The detection of transient subduction zone interface properties using teleseismic data

The properties of the subduction interface are a key influence on seismic processes in subduction zones, governing the transition between stable aseismic slip, episodic slow slip events (SSEs), and catastrophic slip in major earthquakes. One way to study these properties, and their spatial variation, is through the seismic velocity structure of the interface itself, with ultraslow velocity layers (USVLs) along the interface being inferred to result from elevated pore fluid pressures, reducing the shear modulus, reducing fault friction, and facilitating slow slip. Temporal variations in the fluid content, and hence also in the seismic velocity, of such layers offer a route of explain the episodic, transient nature of slow slip in many subduction zones. However, monitoring offshore subduction zones is challenging, due to the inaccessibility of marine environments, where conventional geodetic and passive seismological techniques are logistically difficult. Using the Mexican subduction zone as a test case, we demonstrate that seismic signatures indicative of USVLs, observed using data from small-aperture arrays at teleseismic distances, align in space and time with SSE's documented from geodetic studies, with such signals absent outside of regions experiencing SSEs. Our findings reinforce the idea that the slab geometry modulates the distribution of fluids along the subduction interface along the Mexican subduction zone, with a corresponding geometrical control on the occurrence of slow slip. Our study establishes a replicable, cost-effective framework for observing USVLs and inferred episodic slow slip activity using remote seismic data, enabling the investigation of offshore subduction zones where geodetic methods are impractical. By extending the reach of seismic waveform analysis to underexplored marine environments, our approach offers a paradigm shift in the study of megathrust dynamics, with implications for global seismic and tsunami hazard assessment.