High‐Density Seismic Network for Improved Tomographic Imaging of the Ecuadorian Forearc: Slip Mode Controlled by In Situ Material Heterogeneity

Abstract Large subduction earthquakes often occur along the Ecuadorian margin. Following the 2016 Pedernales megathrust earthquake (Mw 7.8), a temporary network of 82 stations recorded its aftershocks. Data from this international response and the permanent network run by IG‐EPN showed that subducted seafloor topography, accreted terranes, and inherited structures control the slip behavior along the margin. However, the role of fluids remains an open question. Two hundred ninety‐eight nodes were deployed from October to November 2020. From 2021 to 2022, 59 temporary broadband stations overlapped with a 1‐month deployment of 483 nodes in March–April 2022. The broadband stations extend from the foothill of the Andes to the coast while the nodes concentrate along the coast overlapping with the rupture area of the Pedernales earthquake. We built a seismicity catalog from the nodal and 2021–2022 broadband deployment of nearly 4,000 earthquakes with 160,000 P and 173,000 S arrivals. We use P and S‐wave velocity models, derived from a 3D travel time tomography inversion, to infer fluid saturation and crack density in subducting and overriding plates. The collocation of high fracture density and saturation rate with large slip areas on the megathrust (SSE or earthquake) could facilitate the propagation of a rupture. The overriding plate has smaller crack densities than the downgoing one. In the subducting plate, fluid sources are observed at depths ranging from 18 to 40 km. The highly fractured upper downgoing plate could provide pathways for fluid to migrate upward, facilitating fluid seepage on the plate interface and trigger subduction earthquakes.