Cascadia Slow Slip Recorded in PBO Borehole Pore Pressure Data

Abstract:

The Cascadia subduction zone hosts quasi-periodic, large-scale slow slip events (SSE) which influence seismic hazard along the megathrust. SSEs are observed at many subduction zones and are generally detected by geodetic signatures of displacement or spatially-correlated seismic tremor. Here we investigate whether signatures of Cascadia SSEs are also resolvable within continuous borehole pore pressure data resulting from the poroelastic response to fault slip. Our analysis focuses on eleven years of data from nine onshore borehole observatories along the Cascadia subduction zone margin which are part of NSF Earthscope’s Plate Boundary Observatory (PBO) network. For multiple SSEs, we find distinct anomalous pore pressure variations occurring across several boreholes. These signals correspond with geodetic, tremor, and strain transients associated with SSEs. In addition, the direction of the pore pressure changes are consistent with dilatations and contractions observed with co-located strainmeters. Similar to observations from boreholes offshore SW Japan and Costa Rica, and onshore above the Nankai subduction zone, we interpret these borehole pore pressure changes as poroelastic signatures of ongoing slow-slip. Our results suggest that the signatures of SSEs are recognizable within sensitive onshore borehole pore pressure data in Cascadia, and that borehole pore pressure may provide an additional constraining dataset that can help in the real-time detection of SSE within the Cascadia region.