The Seismogenic Structure of the 2017 Mw 6.9 Milin, Tibet, Earthquake: A Possible Newly Active Fault at the Eastern Himalayan Syntaxis

On 18 November 2017, an Mw 6.9 earthquake occurred in Milin, Tibet, with the epicenter at the top of the Namche Barwa syntaxis. This event did not produce surface ruptures, and its seismogenic structure remains unclear or controversial. Using the locations of the Milin mainshock and aftershocks, locations of regional small earthquakes and focal mechanism solutions from 2007 to 2009, this work analyzed the causative fault and tectonic setting of the Milin earthquake and assessed the regional seismic risk. The results suggest that the seismogenic structure of the Milin earthquake was a secondary fault, the southern branch of the XiXingla fault (XXLF). Within 28 hr after the mainshock, the aftershocks of the Milin event spread northeastward to the secondary north branch fault of the XXLF and the secondary south branch fault of the Palong–Pangxin fault. Across the top of the Namche Barwa syntaxis (Namche Barwa block) and the Chayu block in the southeast, an earthquake dense belt (EDB) has developed. This EDB has similar deep structures beneath the two blocks, in which several northeast-dipping structural planes exit, and different portions of the EDB imply a unified tectonic stress field. Combining these data with the foreshock–mainshock–aftershock data for the 1950 Mw 8.6 Chayu, Tibet, earthquake, we speculate that the structural planes produced by the EDB at depth in the two blocks have already been connected or tended to connect, resulting in a new fault system trending northwest and approximately 280 km long. The 2017 Mw 6.9 Milin earthquake occurred at the northwestern end of this fault system. At present, the development stage, maturity, and fine structure of this new fault system remain unclear but should receive additional attention. Based on its maximum rupture area, this new fault system is capable of generating an Mw 7.7 earthquake in the future.

Enhanced Quaternary exhumation in the Namche Barwa syntaxis, eastern Himalaya

The Namche Barwa syntaxis in the eastern Himalaya is rapidly evolving in terms of its tectonics and topography. Here we constrain the exhumation history of the Yigong River to the immediate north of the syntaxis across different time scales using a multidisciplinary approach. Our new thermochronometric data reveal an acceleration of exhumation rates since 2 Ma in the downstream of the Yigong. Cosmogenic nuclides and thermoluminescence thermochronometry analyses confirm persistent rapid exhumation in the lower Yigong over the Quaternary with further increased exhumation in the last 100 ka. Together with the analysis of the morphology of the Yigong River profile, we interpret that northward expansion of the syntaxis together with capture of the Yigong by the Yarlung Tsangpo River during this expansion is responsible for the exhumation history of the Yigong River in the Quaternary.

Supplemental Material: Enhanced Quaternary exhumation in the Namche Barwa syntaxis, eastern Himalaya

Analytical approaches used in this study, analyses parameters, and resulting data.<br>

Supplemental Material: Enhanced Quaternary exhumation in the Namche Barwa syntaxis, eastern Himalaya

Analytical approaches used in this study, analyses parameters, and resulting data.<br>

Geological and Geomorphological Evidence for Activity along the Motuo Fault, Eastern Side of the Namche Barwa Syntaxis, Tibetan Plateau

The Motuo fault (MTF) strikes along the Yarlung Zangbo suture zone on the eastern boundary of the Namche Barwa syntaxis. The movement pattern and Quaternary activity of the MTF remain unclear, which hampers efforts to undertake meaningful seismic hazard assessments near the southeastern part of the Tibetan plateau and to understand the tectonic evolution of the Namche Barwa syntaxis. In this study, the MTF is shown to feature left-lateral strike-slip movements with offset gullies and mountain ridges and appears to have ruptured during the late Pleistocene to Holocene, as evidenced from geological, paleoseismic, and radiocarbon dating investigations. Specifically, at least three surface-rupturing paleoseismic events are revealed; two events occurred after 2606 B.P. and after 18.2 ka. Combining this information with previous Global Positioning System results in southeastern Tibet, we suggest that, as a boundary fault, the MTF regulates the movements of the Namche Barwa and Chayu blocks. The velocity difference between the two blocks advancing to the north is the main mechanism of left-lateral strike-slip motion along the MTF. The accumulation and release of shear stress between the two blocks have led to strong activity along the MTF, since the late Quaternary.