Inelastic dilatancy as a mechanism for coseismic fluid depressurization of a shallow fault zone

TL;DR

This study demonstrates that inelastic dilation during earthquakes significantly depressurizes shallow fault zones, influencing fluid flow and potentially aiding in fault mechanics understanding.

Contribution

The paper introduces a 3-D groundwater flow model that incorporates inelastic dilation effects based on dynamic rupture simulations, highlighting its role in coseismic fault depressurization.

Findings

Inelastic dilation causes notable pore pressure reduction within 1-2 km of the fault at shallow depths.

Model predictions align with field observations of water level drawdowns during earthquakes.

Elastic strain models underestimate water level changes compared to inelastic dilation models.

Abstract

Hydrologic observations and experimental studies indicate that inelastic dilation from coseismic fault damage can cause substantial pore pressure reduction, yet most near-fault hydromechanical models ignore such inelastic effects. Here, we present a 3-D groundwater flow model incorporating the effects of inelastic dilation based on an earthquake dynamic rupture model with inelastic off fault deformation, both on pore pressure and permeability enhancement. Our results show that inelastic dilation causes mostly notable depressurization within 1 to 2 km off the fault at shallow depths (< 3 km). We found agreement between our model predictions and recent field observations, namely that both sides of the fault can experience large magnitude (~tens of meters) water level drawdowns. For comparison, simulations considering only elastic strain produced smaller water level changes (~several meters) and contrasting signs of water level change on either side of the fault. The models show that inelastic dilation is a mechanism for coseismic fault depressurization at shallow depths. While the inelastic dilation is a localized phenomenon which is most pronounced in the fault zone, the pressure gradients produced in the coseismic phase have a broader effect, increasing fluid migration back into the fault zone in the postseismic phase. We suggest field hydrologic measurements in the very near field (1 to 2 km) of active faults could capture damage-related pore pressure signals produced by inelastic dilation, helping improve our understanding of fault mechanics and groundwater management near active faults.