Coupled Transient Processes Govern Intraplate Earthquake Swarm Evolution: Insights from the 2019-2020 Palghar Sequence
Earthquake swarms provide a natural window into fault response to transient perturbations, yet their driving processes are commonly interpreted using fluid-driven and aseismic-slip-driven end-member models. Intraplate swarms offer a unique setting to test these models because low secular tectonic loading heightens the sensitivity of faults to transient, non-tectonic forcing. We use a machine-learning-enhanced catalog of $\sim$50,000 earthquakes from the 2019--2020 Palghar earthquake swarm in western India to test this end-member framework. High-resolution relocations, together with moment tensor solutions, reveal two shallow normal faults in a granitic basement, with seismicity sequentially migrating from the western to the eastern fault before expanding into the intervening damage zone. Although the swarm exhibits an overall diffusion-like expansion, the relocated seismicity reveals a persistent $\sim$5-km-deep localized seismicity band and repeated migration fronts sometimes propagating faster than expected from fluid diffusion alone. The velocity--duration scalings of these intermittent episodes span both fluid- and slow-slip-driven regimes. Sequential fault activation and contrasting migration styles throughout the swarm duration reveal dynamics that cannot be explained by either end-member mechanism alone. Instead, the Palghar swarm evolved through coupled fluid-assisted deformation and transient stress transfer within an interacting fault network, with migration episodes consistent with aseismic deformation. These observations reveal that high-resolution catalogs can disentangle transient processes hidden within apparently diffusive swarm behavior. More broadly, intraplate earthquake swarms provide powerful natural laboratories for resolving how coupled transient processes govern earthquake triggering and fault interaction in stable continental crust.