Estimation of Fault Slip Rates in the Vicinity of the 2008 Wenchuan Earthquake Using Viscoelastic Earthquake Cycle Models, GPS Data, and Geologic Uplift Rates

Low GPS-derived contraction rates across Longmen Shan on the eastern margin of the Tibetan plateau has lead previous workers to conclude that reverse fault slip rates in these region are at most a few mm/yr. This region experienced a M 7.9 earthquake on May 12, 2008 along the Beichuan fault. We modeled the interseismic slip rates along the series of range-bounding faults using GPS data as well as the vertical uplift rates from geologic data and two- and three-dimensional viscoelastic earthquake cycle models. In the earthquake cycle models it is assumed that faulting occurs in an elastic upper crust overlying a viscoelastic lower crust and upper mantle. Using 2D models, the best fit to the data is obtained with an elastic thickness of 40-60 km and a fault dip of 500 with T/Tr = 4 where T is the earthquake recurrence interval and Tr is the characteristic relaxation time of the lower crust and upper mantle. We infer a reverse slip rate of 5-12 mm/year, strike slip rate of 2-8 mm/year and a total slip rate of 5.5 -15 mm/ year. The 3D earthquake cycle model predicts lower slip rates of approximately 2 -7 mm/year of dip slip and 2-10 mm/year of strike slip. These model estimates of slip rates are significantly higher than proposed by previous work using models that neglect the effect of viscous flow in the lower curst and upper mantle. We conclude that the earthquake potential of range-bounding faults is higher than previously suggested.