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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.

With the 2008 MS6.1 Panzhihua earthquake as a case study, we demonstrate that the focal depth of the main shock can be well constrained with two approaches: (1) using the depth phase sPL and (2) using full waveform inversion of local and teleseismic data. We also show that focal depths can be well constrained using the depth phase sPL with single broadband seismic station. Our study indicates that the main shock is located at a depth of 11 km, much shallower than those from other studies, confirming that the earthquake occurs in upper crust. Aftershocks are located in the depth range of 11–16 km, which is consistent with a ruptured near vertical fault whose width is about 10 km, as expected for an MS6.1 earthquake.

To better understand repeatability of strong earthquakes in previously ruptured zones during one seismogenic period, we studied the rupture zones of the doublet of M6 earthquakes in Zhongba region of southcentral Tibet, China, in 11 July 2004 and 7 April 2005, respectively. We focused on the overlapping degree of two strong quakes’ aftershock areas one week after the mainshocks by using the SQH station in China Seismic Network and a 68-stations temporary broadband seismic array, a part of the international HI-CLIMB project. About 115 local earthquakes were recorded in one week after the mainquakes, and we located these earthquakes by master event relative location (MERL) method. We also used this method to relocate 31 other M3.7+ earthquakes from 1 July 2004 to 1 July 2005. Meanwhile, we studied two mainshocks’ coseismic ruptures with satellite interferometric synthetic aperture radar (InSAR). Our results show that the ruptured zones of the two earthquakes do not overlapp substantially, either from early aftershock data or from InSAR inversions.

We use interferometric synthetic aperture radar (InSAR) and broadband seismic waveform data to estimate a source model of the 11th July, 2004 MW6.2 Zhongba earthquake, Tibet of China. This event occurred within the seismically active zone of southwestern Tibetan Plateau where the east-west extension of the upper crust is observed. Because of limitations in one pair of InSAR data available, there are trade-offs among centroid depth, rupture area and amount of slip. Available seismic data tightly constrain the focal mechanism and centroid depth of the earthquake but not the horizontal location. Together, two complementary data sets can be used to identify the actual fault plane, better constrain the slip model and event location. We first use regional seismic waveform to estimate point source mechanism, then InSAR data is used to obtain better location. Finally, a joint inversion of teleseismic P-waves and InSAR data is performed to obtain a distributed model. Our preferred point source mechanism indicates a seismic moment of ∼2.2×1018 N·m (MW6.2), a fault plane solution of 171° (342°)/42°(48°)/−83°(−97°), corresponding to strike/dip/rake, and a depth of 11 km. The fault plane with strike of 171° and dip of 42° is identified as the ruptured fault with the aid of InSAR data. The preferred source model features compact area of slips between depth of 5–11 km and 10 km along strike with maximum slip amplitude of about 1.5 m.

Ground deformation as observed with GPS or InSAR has been broadly inverted in constraining source parameter of earthquakes. However, for earthquakes occurring beneath sedimentary basins, the very slow sub-surface shear velocity (vS, down to 200 m/s) may cause substantial bias to earthquake source inversion if simple crustal models are used. For Bohai basin, Sichuan basin and rock-sites, we test effects of sub-surface shear velocity structure on ground deformation, and find that up to a factor of 2 overestimate of seismic moment could be generated by the basin structures. Therefore, the very slow sub-surface velocity has to be taken into account before accurate source inversion can be applied.