The Dual Role of Ductile Barriers: From Stress-Concentrating Seals to Coseismic Valves in Induced Seismicity

Hydraulic fracturing often induces complex seismic sequences that migrate across stratigraphically distinct formations. However, the mechanisms governing delayed triggering and vertical interaction through lithological boundaries remain poorly understood. In this study, we report a novel "coseismic valve" mechanism observed in the Weiyuan shale gas field, southern Sichuan Basin, where the multi-stage evolution of seismicity was strictly governed by pre-existing 3D mechanical stratigraphy.

Using a dense local monitoring array, we constructed a high-resolution catalog by the deep-learning-based LOC-FLOW workflow. This catalog revealed a vertically partitioned fault system, where the deep and shallow seismicity clusters are distinctly separated by a ~400 m thick low-velocity ductile barrier.This barrier mechanically isolated a deep, critically stressed segment (characterized by a low b-value) from a shallower, compliant damage zone. Our analysis reveals a paradox in the role of ductile layers: initially, the barrier acted as a "pressure seal," preventing fluid leak-off and facilitating high differential stress accumulation in the underlying reservoir. This confinement culminated in the nucleation of an Mw 3.6 mainshock with an anomalously high stress drop.

Crucially, finite fault inversion and isochrone back-projection demonstrate that the mainshock rupture propagated upward, dynamically breaching the ductile barrier. This mechanical breach effectively functioned as a valve, establishing a vertical conduit for hydraulic connectivity. Following a distinct 6-day delay, a diffusive seismic swarm erupted in the previously quiescent shallow segment, driven by the upward surge of overpressured fluids through the newly created fracture network.

Our findings challenge the conventional view of ductile layers merely as passive aseismic buffers. We demonstrate that they can play a dual role: serving as stress-concentrating seals that prime the system for nucleation, and as structural valves that, once ruptured, enable cascading seismic hazards. This dynamic interaction highlights the necessity of integrating 3D structural frameworks into seismic risk assessment for geo-energy projects.