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The Delaware basin in Texas, one of the largest oil and gas production sites in the United States, has been impacted by widespread seismicity in recent years. The M 5.0 earthquake that occurred in March 2020 near the town of Mentone is one of the largest induced earthquakes recorded in this region. Characterizing the source parameters and triggering mechanism of this major event is imperative to assess and mitigate future hazard risk. A former study showed that this event may be attributed to the deep injection nearby. Interestingly, the earthquake is in proximity to shallow injection wells with much larger total injection volume. In this study, we investigate the role of these shallow injection wells in the triggering of the M 5.0 event despite their farther distance from the mainshock. We perform source-parameter inversion and earthquake relocation to determine the precise orientation of the south-facing normal-fault plane where the mainshock occurred, followed by fully coupled poroelastic stress modeling of the change of Coulomb failure stress (ΔCFS) on the fitted fault plane caused by shallow injection in the region. Results show that shallow wells caused up to 20 kPa of ΔCFS near the mainshock location, dominated by positive poroelastic stress change. Such perturbation surpasses the general triggering threshold of faults that are well aligned with the local stress field and suggests the nonnegligible role of these shallow wells in the triggering of the mainshock. We also discuss the complex effect of poroelastic stress perturbation in the subsurface and highlight the importance of detailed geomechanical evaluation of the reservoir when developing relevant operational and safety policies. [ABSTRACT FROM AUTHOR]

Numerous studies have reported the occurrence of aseismic slip or slow slip events along faults induced by fluid injection. However, the underlying physical mechanism and its impact on induced seismicity remain unclear. In this study, we develop a numerical model that incorporates fluid injection on a fault governed by rate‐and‐state friction to simulate the coupled processes of pore‐pressure diffusion, aseismic slip, and dynamic rupture. We establish a field‐scale model to emulate the source characteristics of induced seismicity near the Dallas‐Fort Worth Airport (DFWA), Texas, where events with lower‐stress drops have been observed. Our numerical calculations reveal that the diffusion of fluid pressure modifies fault criticality and induces aseismic slip with lower stress drop values (<1 MPa), which further influence the timing and source properties of subsequent seismic ruptures. We observe that the level of pore‐pressure perturbation exhibits a positive correlation with aseismic‐stress drops but a reversed trend with seismic‐stress drops. Simulations encompassing diverse injection operations and fault frictional parameters generate a wide spectrum of slip modes, with the scaling relationship of moment (M0) with ruptured radius (r0) following an unusual trend, M0∝r04.4 ${M}_{0}\propto {r}_{0}^{4.4}$, similar to M0∝r04.7 ${M}_{0}\propto {r}_{0}^{4.7}$ observed in the DFWA sequence. Based on the consistent scaling, we hypothesize that the lower‐stress‐drop events in the DFWA may imply less dynamic ruptures in the transition from aseismic to seismic slip, located in the middle of the broad slip spectrum, as illustrated in our simulations. Plain Language Summary: Injection‐induced earthquakes have presented significant obstacles to developing energy resources related to fluid injection, such as enhanced geothermal systems and shale gas development. Despite their prevalence, the causes and impact of these earthquakes are not fully understood. Aseismic slip, characterized by slower velocities and longer durations than typical earthquakes, has been observed in induced earthquake studies. In this study, we use a numerical model to investigate how fluid pressures influence the slip properties of induced seismicity near the Dallas‐Fort Worth airport (DFWA), Texas. Our model shows that elevated fluid pressure induces aseismic slip and advances or delays fast slip (i.e., earthquakes). The pore‐pressure perturbation alters the source characteristics of both aseismic‐slip events and seismic ruptures, enhancing aseismic‐stress release while diminishing seismic‐stress release. Simulations involving various fault frictional properties reveal a wide spectrum of slip modes, ranging from slow to rapid slip, which are different from the stress‐release processes that drive globally observed natural earthquakes, but exhibit similarities to observations in the DFWA. Consequently, we infer that the DFWA events may exhibit reduced dynamic characteristics akin to slow slip events positioned in the middle of the broad spectrum generated in our modeling. Key Points: Pore‐pressure change induces aseismic slip with lower stress drops, either advancing or delaying seismic rupturesPore‐pressure perturbation exhibits a positive correlation with aseismic‐stress drops but a reversed trend with seismic‐stress dropsSimulations show a wide spectrum of induced‐slip behavior, exhibiting a similar source scaling to observations [ABSTRACT FROM AUTHOR]

