Fluid pressure fluctuations and the seismic signature of a fault-controlled fluid migration pulse

Fluids in the crust may build pressure through a variety of mechanisms. This buildup can activate nearby faults, which then serve as conduits for fluid flow and as valves for pressure release. The rapid pressure drop promotes mineral precipitation and fault sealing, allowing the cycle to restart. While evidence of this cyclical interplay between faults and fluids is abundant in exhumed faults, whether these processes can be identified in the seismological record remains unknown. Here, we have detected a complete sequence of fluid pressure build-up, fluid migration through local faults, and pressure release by means of a joint magnetotelluric and seismicity survey in the Southern Andes. We identified intense seismicity along a previously unmapped WNW-striking fault, with seismic swarm activity concentrated at the edge of a low-resistivity crustal zone, interpreted as a fluid reservoir. Hypocenters delineate two fault planes from the reservoir toward higher-resistivity crust. We linked distinct seismicity patterns to each sequence stage: fluid pressure buildup is marked by relatively high b -values (1.2–1.4) and low magnitudes ( M L < 1.5 ), while fluid migration and pressure release are characterized by a sudden drop in b -values (<1). Our findings capture the seismic signature of a fluid migration pulse along faults, linking active fault-fluid interactions with long-term geological evidence of fluid compartmentalization and cyclical along-fault fluid flow. • A joint magnetotelluric and seismicity survey was employed to detect coupled fault-fluid processes in the Southern Andes. • A seismic swarm with delineated hypocenters revealed along-fault fluid migration from a low-resistivity fluids reservoir. • Fluid migration is associated with a sudden fluid pressure drop, recognized in the sequence from a drastic drop in b-values.