Sensitivity of Slow Slip and Tremor to Tidal Stress Changes and Implications for Physical Conditions Deep in Subduction Zones

Abstract:

Solid Earth and ocean tides generate tiny stress changes that strongly modulate slow slip and tremor on deep plate boundaries. Analysis of tidal influence on tremor during large ETS in the Cascadia subduction zone (Houston 2015 NatGeo) finds that the sensitivity of tremor to tides increases markedly over the several days of tremor and slip at a point, and is an exponential function of tidal stress. Furthermore, a direct in situ image of the Coulomb sliding line shows intrinsic friction <~0.1, far lower than lab values. A strength-threshold model for failure, in which slip and tremor occur when tidally-modulated stress exceeds strength, can explain the evolving, increasing, exponential sensitivity to tides during ETS. Alternately, in the context of rate-state friction with varying pore pressure, the evolution of tidal sensitivity implies pore pressures grow during ETS to very near–lithostatic values for typical rate-state parameter choices.

Extending analysis to the entire slow slip “seismic cycle” including periods between large ETS shows that tidal response also evolves between ETS - decaying over the first quarter of the cycle to low values then climbing back up in the second half of the cycle towards the strong response which occurs late during the ETS. To extend the model for threshold failure strength, I posit that after the stress and strength drop of an ETS, stress rebuilds linearly by tectonic loading, whereas strength rebuilds as log(time) (Vidale et al, 1994). Model stress and strength diverge the most midway through the cycle, the period of weakest observed tidal sensitivity. Tidal stresses are more effective in triggering tremor later in the cycle as linearly-growing stress draws near logarithmically-growing strength. This model broadly fits my observed evolution of tidal response. However, dip-dependence of tidal sensitivity, ETS initiation, and LFEs shows the effect of properties that vary with dip, such as pressure, temperature, or permeability evolution, and the value of integrating lab constraints, physics, and observations. The evolution of tremor’s tidal sensitivity over the slow slip seismic cycle illuminates the competition between healing on the plate interface and reloading with tectonic stress, and can help constrain and even monitor, physical conditions on deep subduction zones.