WHY BLIND THRUST FAULTS DO NOT PROPAGATE TO THE EARTHS SURFACE - NUMERICAL MODELING OF COSEISMIC DEFORMATION ASSOCIATED WITH THRUST-RELATEDANTICLINES

High fault-tip stress concentrations are associated with coseismic sli p on blind thrust faults and suggest that these structures should read ily propagate to the Earth's surface. Seismic profiles of blind-thrust -related earthquakes reveal diffuse zones of aftershocks surrounding t he fault tip which are attributed to inelastic deformation, such as fl exural-slip or extensional fracturing. The complex interaction between blind thrust faults and secondary structures may control the evolutio n of blind thrust systems. The influence of bedding-plane slip on faul t propagation is simulated with numerical models using the boundary el ement method. We use two parameters to estimate the tendency for thrus t fault propagation, (1) the mode II stress intensity factor and (2) t he maximum Coulomb stress near the fault tip. Calculations from both a nalyses suggest that shallow thrust faults may exhibit an increased te ndency to propagate as a result of interaction with the Earth's surfac e and slip along bedding planes above the fault tip and a decreased te ndency to propagate due to slip along bedding planes at or below the f ault tip. Our results demonstrate that the magnitude and style of inel astic deformation in active fault systems control fault propagation.