Deformation of the Haiyuan-Liupanshan fault zone inferred from the denser GPS observations

The Haiyuan-Liupanshan fault, an active tectonic feature at the Tibetan Plateau's northeastern boundary, was ruptured by two M8 earthquakes (1920 and 1927) bracketing an unbroken section (the Tianzhu seismic gap). A high seismic hazard is expected along the gap.To monitor deformation characteristics and do a seismic risk assessment, we made measurements at two newly built campaign-mode Global Positioning System (GPS) stations and 13 pre-existing stations in 2013 and 2014.Adding existing data from 1999 to 2014, we derived a new velocity field.Based on the horizontal velocity, we used three block models to invert the deformation of four crustal blocks. The results suggest non-uniform deformation in the interior of the Lanzhou block, the Ordos block and the Alaxan block, but uniform deformation in the Qilian block.Fault slip rates derived from block models show a decreasing trend from west to east, (2.0-3.2 mm/a on the Haiyuan fault to 0.9-1.5 mm/a on the Liupanshan fault).The Haiyuan fault evidences sinistral striking-slip movement, while the Liupanshan fault is primarily thrusting due to transformation of the displacement between the strike-slip and crustal shortening.The locking depth of each segment along the Haiyuan fault obtained by fitting the fault parallel velocities varies drastically from west to east (21.8-7.1 km).The moment accumulation rate, calculated using the slip rate and locking depth, is positively correlated with the locking depth.Given the paucity of large seismic events during the previous millennium, the Tuolaishan segment and the Maomaoshan segment have higher likelihood of nucleation for a future event.