Estimating Effective Normal Stress During Slow Slip Events From Slip Velocities and Shear Stress Variations

Theoretical analyses using a rate‐ and state‐dependent friction law have shown quasistatic and stable slip behaviors following velocity weakening. We estimated spatiotemporal slip distributions for six slow slip events (SSEs) off the Boso Peninsula, Japan, from Global Navigation Satellite System data and plotted their shear stress variations vs. slip velocity, here called “trajectories.” Most trajectories are similar to theoretical stable trajectories, implying that the Boso SSEs represent a quasistatic and stable process, and that the gradient of the trajectories during slip acceleration is equal to the product of the friction coefficients of the slipping plates ( a–b ) and the effective normal stress (). Taking ( a–b ) = −0.003, we estimate that the effective normal stresses during the SSEs were 10–50 MPa at 12.5–20.5 km depth. These values are much smaller than the lithostatic pressures at those depths, implying high pore pressures on the slow slip interface.