On the Common Features of Reservoir Water-Level Variations and Their Influence on Earthquake Triggering: An Inherency of Physical Mechanism of Reservoir-Triggered Seismicity

Abstract

ABSTRACT Impoundment of hydroelectric water reservoir influences the stability of nearby faults that may lead to reservoir-triggered seismicity (RTS). Various qualitative empirical relations, relating reservoir water-level variations with earthquake triggering and their frequency, have been deduced. With the goal to give a theoretical causation (in terms of time) to these empirical relations, a detailed theoretical analysis of the physical mechanism of RTS phenomenon, in terms of mechanical loading and changes in the pore-fluid boundary condition in the underlying rockmass, is undertaken. Three components, namely elastic stress, diffusion pore pressure, and stress-induced pore pressure, are simulated by considering a simple and schematic reservoir water-level time series using the Green’s function solution of poroelastic equations and frictional failure criterion. Various factors may influence the occurrence of RTS, but here definite role and nature of the poroelastic components in governing the empirical relations are simulated. The analysis suggests that (1) all the components contribute in RTS cases that are associated with higher reservoir water level, (2) diffusion pore pressure contributes mainly in RTS cases that are associated with longer duration of high reservoir water level, and (3) contribution of stress-induced pore pressure dominates in the RTS cases that are associated with rate of change of reservoir water level. Further, detailed simulations corroborate that the rapid type of earthquake triggering in RTS cases is mainly influenced by the immediate increase of stress and stress-induced pore pressure, whereas delayed type of triggering is mainly influenced by the diffusion pore pressure, and continuing type of triggering, inter alia, is influenced by the effect of dynamic changes in seasonal water cycle on the three components. The analyses lead us to conclude that the empirical relations are governed by the physical mechanism of RTS within the ambit of poroelastic theory.