As early as in 1733, an M~7.8 earthquake (1733DC-M7.8 earthquake) ruptured the 110km-long northern segment of the Xiaojiang fault zone, on the southeastern boundary of Tibetan plateau, and 100 years later, in 1833, another M~8.0 earthquake (1833SM-M8.0 earthquake) broke the 130 km-long middle segment. Here we show a new insight into the source of the 1733DC-M7.8 earthquake through the latest regional GNSS measurements plus our near-fault seismic and GNSS observations since February 2012. An inversion of the regional GNSS measurements resolved an asperity for the 1733DC-M7.8 earthquake that is bearing more slip deficit in comparison with the 1833SM-M8.0 earthquake, implying an Mw7.2+ earthquake, and the near-fault GNSS and seismic observations not only justified this asperity with localized deformation and multi-scaled seismicity but also are declaring an impending rupture of the asperity. More importantly, this work contributes a typical example to the recently proposed model for the generation of large earthquakes.