Understanding the relationship between earthquake rupture processes and injection locations can shed lights on the underlying triggering mechanisms of induced seismicity. Rupture directivity, in particular, has strong effects on the resulting ground motions. Here, we constrain rupture directivity of four major induced earthquakes (⁠Mw≥5.0⁠) in the central United States that occurred between 2011 and 2016. We utilize the rich pool of broadband and strong-motion seismic data and select smaller earthquake recordings as empirical Green's functions (EGFs) to forward-model the rupture directions of the target events assuming the 1D Haskell model. The typical notion is that rupture tends to propagate away from the injection site where fluid pressure is the highest. Our analysis of four target earthquakes indicates various rupture styles with respect to the location of injection wells. The 2011 Mw 5.7 Prague and 2016 Mw 5.0 Cushing earthquakes ruptured away from the injection wells, whereas the 2016 Mw 5.1 Fairview earthquake ruptured toward the injection. The 2016 Mw 5.8 Pawnee earthquake, potentially due to a downward initial rupture, shows weak to no bias of rupture directivity relative to the injection source in our analysis. In light of theoretical models of induced earthquake rupture, we find that high-pressure injection and low initial shear stress on the fault, which are well suited to describe the Fairview region, may favor rupture toward the injection well if earthquakes are primarily induced by pore-pressure change on the fault. [ABSTRACT FROM AUTHOR]

Recent studies emphasize the rapid assessment of earthquake source properties, such as moment magnitude, to help alleviate the impact of earthquakes. Depending on local crustal structure, earthquakes occurring at different depths can differ greatly in high-frequency motions, which emphasizes the importance in constraining focal depth for the predictions of strong motions. For large earthquakes, assessing rupture directivity is also essential in estimating ground-motion effects throughout the source region. In this article, we perform an in-depth study on a group of recent earthquakes near the intersection of the San Jacinto and San Andreas fault systems in southern California. We develop a systematic method to accurately estimate moment magnitude and focal mechanism within 3-6 s after the first P arrival. Focal depth can also be constrained within ~10 s upon the arrival of S waves. To determine the direction of fault rupture, we implement a forward-modeling method, which takes smaller earthquake recordings as empirical Green's functions to simulate the rupture direction of the beginning motion generated by larger events. With a small event nearby, we resolve the rupture characteristic of the 2014 Mw 4.4 event using information at stations within 35 km from the epicenters and successfully predict the ground-motion response at stations at farther distances, where directivity effect is significant. Rupture direction of simulated earthquakes with larger magnitudes can also be accurately resolved using the method proposed, opening a possibility to predict ground motions ahead of time, in particular for hazardous regions. [ABSTRACT FROM AUTHOR]

Repeating earthquake sequences have been actively investigated to clarify many aspects of earthquake physics. The two particularly well‐studied sequences, known as the Los Angeles and San Francisco repeaters, have several intriguing observations, including their long (for the seismic moment) recurrence times that would suggest stress drops of 300 MPa based on typical assumptions, near‐syncronized timing prior to 2004, and higher than typical inferred stress drops (of 25 to 65 MPa, up to 90 MPa locally), but not as high as the recurrence times suggest. Here we show that all these observations are self‐consistent, in the sense that they can be reproduced in a single fault model. The suitable models build on the standard rate‐and‐state fault models, with velocity‐weakening patches imbedded into a velocity‐strengthening region, by adding either enhanced dynamic weakening during seismic slip or elevated normal stress on the patches, or both, to allow for the higher stress drops. Such models are able to match the observed average properties of the San Francisco and Los Angeles repeaters, as well as the overall nontrivial scaling between the recurrence time and seismic moment exhibited by many repeating sequences as a whole, for reasonable parameter choices based on experiments and theoretical studies. These models are characterized by the occurrence of substantial and variable aseismic slip at the locations of the repeating sources, which explains their atypical relation between recurrence interval and seismic moment, induces variability in the repeating source properties as observed, and results in their neither slip‐ nor time‐predictable behavior. Key Points: Enhanced dynamic weakening and/or elevated normal stress are used to reproduce observationsHigh stress drops, interaction, and repeat times of SF/LA repeaters are simultaneously reproducedAseismic slip between repeating events is key to explaining their source properties and variability [ABSTRACT FROM AUTHOR]