MULTISCALE SPATIOTEMPORAL ANALYSES OF SEISMICITY AND WASTEWATER INJECTION IN OKLAHOMA

The increase in seismicity in Oklahoma since 2009 has been primarily attributed to wastewater disposal, however, the extent and nature of the relationship is not clear. On the state level, there is no statistically significant relationship between injection volume and nearby seismic activity for individual wells. However, there is a clear trend between combined regional injection volumes and seismicity. This relationship requires a minimum association distance of 25 km to emerge and 40 km to stabilize. Analysis of the temporal relationship seen between injection and seismicity shows a clear 215-day delay at the state level and time delay between 0 and 180 days at a 20 km spatial scale. The time delay exhibits spatial variability, which may suggests spatial heterogeneity in the hydraulic fluid properties. Furthermore, clusters of seismicity are used to understand the underlying triggering processes and fluid movement in Oklahoma. Around 52% of the seismic clusters in Oklahoma show statistically significant diffusive migration, which is an indicator of pore pressure triggering, with a mean diffusivity around 0.05 m2/s. At the regional scale, there are also signs of diffusive migration away from areas with high injection rates with an average diffusivity of 1.8 m2/s. The large-scale diffusion likely reflects the pressure front propagation within the Arbuckle Group, compared to the cluster based diffusion showing pressure movement in the crystalline basement within a single fault system. Finally, the individual clusters show the majority of earthquakes occur along steeply dipping seismogenic faults that have strike directions between 30° and 120° E. Many of these seismogenic faults occur on sub-optimal fault orientations based on fault hazard maps, suggesting current fault hazard assessment may not properly represent the actual hazard

The Pawnee earthquake as a result of the interplay among injection, faults and foreshocks

The Pawnee M5.8 earthquake is the largest event in Oklahoma instrument recorded history. It occurred near the edge of active seismic zones, similar to other M5+ earthquakes since 2011. It ruptured a previously unmapped fault and triggered aftershocks along a complex conjugate fault system. With a high-resolution earthquake catalog, we observe propagating foreshocks leading to the mainshock within 0.5 km distance, suggesting existence of precursory aseismic slip. At approximately 100 days before the mainshock, two M ≥ 3.5 earthquakes occurred along a mapped fault that is conjugate to the mainshock fault. At about 40 days before, two earthquakes clusters started, with one M3 earthquake occurred two days before the mainshock. The three M ≥ 3 foreshocks all produced positive Coulomb stress at the mainshock hypocenter. These foreshock activities within the conjugate fault system are near-instantaneously responding to variations in injection rates at 95% confidence. The short time delay between injection and seismicity differs from both the hypothetical expected time scale of diffusion process and the long time delay observed in this region prior to 2016, suggesting a possible role of elastic stress transfer and critical stress state of the fault. Our results suggest that the Pawnee earthquake is a result of interplay among injection, tectonic faults, and foreshocks

Statistical seismicity analysis methods for the detection of fault activation during fluid injection

Fluid injection operations and the connected increase in pore pressure can have undesirable side effects such as induced seismic activity, fault slip and wellbore damage. Here, we present two statistical methods that allow for an identification of fault activation and induced seismic activity. First, we differentiate induced from tectonic seismicity based on a significant increase in background seismicity rates. We determine temporal variations in background rates by fitting earthquake interevent-times with a two-parametric gamma distribution. The corresponding parameters provide insight into short-period aftershock clustering and longer period background seismicity rate changes. We show that temporal changes in background rates can be used to identify regions with induced seismicity in the central United. Second, we identify fault activation processes by analyzing temporal variations in Gutenberg-Richter b-value A significant drop in b-value can potentially be indicative of fault activation during continuous injection operations. Adjusting injection operations in response to jumps in background rates and decreasing b-values may help control fault activation and induced earthquake activity

The 2016 Mw5.1 Fairview, Oklahoma earthquakes: Evidence for long-range poroelastic triggering at \u3e40 km from fluid disposal wells

