ABSTRACT Weak-amplitude seismic waves from distant earthquakes have been observed to cause triggered earthquakes and hydrogeological permeability changes in fluid rich regions such as in geothermal and volcanic regions. Because the stress of the incident wave is weak (∼1 kPa), many researchers have speculated that pore pressure amplification could have occurred, but the exact mechanism has not been pinpointed. It is natural to ask if dynamic seismic waves interacting with water-saturated rocks of spatially variable porosity can amplify pore fluid pressure. To answer this question, we numerically solve Biot’s poroelastic equations via a staggered-grid velocity–stress finite-difference method with perfectly matched layer boundary conditions for elastic wave propagation in poroelastic media and the fluid pore pressure. Our modeling results show that pore fluid pressure can be amplified moderately by more than 3.5 times from focusing of the slow P-wave in regions of strong porosity gradients, such as fault and fracture zones. Such amplification increases pore pressure and reduces effective normal stress, which may promote failure under Coulomb conditions. Our results show that this mechanism is distinct from the previously identified pressure surge phenomenon in which the amplification factor can be significantly larger.
McNease et al. (Thu,) studied this question.