Collective properties of injection‐induced earthquake sequences: 2. Spatiotemporal evolution and magnitude frequency distributions

Abstract

Probabilistic seismic hazard assessment for induced seismicity depends on reliable estimates of the locations, rate, and magnitude frequency properties of earthquake sequences. The purpose of this paper is to investigate how variations in these properties emerge from interactions between an evolving fluid pressure distribution and the mechanics of rupture on heterogeneous faults. We use an earthquake sequence model, developed in the first part of this two‐part series, that computes pore pressure evolution, hypocenter locations, and rupture lengths for earthquakes triggered on 1‐D faults with spatially correlated shear stress. We first consider characteristic features that emerge from a range of generic injection scenarios and then focus on the 2010–2011 sequence of earthquakes linked to wastewater disposal into two wells near the towns of Guy and Greenbrier, Arkansas. Simulations indicate that one reason for an increase of the Gutenberg‐Richter b value for induced earthquakes is the different rates of reduction of static and residual strength as fluid pressure rises. This promotes fault rupture at lower stress than equivalent tectonic events. Further, b value is shown to decrease with time (the induced seismicity analog of b value reduction toward the end of the seismic cycle) and to be higher on faults with lower initial shear stress. This suggests that faults in the same stress field that have different orientations, and therefore different levels of resolved shear stress, should exhibit seismicity with different b‐values. A deficit of large‐magnitude events is noted when injection occurs directly onto a fault and this is shown to depend on the geometry of the pressure plume. Finally, we develop models of the Guy‐Greenbrier sequence that captures approximately the onset, rise and fall, and southwest migration of seismicity on the Guy‐Greenbrier fault. Constrained by the migration rate, we estimate the permeability of a 10 m thick critically stressed basement fault to be 5 × 10−12 m2. We also consider alternative scenarios in which only one of the two disposal wells operated and suggest that, due to the wells interacting, total seismicity may be greater than that attributable to each well alone.

Physics‐based forecasting of induced seismicity at Groningen gas field, the Netherlands

“…Assuming that the source radius is L /2, then for stress drop, Δ τ , the moment magnitude is given [ Kanamori and Anderson , ; Hanks and Kanamori , ] While this magnitude definition is not wholly satisfactory given the 1‐D nature of our model, it nevertheless generates event magnitudes that exhibit a power law scaling very similar to the Groningen events (Figure f). For further discussion of source scaling and stress drop independence in our model, see Dempsey et al []. Faults are discretized to δ x ∼ 10m, which is sufficient to resolve earthquakes down to M 1.…”

“…Rather, we are replicating the collective behavior of all events, which in turn reflects the underlying physical drivers, such as elevated seismicity during periods of rapid pressure or stress change. In addition, the ensemble approach ensures that the synthetic earthquakes replicate some empirical features of real seismicity, such as an exponential interevent time distribution and the Gutenberg‐Richter magnitude frequency distribution (MFD) [ Dempsey et al , ] (see also supporting information). The MFD shape is sensitive to the parameterization of stress heterogeneity on faults, and calibration of this distribution to replicate Gutenberg‐Richter power law scaling is described in Dempsey et al [].…”

Microseismicity Cloud Can Be Substantially Larger Than the Associated Stimulated Fracture Volume: The Case of the Paralana Enhanced Geothermal System

“…Furthermore, there is little evidence that the size of an event carries much information about in situ pressure conditions, whereas its hypocenter location and timing certainly do: In this sense, large and small earthquakes contain equal information. A notable exception to this is the ongoing discussion around the maximum magnitude‐induced earthquakes, where the very largest events may carry information about the dimensions of the stimulated volume (Shapiro et al, ), the crustal stress state (Dempsey, Suckale et al, ; Goertz‐Allmann & Wiemer, ), or a combination of those two elements (Galis et al, ).…”

3‐D Modeling of Induced Seismicity Along Multiple Faults: Magnitude, Rate, and Location in a Poroelasticity System

“…At the geothermal stimulation site at Basel, Switzerland, the b value varies in the range from 1.1 to 1.6 corresponding to the location relative to the injection point and the phase of injection operations (higher b value closer to the injector during the injection period). The b value of induced earthquakes is generally higher than normal tectonic events (Shapiro et al, 2011), and this study assumes b = 1.38 (mean b value from Table 1 in ; Dempsey et al, 2016) for the prediction of earthquake magnitudes.…”