THE HYDROLOGY OF THE SEISMIC CYCLE PART II: SPATIAL AND TEMPORAL SCALES OF PORE-FLUID PRESSURE CHANGE DURING POST-SEISMIC FLUID ADVECTION

Pore-fluid pressure cycling (i.e., fault-valve behavior) is one mechanism thought to govern the seismic cycle. However, beyond conceptualizing the influence of changing pore-fluid pressure on the seismic cycle, little has been done to quantitatively assess the rates and magnitudes at which pore-fluid pressure changes and how this may affect earthquake mechanics. We report results of a simple model to evaluate spatial and temporal changes in pore fluid pressure at depth following earthquakes. Our 2D finite volume model is built on basic hydrologic principles and features a post-failure rupture that breaches the boundary between shallower hydrostatic and deeper over-pressured sections of crust. We systematically varied both the size and hydraulic conductivities of these primary elements. No combination of hydraulic conductivities produces the instantaneous drop in pore-fluid pressure in the deeper, over-pressured crust predicted by the fault-valve conceptual model. Rather, the change in pore-fluid pressure occurs over the scale of years to tens or hundreds of years. The magnitude of pore-fluid pressure change is also spatially variable, being greatest at the boundary between the hydrostatic and over-pressured fluid reservoirs. The pore-fluid pressure throughout most of the fault zone, however, typically remains largely unchanged from its pre-failure state. We suggest that pore-fluid pressure, though it must contribute to fault weakness, does not control the periodicity of the seismic cycle. Our model demonstrates that earthquake rupture does not produce a large, rapid decline in pore-fluid pressure followed by a gradual rebuilding of pressure during the interseismic period. These results suggest that pore-fluid pressure more likely acts as a continual influence on fault mechanics at hypocentral depths. We therefore submit that fault-valve behavior is neither as widespread nor applicable as previously thought, necessitating a redirection in how we conceptualize the role of pore-fluid pressure in earthquake mechanics.