Constraints on the seismogenic faults of the 2003–2004 Delingha earthquakes by InSAR and modeling

• Use the 3D finite element method to model the InSAR deformation interferograms.
• Determine the seismogenic fault through comparing the model results with InSAR observations.
• Analayze the shallow slip deficit of the co-seismic deformation of the 2003 Delingha earthquake.

The 2003 Mw6.3 Delingha earthquake happened at the Dacaidan-Zongwulong fault system, which is an important active tectonic belt on the northeastern margin of Tibetan plateau. Until the end of 2004, 6 approximately Mw5.0 aftershocks occurred on almost the same seismogenic fault. The conventional viewpoint is that the seismic hazard near the continental seismogenic fault will be reduced in decades or centuries after rupture. Why did so many large earthquakes happen on almost the same seismogenic fault within a short temporal interval? The seismogenic fault information and the character of the co-seismic slip distribution are the keys to answer this problem. In this paper, the co-seismic slip distribution of the 2003 Mw6.3 Delingha earthquake was inverted using 18 teleseismic broadband P waveforms, 8 SH waveforms and 28 long period surface waveforms data. The peak slip concentrates at 12 km depth beneath the epicenter, and the slip decreases gradually from 6 km depth to surface and beneath 12 km depth. The shallow slip deficit of the co-seismic slip distribution may be caused by inelastic failures in the uppermost crust. Beneath 12 km depth, the strain energy may be accumulated by the velocity-strengthening frition behavior and may trigger aftershock. Three post-seismic InSAR interferograms spanning the most aftershocks were formed. Through analysis on the aftershock distribution and the focal mechanisms of Mw ⩾ 4.9 aftershocks, two 3D finite element models containing topography were built and used to simulate the InSAR observations. Comparing the modeled results with the InSAR observations, we obtained the seismic source of the 2003–2004 Delingha earthqakes. It consists of two southwest-dipping seismogenic faults, named the west-fault and the east-fault. The east-fault extends beneath the west-fault. The 2003 Mw6.3 Delingha earthquake occurred on the west-fault, while 6 large aftershocks occurred on the east-fault. This shows that the complex spatial location relationship between the seismogenic faults may be an important factor to the large earthquake occurrence with high frequency at almost the same locality. Due to special tectonic setting and complex structure, the Dacaidan-Zongwulong fault system will be a high seismic risk zone in the future.