Brittany A. Erickson1 and Eric M. Dunham2, 3

1. Department of Geological Science, San Diego State University
2. Department of Geophysics, Stanford University
3. Institute for Computational and Mathematical Engineering, Stanford University

A Finite Difference Method for Earthquake Cycles in Heterogeneous Media: Alternating Sub-basin and Surface-rupturing Events on Faults Crossing a Sedimentary Basin

We present a numerical method for earthquake cycle simulations that includes a volume discretization of the off-fault material to accommodate spatially variable elastic properties. The method is developed for the two-dimensional antiplane shear problem of a vertical strike-slip fault with rate-and-state friction. The equations of linear elasticity are discretized with summation-by-parts finite differences. The system is loaded at the remote boundaries to capture the effect of slow tectonic forcing. To facilitate efficient solution, we employ the radiation-damping approximation, rather than rigorously including inertial dynamics. We compare earthquake cycles in a homogeneous half-space with those in which the upper portion of the fault cuts through a sedimentary basin of varying basin depth and basin shear modulus. In a homogeneous half-space, events rupturing the full seismogenic depth occur periodically. Event sequences are more complex in basin models, with one or several sub-basin events confined to the lower section of the fault followed by a much larger, surface-rupturing event that breaks through the basin. Greater basin depth or more compliant basin material promote the emergence of these sub-basin ruptures, which nucleate in the seismogenic zone below the basin but are unable to penetrate through the sedimentary basin. We found that these sub-basins ruptures leave stress concentrations near the lower edge of the basin, which facilitate subsequent events to reach Earth’s surface. Predicted surface velocities are essentially identical before sub-basin events and surface-rupturing events, suggesting that geodetic observations would not be useful in predicting the rupture mode. The alternating sequence of sub-basin and surface-rupturing events would complicate interpretation of paleoseismic data. This finding might help explain discrepancies between geodetic and geologic estimates of recurrence intervals on faults passing through basins, such as the San Jacinto and San Andreas faults in Southern California. We also speculate that a similar phenomenology might occur in subduction zones, with most megathrust events stopping at the down-dip limit of the accretionary prism and only a few breaking through to the trench.

2013 SCEC Annual Meeting