north of epicenter than in the south, extending in NNW overall. As the Lenglong Ling fault (LLF) is dominated by horizontal movement from high resolution remote images and field observation, it should not be responsible for this event of pure thrusting. North of the Lenglong Ling fault, there is a fault striking in 140°, which is north-dipping on the surface according to high resolution remote sensing, similar to the nodal plane I of focal mechanism solutions from deferent institutes. The long axis' direction of the overall distribution of aftershocks has also a difference of 20° with LLF, while almost parallel to the fault on its north. Thus, we infer that this fault is most likely the seismogenic structure of the Menyuan earthquake. Integrating the characteristics of aftershock distribution at depth, main shock focal mechanism solutions, geometric distribution and activity of the seismogenic fault on surface, in combination with the magnetotelluric sounding and related geophysical data around the earthquake zone, a cartoon model of the seismogenic mechanism is established. We speculate this earthquake is one of the events occurring during the pushing growth process of the Tibetan Plateau blocks in NE direction following the 2008 MS7.4 and 2014 MS7.3 earthquakes in Yutian, Xinjiang.

The nearly EW-trending East Kunlun fault zone is the north boundary of the Bayan Har block. The activity characteristics and the position of the eastern end of its eastward extension are of great significance to probing into the dynamic mechanism of formation of the east edge of the Tibetan Plateau, and also lay the foundation for seismic risk assessment of the fault zone. The following results are obtained by analysis based on satellite image interpretation of landforms, surface rupture survey, terrace scarp deformation survey, and terrace dating data on the eastern part of the East Kunlun fault zone: (1) the Luocha segment is a Holocene active fault, where a reverse L-shape paleoearthquake surface rupture zone of about 50 km long is located; (2) the Luocha segment is characterized by left-lateral slip movement under the compression-shear condition since the later period of the Late Pleistocene, with a rate of 7.68–9.37 mm/a and a vertical slip rate of 0.7–0.9 mm/a, which are basically in accord with the activity rate of segments on its west side. The results indicate that it is a part of eastward extension of the East Kunlun fault zone; (3) the high-speed linear horizontal slip of the nearly EW-trending East Kunlun fault zone is blocked by the South China block at east, and transforms into the vertical movement of the nearly SN-NNE trending Minjiang fault zone and the Longmenshan fault zone, and the uplift of Longmenshan and Minjiang. The area where transform of the two tectonic systems occurred confines the position of the east end; (4) Luocha segment and Maqu segment constitute the “Maqu seismic gap”, so, seismic risk at Maqu segment is higher than that at Luocha segment, which should attract more attention.