The Castanhão Reservoir, NE Brazil: Reservoir-Triggered Seismicity in secondary transtensional tectonic settings within a major strike-slip system

We investigate Reservoir-Triggered Seismicity (RTS) associated with the Castanhão Reservoir in northeastern Brazil, located within an intraplate region. Using continuous waveform data from a local seismic network (2009–2010), we construct a high-resolution earthquake catalogs of 227 events through template matching (PyMPA) and probabilistic location methods, with subsequent double-difference relocation improving spatial resolution for 103 events. The seismicity forms two clusters delineating a steeply dipping (∼80°) fault zone characterized as a reactivated strike-slip system with secondary transtensional/stepover structures within the Jaguaribe Shear Zone corridor, with moment magnitudes ranging from Mw 0.0 up to Mw 2.7. To assess triggering mechanisms, we model pore-pressure diffusion induced by reservoir impoundments. The estimated hydraulic diffusivity (∼0.080 m²/s) and spatiotemporal evolution of seismicity indicate a response governed by fluid pressure diffusion. Pore-pressure increases on the order of tens of kPa are sufficient to activate critically stressed fault segments, consistent with previous RTS observations. The seismic sequence exhibits swarm-like behavior, interpreted as progressive, fluid-driven rupture within a segmented fault network. We further analyze the temporal evolution of seismic strain using Seismic Strain Dynamics (SSD) and seismic regularity (dc) metrics, revealing patterns consistent with a multi-stage earthquake preparatory process. The largest event in the main cluster (Mw 2.1) occurs during a late-stage failure phase, following prolonged fault conditioning. Our results highlight the role of reactivated, critically stressed secondary strike-slip fault networks as hydraulically favorable conduits for pore-pressure diffusion and RTS. The study demonstrates that even moderate pore-pressure perturbations can induce delayed, swarm-type seismicity in intraplate settings, emphasizing the importance of integrating geological structure, hydromechanical processes, and seismic monitoring for improved hazard assessment near large reservoirs.