The 2008/01/09-22 Ms=6.4-5.4 earthquakes in southern central Tibet from InSAR observations and numerical modeling

A sequence of three earthquakes of Ms=6.4, 5.9, and 5.4 occurred within two weeks in January 2008 near the center of south Tibet (85.3°E and 32.3°N). We use interferometric Synthetic Aperture Radar (InSAR) observations to study the fault structure and coseismic displacement of these events. Two pairs of ascending (Track 341 and Track 427) and one pair of descending (Track 348) Envisat data covering the three events are processed using the ROI-PAC software. The observed coseismic deformation shows that these events occurred on two approximately parallel normal faults trending ~25-30° east of north and dipping to the west. We modeled these events using two planar faults embedded in an elastic half-space and divided into 10x10 and 5x5 ~4.5 km- and 3 km-wide patches, respectively. The variable slip solution is obtained by least-square inversion of the observed displacement in the three interferograms while exploring a range of possible geometry and fault dip angles. The minimum misfit between the modeled displacement and InSAR data is obtained with the main rupture fault dipping ~65° and the secondary fault ~70°, both to the west. The coseismic displacement concentrates in the center of the main fault in the 2-16 km depth range with a maximum displacement of ~1.1 m. This slip distribution corresponds to a moment release of ~5.51 1018 Nm, consistent with the seismic moment released by the largest event on January 9. The second fault is located in the hanging wall of the main fault, ~8.5 km away to the west. Coseismic displacement reaches ~0.55 m on this fault, within a depth range of 1-6 km. This slip corresponds to a geodetic moment of ~8.44 1017 Nm, close to the seismic moment released by the January 16 event. The two ruptures could be responsible for the three main earthquakes of the January 2008 cluster, although the location of the January 22 event could not be identified in the radar data depicting the collective contributions of the three events. These observations suggest that active normal faulting in southern Tibet bears relatively high dipping angles and distributes mainly within the upper crust. To investigate the mechanical relationship between the two ruptures, we develop 3-D finite element models to compute the Coulomb stress change on the secondary fault after the main fault event. We explore both purely elastic and poroelastic solutions. Model results show that the Coulomb stress increased at shallow depth in the hanging wall of the main fault after the first event, favoring slip on the secondary fault plane. The poroelastic solution results in larger Coulomb stress changes in the early post-seismic period compared to the purely elastic solution, suggesting that pore fluids in the crust may play an important role in triggering earthquakes at shallow depth.