Abstract

A sequence of three earthquakes of Ms = 6.4, 5.9, and 5.4 occurred in southern central Tibet on 9, 16, and 22 January 2008. Interferometric satellite aperture radar (InSAR) observations indicate that the two largest events occurred on two northeast-trending, northwest-dipping normal faults, separated by ∼9.5 km, with slip reaching 1.2 m on the main fault and 0.75 m on the second. We develop a three-dimensional finite element model to calculate the Coulomb stress change due to poroelastic deformation in the crust after the first event. Using a uniform permeability of 8 × 10−15 m2 for faults and the host rock leads to negligible temporal pore pressure change in the two weeks after the first event. However, assuming higher permeability along faults (10−14 to 10−11 m2), the second fault channels the flow of pore fluids upward from the deeper region where the pore pressure increase was the largest immediately after the first event. In this case, the poroelastic Coulomb stress on the second fault increased suddenly by ∼1 × 105 Pa after the first earthquake and continued to increase by as much as 70% in the subsequent week. Furthermore, the distribution of Coulomb stress change on the second fault correlates precisely with the slip distribution inferred for the second event. These findings suggest that the first earthquake not only triggered the second event, but the Coulomb stress change it induced on the second fault plane controlled the extent of the second rupture.

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