Maps of Secondary Slip Structures within Fault Steps & Mechanical Models of their Development and Influence on Multifault Earthquake Ruptures

Abstract:

Following an earthquake, complex fault arrays and distributed damage may be mapped at the surface and at depth from aftershocks, particularly in steps between faults. For example, a releasing step between two faults involved in the 1992 Landers, CA earthquake hosts a complex array of surface faulting and numerous aftershocks at depth, many of which cluster along distinct surfaces. Paleoseismology and observations from the 1979 Homestead Valley earthquake sequence located within this step suggest structural development in prior earthquakes as well. We ask: how important are these structures in allowing slip to continue through a releasing step and how do they develop over geologic time? We analyze slip structures within releasing steps from several earthquakes and use these data to constrain 2D, slip-weakening models of linear, 20 km faults separated by releasing steps with widths of 0.5 to 5 km. We assess if and when slip continues across these steps, both without and with the development of slip structures within the step. Perhaps unsurprisingly, the development of new structures within a step promotes slip through the step. However, both the potential for the continuation of slip and the extent of the structural development there depend on different aspects of the step geometry. Prior to the development of additional slip structures, all sets of faults with step distances of <1 km show continuous slip through the step, but only overlapping fault sets with step distances of ≥1 km do so. This threshold distance also depends on stress drop along model faults. When slip structures are allowed to develop, they do so preferentially within steps between overlapping faults and the continuation of slip becomes more dependent on fault overlap than step distance. This suite of models reveals that both fault step geometry and secondary slip structures are key characteristics of releasing steps and that accounting for these features is critical to assessing the seismic hazard of segmented fault systems.