Aftershock Triggering and Spatial Aftershock Zones in Fluid‐Driven Settings: Discriminating Induced Seismicity From Natural Swarms.

Aftershock cascades play an important role in forecasting seismicity in natural and human‐made situations. While their behavior including the spatial aftershock zone has been the focus of many studies in tectonic settings, this is not the case when fluid flows are involved. Using high‐quality seismic catalogs, we probe aftershocks dynamics in five settings influenced by fluids: (a) induced seismicity in Oklahoma and Kansas, (b) natural swarms in California and Nevada, and (c) suspected swarms in the Yuha Desert (California). All settings exhibit significant aftershock behavior highlighting the importance of event‐event triggering processes. The spatial aftershock zones scale with mainshock magnitude as expected based on the rupture length. While (a) and (b) show a rapid decay beyond their rupture length, (c) exhibits long‐range behavior suggesting that fluid migration might not be the dominant mechanism. We also find that the scaling of aftershock productivity with mainshock magnitude together with the Gutenberg‐Richter b‐value might allow to distinguish between natural swarms and induced seismicity. Plain Language Summary: While it is known that fluid injection operations can induce seismic activity, it has remained unclear how this activity compares to their natural counterpart, seismic swarms driven by natural fluid flows. The latter are typically characterized by the absence of a dominant event within the seismic sequence, while exhibiting other characteristics consistent with tectonic sequences including aftershock triggering. Our analysis of high‐quality seismic catalogs for both types of fluid‐driven seismicity shows that both exhibit a significant amount of aftershocks arising from "secondary" processes (i.e. stress‐based event‐event triggering as an indirect consequence of fluid injections) leading to spatially localized aftershocks zones. Yet, the trade‐off between the seismic productivity relation, which refers to the average increase in the number of aftershocks with the magnitude of their trigger, and the distribution of earthquake magnitudes controls the relative role of small compared to large triggers and we find that aftershock triggering is much more dominated by smaller events in the induced setting. Both findings are of direct importance for earthquake forecasting and seismic hazard assessment. Key Points: Significant event‐event triggering is present in both natural swarms and induced seismicityBoth fluid‐driven settings are characterized by narrow aftershock zonesAftershock triggering is dominated by smaller triggers in induced seismicity but much less so for natural swarms [ABSTRACT FROM AUTHOR]