Quantitative interpretation of the tidal response of water levels measured in wells has long been made either with a model for perfectly confined aquifers or with a model for purely unconfined aquifers. However, many aquifers may be neither totally confined nor purely unconfined at the frequencies of tidal loading but behave somewhere between the two end‐members. Here we present a more general model for the tidal response of groundwater in aquifers with both horizontal flow and vertical leakage. The model has three independent parameters: the transmissivity (T) and storativity (S) of the aquifer and the specific leakage (K′/b′) of the leaking aquitard, where K′ and b′ are the hydraulic conductivity and the thickness of the aquitard, respectively. If T and S are known independently, this model may be used to estimate aquitard leakage from the phase shift and amplitude ratio of water level in wells obtained from tidal analysis. We apply the model to interpret the tidal response of water level in a US Geological Survey (USGS) deep monitoring well installed in the Arbuckle aquifer in Oklahoma, into which massive amount of wastewater coproduced from hydrocarbon exploration has been injected. The analysis shows that the Arbuckle aquifer is leaking significantly at this site. We suggest that the present method may be effective and economical for monitoring leakage in groundwater systems, which bears on the safety of water resources, the security of underground waste repositories, and the outflow of wastewater during deep injection and hydrocarbon extraction.

The 2016 M w 5.8 Pawnee earthquake occurred in a region with active wastewater injection into a basal formation group. Prior to the earthquake, fluid injection rates at most wells were relatively steady, but newly collected data show significant increases in injection rate in the years leading up to earthquake. For the same time period, the total volumes of injected wastewater were roughly equivalent between variable-rate and constant-rate wells. To understand the possible influence of these changes in injection, we simulate the variable-rate injection history and its constant-rate equivalent in a layered poroelastic half-space to explore the interplay between pore-pressure effects and poroelastic effects on the fault leading up to the mainshock. In both cases, poroelastic stresses contribute a significant proportion of Coulomb failure stresses on the fault compared to pore-pressure increases alone, but the resulting changes in seismicity rate, calculated using a rate-and-state frictional model, are many times larger when poroelastic effects are included, owing to enhanced stressing rates. In particular, the variable-rate simulation predicts more than an order of magnitude increase in seismicity rate above background rates compared to the constant-rate simulation with equivalent volume. The observed cumulative density of earthquakes prior to the mainshock within 10 km of the injection source exhibits remarkable agreement with seismicity predicted by the variablerate injection case. Electronic Supplement: Animations of the evolution of pore pressure, vertical displacement, and cylindrical tensor strains in the injection simulation domain, and a set of input commands for running simulation A using poel in ASCII format.

Wastewater disposal is generally accepted to be the primary cause of the increased seismicity rate in Oklahoma within the past decade, but no statewide analysis has investigated the contribution of hydraulic fracturing (HF) to the observed seismicity or the seismic hazard. Utilizing an enhanced seismicity catalog generated with multistation template matching from 2010 to 2016 and all available hydraulic fracturing information, we identified 274 HF wells that are spatiotemporally correlated with bursts of seismicity. The majority of HF‐induced seismicity cases occurred in the SCOOP/STACK plays, but we also identified prominent cases in the Arkoma Basin and some more complex potential cases along the edge of the Anadarko Platform. For HF treatments where we have access to injection parameters, modeling suggests that poroelastic stresses are likely responsible for seismicity, but we cannot rule out direct pore pressure effects as a contributing factor. In all of the 16 regions we identified, ≥75% of the seismicity correlated with reported HF wells. In some regions, >95% of seismicity correlated with HF wells and >50% of the HF wells correlated with seismicity. Overall, we found ~700 HF‐induced earthquakes with M ≥ 2.0, including 12 events with M 3.0–3.5. These findings suggest state regulations implemented in 2018 that require operators in the SCOOP/STACK plays to take action if a M > 2 earthquake could have a significant impact on future operations.

The seismicity rate in the Delaware Basin, located in western Texas and southeastern New Mexico, has increased by orders of magnitude within the past~5 years. While no seismicity was reported in the southern Delaware Basin during 1980-2014, 37 earthquakes with M > 3 occurred in this area during 2015-2018. We generated an improved catalog of~37,000 earthquakes in this region during 2009-2018 by applying multistation template matching at both regional and local stations using all earthquakes in the Advanced National Seismic System (ANSS) and TexNet catalogs. We found that the vast majority of the seismicity is most likely associated with wastewater disposal, while at least~5% of the seismicity was induced directly by hydraulic fracturing. We inferred far-field effects of wastewater disposal inducing earthquakes over distances >25 km. The spatial limits of seismicity correlate with geologic structures that include the Central Platform and Grisham Fault, suggesting hydrologic compartmentalization by low-permeability boundaries. Given that the seismicity rate increased throughout the duration of the study, if industry operations continue unaltered, it is likely that both the seismicity rate and number of M > 3 earthquakes may continue to increase in the future.

The cumulative seismic moment is a robust measure of the earthquake response to fluid injection for injection volumes ranging from 3,100 to about 12 million m3. Over this range, the moment release is limited to twice the product of the shear modulus and the volume of injected fluid. This relation also applies at the much smaller injection volumes of the field experiment in France reported by Guglielmi et al. (2015, https://doi.org/10.1126/science.aab0476) and laboratory experiments to simulate hydraulic fracturing described by Goodfellow et al. (2015, https://doi.org/10.1002/2015GL063093). In both of these studies, the relevant moment release for comparison with the fluid injection was aseismic and consistent with the scaling that applies to the much larger volumes associated with injection‐induced earthquakes with magnitudes extending up to 5.8. Neither the microearthquakes, at the site in France, nor the acoustic emission in the laboratory samples contributed significantly to the deformation due to fluid injection.