Wastewater disposal in the central U.S. is likely responsible for an unprecedented surge in earthquake activity. Much of this activity is thought to be driven by induced pore pressure changes and slip on pre-stressed faults, which requires a hydraulic connection between faults and injection wells. However, direct pressure effects and hydraulic connectivity are questionable for earthquakes located at large distances and depths from the injectors. Here, we examine triggering mechanisms of induced earthquakes, which occurred at more than 40 km from wastewater disposal wells in the greater Fairview region, northwest Oklahoma, employing numerical and semi-analytical poroelastic models. The region exhibited few earthquakes before 2013, when background seismicity started to accelerate rapidly, culminating in the Mw5.1 Fairview earthquake in February 2016. Injection rates in the ∼2–2.5 km deep Arbuckle formation started to increase rapidly in 2012, about two years before the start of seismicity-increase. Most of the injection activity was concentrated toward the northeast of the study region, generating a relatively cohesive zone of pressure perturbations between 0.1 and 1 MPa. Much of the near-injection seismicity was likely triggered by pressure effects and fault-assisted pressure diffusion to seismogenic depth. Outside of the high-pressure zone, we observed two remarkably detached, linear seismicity clusters, which occurred at 20 to 50 km distance from the initial seismicity and 10 to 40 km from the nearest, high-rate injector. Semi-analytical models reveal that poroelastically-induced Coulomb-stress-changes surpass pore pressure changes at these distances, providing a plausible triggering mechanism in the far-field of injection wells. These results indicate that both pore-pressures and poroelastic stresses can play a significant role in triggering deep and distant earthquakes by fluid injection and should be considered for seismic hazard assessment beyond the targeted reservoir

Temporal Correlation Between Seismic Moment and Injection Volume for an Induced Earthquake Sequence in Central Oklahoma

The rapidly increased earthquake rate in the central United States has been linked with wastewater injection. While the overall understanding appears clear at large scales, the interaction between injection and faulting at smaller scales within individual sequences is still not clear. For an earthquake sequence in central Oklahoma, we conduct finer-scale analysis of the spatiotemporal evolution of seismicity and pore pressure modeling. The pore pressure modeling suggests that nearby wells show much stronger correlation with earthquake sequence evolution. Detailed temporal analysis found correlation between earthquake rate, seismic moment, and injection rates from wells in close proximity. However, the observed maximum magnitude (Mmax) is about 1 order of magnitude smaller than expected based on a theoretical relationship between Mmax and cumulative volume. This discrepancy may point toward additional parameters, such as fault size and stress, which influence Mmax. The lower Mmax is consistent with the truncated Gutenberg-Richter distribution observed from matched filter detected catalog. Overall, the detailed observations suggest that it is possible to resolve relationships between individual disposal wells and induced earthquake sequences

Temporal Correlation Between Seismic Moment and Injection Volume for an Induced Earthquake Sequence in Central Oklahoma

The rapidly increased earthquake rate in the central United States has been linked with wastewater injection. While the overall understanding appears clear at large scales, the interaction between injection and faulting at smaller scales within individual sequences is still not clear. For an earthquake sequence in central Oklahoma, we conduct finer-scale analysis of the spatiotemporal evolution of seismicity and pore pressure modeling. The pore pressure modeling suggests that nearby wells show much stronger correlation with earthquake sequence evolution. Detailed temporal analysis found correlation between earthquake rate, seismic moment, and injection rates from wells in close proximity. However, the observed maximum magnitude (Mmax) is about 1 order of magnitude smaller than expected based on a theoretical relationship between Mmax and cumulative volume. This discrepancy may point toward additional parameters, such as fault size and stress, which influence Mmax. The lower Mmax is consistent with the truncated Gutenberg-Richter distribution observed from matched filter detected catalog. Overall, the detailed observations suggest that it is possible to resolve relationships between individual disposal wells and induced earthquake sequences