Imaging the migration of seismicity along the Zongwulongshan thrust fault zone in Qinghai, China from 2003 to 2009 using spaceborne radar interferometry

Three MW ~6.3 earthquakes followed by a series of large aftershocks occurred on the north margin of Qaidam Basin in Qinghai, China during the period from 2003 to 2009. All the observed events were located within a 150-km-long and 50-km-wide zone along the Zongwulongshan thrust fault. In this study, over 400 unwrapped interferograms covering the entire earthquake areas from 5 ENVISAT ASAR tracks were generated by using the JPL/Caltech ROI_PAC software. We attempted to retrieve surface displacements in different phases (coseismic and postseismic) , rather than to obtain a simple mean velocity. A novel iterative time series (TS) analysis method was used to separate surface displacements from various sources of errors (e.g. orbital ramps, DEM errors and atmospheric delays (APS)) by fully utilizing the datapoints from overlapping regions. Combining a network strategy, a wavelet-based algorithm was employed to estimate topography-related APS. Five independent coseismic interferograms resulting from 6 different events were generated, of which one displacement map resulted from the 28th Aug 2009 MW 6.3 earthquake and its latest largest aftershock 3 days after the mainshock together. Significant postseismic displacements following all the three mainshocks were observed even during the period from Nov 2008 to Aug 2009. In the first three months after the Nov 2008 mainshock, a maximum accumulative displacement of ~ 3 cm was detected. From then until the 2009 mainshock, no clear surface displacement was found. Using geodetic modeling, the fault parameters of three mainshocks were determined, which are consistent with seismic solutions. The principal thrust slips characterized the Qilianshan-nanshan compressional belt caused by the collision of India to Eurasian plates. However, the oblique slip mechanisms of another three large aftershocks were not consistent with their mainshocks, suggesting that the main shocks had possibly changed the stress state in the vicinity of the hypocentres. A simple numerical experiment indicated that the westward migration of seismicity along the Zongwulongshan fault from 2003 to 2009 was likely controlled by a large creep along the Altyn Tagh fault. Tectonic settings and coseismic interferograms associated with three Mw6.3 Earthquakes and their large aftershocks during the period of 2003-2009. The gray lines indicate the distribtuion of active faults in the study area. The broken polygon suggests the spatial location of the sub-figure which suggests the coseismic surface changes resulting from the 2003 Mw6.3 Delingha Earthquake.