Abstract
A method is described for the prediction of site-specific surface ground motion due to induced earthquakes occurring in predictable and well-defined source zones. The method is based on empirical Green’s functions (EGFs), determined using micro-earthquakes at sites where seismicity is being induced (e.g., hydraulic fracturing and wastewater injection during shale oil and gas extraction, CO2 sequestration, and conventional and enhanced geothermal injection). Using the EGF approach, a ground-motion field (e.g., an intensity map) can be calculated for a potentially felt induced event originating within the seismic zone. The approach allows site- and path-specific effects to be mapped into the ground-motion field, providing a local ground-motion model that accounts for wave-propagation effects without the requirement of 3D velocity models or extensive computational resources. As a test case, the ground-motion field for the mainshock (ML = 3.4, M = 3.2) resulting from the Basel Enhanced Geothermal System (EGS) was simulated using only seismicity recorded prior to the event. We focussed on peak ground velocity (PGV), as this is a measure of ground motion on which Swiss norms for vibration disturbances are based. The performance of the method was significantly better than a previously developed generic ground-motion prediction equation (GMPE) for induced earthquakes and showed improved performance through intrinsic inclusion of site-specific effects relative to predictions for a local GMPE. Both median motions and the site-to-site ground-motion variability were captured, leading to significantly reduced misfit relative to the generic GMPE. It was shown, however, that extrapolation beyond units of a couple of magnitude leads to significant uncertainty. The method is well suited to a real-time predictive hazard framework, for which shaking estimates are dynamically updated in light of newly recorded seismicity.
Highlights
The exploitation of resources in the upper crust has the potential for inducing seismicity due to changes in pore pressure or the stress field [1]
This has been demonstrated for the Basel M = 3.2 event related to fluid injection during stimulation of the enhanced geothermal system
In the case that a local ground-motion prediction equation (GMPE) is available, it may be used alongside empirical Green’s functions (EGFs) predictions to provide robust probabilistic estimates, in the early stages of induced seismicity
Summary
The exploitation of resources in the upper crust has the potential for inducing seismicity due to changes in pore pressure or the stress field [1]. Recent advances have led to an increase in activity that may induce seismic events, where fluid injection is involved [3] One such example is shale oil and gas extraction through hydraulic fracturing (“fracking”). As a result of the potential for induced seismicity when injecting a fluid into the subsurface, a challenge for operators is to define how earthquake shaking can be monitored, accurately forecast, and conveyed to regulators and the public [14]. In this sense, there is little difference between the source of induced seismicity, as there is currently no way to reliably predict events on the basis of physical processes. The approach is focussed on data collected during micro-seismic monitoring of the site, which, given a sufficiently dense monitoring network, contains all the information required to make accurate and high-spatial-resolution predictions for potential future events
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