The effects of fault stepovers on ground motion

Abstract Using 3D dynamic models, we investigate the effect of fault stepovers on near-source ground motion. We use the finite-element method to model the rupture, slip, and ground motion of two parallel strike-slip faults with an unlinked overlapping stepover of variable width. We model this system as both an extensional and a com-pressional stepover and compare the results to those of single planar faults. We find that, overall, the presence of a stepover along the fault trace reduces the maximum ground motion when compared to the long planar fault. Whether the compressional or extensional stepover exhibits higher ground motion overall depends on the width of the separation between the faults. There is a region of reduced ground motion at the end of the first fault segment, when the faults are embedded in a homogeneous material. We also experiment with stress fields leading to supershear and subshear rupture velocities, and with different stress drops within those conditions. We find that subshear rupture produces stronger motions than supershear rupture, but super-shear ruptures produce that maximum over a larger area than subshear areas, even though the overall area that experiences any shaking at all is not drastically different between the two cases. Lastly, we experiment with placing realistic materials along and around the faults, such as a sedimentary basin in an extensional stepover, a dam-age zone around the fault, and a soft rock layer on top of bedrock through the entire model area. These configurations alter the pattern of ground motion from the homo-geneous case; the peaks in ground motion for the bimaterial cases depend on the materials in question. The results may have implications for ground-motion prediction in future earthquakes on geometrically complex faults. Online Material: MPEG-4 movies of models of dynamic rupture of fault stepovers embedded in heterogeneous material settings