Cycle-Up of Multiple Rifting Event Models: How Long Does it Take to Reach A Steady State Stress?

Many numerical models are initiated with a background stress state of zero. Often these numerical results are compared directly to geodetic data. Recent work has shown that modeled deformation rates can change as the model is `cycled-up' following repeated earthquakes or rifting events. In this study, we investigate model cycle-up in the context of time-dependent deformation following rifting during the 1975-1984 Krafla eruption in Iceland. We consider the number of rifting cycles required for complete cycle-up, variations in cycle-up time at different locations in the model, background stress magnitudes in fully cycled-up models, and errors incurred when the models are not properly cycled-up. The modeling is done using the commercial software ABAQUS. An ABAQUS user-subroutine is used to apply repeated rifting events within the finite element model. We have generated various 3D models with different fault/rift geometries. The models include (1) a straight rift oriented perpendicular to the far-field velocity boundary conditions, (2) a rift oriented at an angle to the far-field velocities, (3) a model containing two intersecting rifts, one perpendicular to the far-field velocities and the other rift intersecting the first at an angle, and (4) overlapping rift segments in which the overlapped region is bounded by strike-slip faults. We find that different locations in the model have different cycle-up times and steady-state stresses. There are different factors that contribute to model cycle-up. Changes in rheology, far-field boundary conditions, and rifting pattern cause variations in the cycle-up time at different locations in the model. For example, for points in the viscoelastic half-space of the straight rift model, the cycle-up time varies from 4 cycles to 16 cycles as we descend along a vertical line from the base of the rift to the bottom of the model. For a viscosity of ~1.2*1019 Pa-s, steady-state stress values for the same points varies from 0.1 to 0.8 MPa. Similar variations (1.2-3.0 MPa) can be seen for different points in the elastic crust. American Geophysical Union 2004 Fall Meeting San Francisco, California