The weight of the mountains: Constraints on tectonic stress, friction, and fluid pressure in the 2008 Wenchuan earthquake from estimates of topographic loading

摘要

Though it is widely recognized that large mountain ranges produce significant stresses in the Earth's crust, these stresses are not commonly quantified. Nonetheless, near large mountains topography may affect fault activity by changing the stress balance on the faults. In this work, we calculate the stress field from topography in the Longmen Shan (Sichuan, China) and resolve those stresses on several models of the faults that ruptured in the 2008 M-w 7.9 Wenchuan earthquake. We find that the topography results in shear stresses up to 20 MPa and normal stresses up to 80 MPa on the faults, with significant variability across the faults. Topographic stresses generally load the fault in a normal and left-lateral shear sense, opposite to the inferred coseismic slip sense, and thus inhibit the coseismic slip. We estimate the tectonic stress needed to overcome topographic and lithostatic stresses by assuming that the direction of maximum shear accumulated on the faults is roughly collinear with the inferred coseismic slip. We further estimate the static friction and pore fluid pressure assuming that the fault was, on average, at Mohr-Coulomb failure at the time of the Wenchuan earthquake. We use a Bayesian inversion strategy, yielding posterior probability distributions for the estimated parameters. We find most likely estimates of maximum tectonic compressive stress near 0.6 gz and oriented approximate to E-W, and minimum tectonic stress near 0.2 gz. Static friction on the fault is near 0.2, and pore fluid pressure is between 0 and 0.4 of the total pressure.