ETAS Space–Time Modeling of Chile Triggered Seismicity Using Covariates: Some Preliminary Results

Chilean seismic activity is one of the strongest in the world. As already shown in previous papers, seismic activity can be usefully described by a space–time branching process, such as the ETAS (Epidemic Type Aftershock Sequences) model, which is a semiparametric model with a large time-scale component for the background seismicity and a small time-scale component for the triggered seismicity. The use of covariates can improve the description of triggered seismicity in the ETAS model, so in this paper, we study the Chilean seismicity separately for the North and South area, using some GPS-related data observed together with ordinary catalog data. Our results show evidence that the use of some covariates can improve the fitting of the ETAS model.

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 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.

Slow slip-driven seismic signature triggered by water-reservoir impoundment

One of the most important and widely used renewable energy sources is hydroelectric energy produced via Water Reservoir Impoundment (WRI). WRI can trigger strong earthquakes under favourable geological conditions. Thus, the socio-economic impact of reservoir triggered seismicity is very significant. Although many studies have investigated the relationship between the pore pressure changes due to WRI and the observed seismicity, hydromechanical models that explain the observed processes are rare. Here, we investigate the role of hydromechanical interactions during fault deformation to understand earthquake swarm bursts under pore pressure changes due to WRI. As a natural laboratory, we selected the Song Tranh 2 Reservoir in Vietnam. Because the analysed triggered seismicity has swarm characteristics, our work contributes to the further investigation of the physical mechanisms responsible for earthquake swarms and their relationship to slow slip. We conclude that the small high-frequency seismic swarms accompanying WRI are driven by slow slip along a fault; they occur due to the temperature-controlled frictional fault heterogeneity, and their rate and magnitude depend on the sizes of these heterogeneities. Swarm earthquakes are the effect of slip acceleration on the seismic radiation level. The nucleation fronts expand the nucleation regime and may transition into stronger earthquakes. These results provide insights into the physical mechanisms of seismic processes triggered by WRI, which may have implications for assessing the seismic hazards associated with hydroelectric energy production.