Abstract The physical properties of subduction zone interfaces govern the transition between stable aseismic slip, episodic slow slip events (SSEs), and large earthquakes. Ultraslow velocity layers (USVLs) along the megathrust are commonly interpreted as indicators of elevated pore‐fluid pressures that promote slow slip, but their temporal variability remains poorly constrained, particularly offshore. Using the Mexican subduction zone as a test case, we show that seismic signatures consistent with USVLs can be detected using small‐aperture seismic arrays at teleseismic distances. These high‐reflectivity phases correlate spatially with independently documented SSEs, show temporal consistency with SSE occurrence and are absent outside SSE‐prone regions. Our results suggest that slab geometry modulates fluid distribution along the megathrust and exerts a first‐order control on slow slip occurrence. This study demonstrates a replicable, cost‐effective approach to remotely probing transient megathrust properties, providing new opportunities to investigate offshore subduction zones where conventional geodetic observations are limited.