Causal factors for seismicity near Azle, Texas

Abstract

In November 2013, a series of earthquakes began along a mapped ancient fault system near Azle, Texas. Here we assess whether it is plausible that human activity caused these earthquakes. Analysis of both lake and groundwater variations near Azle shows that no significant stress changes were associated with the shallow water table before or during the earthquake sequence. In contrast, pore-pressure models demonstrate that a combination of brine production and wastewater injection near the fault generated subsurface pressures sufficient to induce earthquakes on near-critically stressed faults. On the basis of modelling results and the absence of historical earthquakes near Azle, brine production combined with wastewater disposal represent the most likely cause of recent seismicity near Azle. For assessing the earthquake cause, our research underscores the necessity of monitoring subsurface wastewater formation pressures and monitoring earthquakes having magnitudes of ∼M2 and greater. Currently, monitoring at these levels is not standard across Texas or the United States.

The importance of characterizing uncertainty in controversial geoscience applications: induced seismicity associated with hydraulic fracturing for shale gas in northwest England

“…If this is the correct explanation for this instance of induced seismicity, a very simple mitigation measure is feasible, namely to switch the wastewater injection to the footwall and/or to balance it in future on both sides of the fault so no differential loading effect, which would act to unclamp the fault, can develop. Clearly, if, instead, Hornbach et al (2015) have identified the correct explanation, then this would not work as a mitigation measure; however, the possibility seems worthy of consideration. Regardless of the correct explanation, it is evident that in this region the differential stress is much greater in the basement than at shallower (~2 km) depths in the Barnett Shale, as is to be expected from the increase in differential stress with depth that typifies most regions.…”

“…A potential difficulty with this interpretation concerns the geometry (illustrated in Fig. 2 of Hornbach et al, 2015), the water load arising from this instance of wastewater injection having been concentrated over the hanging-wall of the basement normal fault, which has slipped in the induced seismicity in a normal sense. Since this basement normal fault is very steep (dip ~70°), from previous experience (e.g., Westaway, 2002Westaway, , 2006) the main effect of this load will be to increase the shear stress (in the sense conducive to normal faulting) across this fault, thus unclamping it.…”

“…Nonetheless, induced earthquakes, albeit unrelated to shale gas production, large enough to be felt have occurred in the vicinity. The largest of these to date, the M L 3.6 Azle, Texas, event of 8 December 2013, occurred in Precambrian basement beneath the local Palaeozoic sedimentary sequence (Hornbach et al, 2015).…”

“…According to Hornbach et al (2015), this population of induced earthquakes was caused by injection of industrial wastewater into the Ellenberger Formation, consisting of highly permeable, karstified, Ordovician limestone (e.g., Loucks, 2006); in the view of these authors, some of this water leaked from this sediment into the underlying basement, where it facilitated reactivation of an ancient normal fault to a depth of ~8 km. A potential difficulty with this interpretation concerns the geometry (illustrated in Fig.…”

Temporal variation in the magnitude‐frequency distribution during the Guy‐Greenbrier earthquake sequence

“…There could be a loss of pressure due to friction in the pipe for each injection well. Hornbach et al [2015] gives an estimation of 1.25 MPa for a 2427 m long pipe with an internal diameter of 0.102 m and a Darcy friction coefficient of 0.02. It is~10% of the average injection pressure of well #1 and well #5.…”