EXAMPLES OF SEISMICITY MIGRATION ON 3D SEGMENTED FAULTS

Fault segmentation exerts a critical control on seismicity. In particular, intervening steps can limit rupture length and control earthquake magnitude and event distribution. Since our understanding of this control largely relies on 2D observations of surface rupture of large earthquakes (e.g. Biasi and Wesnousky, BSSA, 2016), our ability to evaluate the role of the 3D nature of fault segmentation for a range of fluid injection operations remains limited. In this study, we examine a series of fault steps imaged by seismicity associated with underground fluid movement within pre-existing faults to unravel the role of 3D fault segmentation on seismicity.

We study 10 steps observed in 4 swarms, including the 2016-19 Cahuilla swarm, California (Ross et al., Science, 2019), the 2010 Madison Plateau swarm (Shelly et al., JGR, 2013), Wyoming, and the 2008 and 2014 Bohemia swarms, Czech Republic (Hainzl et al., JGR, 2016). We selected those data for their high accuracy event location and the clarity of the geometrical features shown by the seismicity. For all the studied steps, event distributions image two segments that are stepping in 3D (e.g. along strike, down dip or oblique to slip), with geometries that are reminiscent of relay zones mapped from seismic reflection surveys (e.g. Roche et al., ESR, 2021). Analysed steps include bifurcating, cylindrical, and lens geometries where the two bounding segments are merging, disconnected and connected along branch lines, respectively.

Detailed mapping of the spatio-temporal migration of seismicity shows that the 3D segment geometry controls event migration, with events migrating either along the segment boundaries, through connected areas, or across the step via a deformation transfer mechanism. Further quantitative analyses suggest that across-step transfer is promoted by high magnitude to separation ratios and high migration rates - conditions which are more prevalent in tectonic-dominated sequences. By contrast, around-step migration is more typical of low magnitude to separation ratios and slow migration rates, conditions which are more characteristic of fluid-dominated sequences. Around-step migration therefore appears to be the dominating mechanism for seismicity development on 3D segmented faults during fluid injection operations.