The Role of the Three-Dimensional Geometry of Fault Steps on Event Migration During Fluid-Induced Seismic Sequences

Abstract

This study analyses how fault segmentation influences seismicity using 3 high-resolution earthquake catalogs from tectonically different areas. The studied event patterns reveal 8 fault steps with different 3D geometries reminiscent of relay zones, which refer to the area of displacement transfer between stepping and overlapping segments due to fault segmentation, including cylindrical, bifurcating, dip, strike and oblique relay zones as mapped from seismic reflection surveys. After detailed mapping of the spatiotemporal event migration, we analyze how 3D fault geometry, and in particular internal segmentation, controls event migration. First, we show that events can migrate continuously between segments via connected areas, producing along-step, around-step and bidirectional migrations, with steps acting as a barrier. Second, we observe seismicity that hops across bounding segments if a sufficiently strong magnitude event, with a relatively large rupture length compared to the step size, occurs near the step. Thus, fault segmentation, inherited from the early stage of fault development, controls earthquake migration patterns, if coupled with the type of forces driving seismicity. Specifically, tectonic-dominated sequences with relatively high magnitude seismicity and critically stressed segments promote inter-segment hopping. In contrast, fluid-dominated sequences, producing lower-magnitude events, are strongly channeled by connected segments.