Katie M. Keranen

Potentially induced earthquakes in Oklahoma, USA: Links between wastewater injection and the 2011 Mw 5.7 earthquake sequence

Significant earthquakes are increasingly occurring within the continental interior of the United States, including five of moment magnitude (Mw) ≥ 5.0 in 2011 alone. Concurrently, the volume of fluid injected into the subsurface related to the production of unconventional resources continues to rise. Here we identify the largest earthquake potentially related to injection, an Mw 5.7 earthquake in November 2011 in Oklahoma. The earthquake was felt in at least 17 states and caused damage in the epicentral region. It occurred in a sequence, with 2 earthquakes of Mw 5.0 and a prolific sequence of aftershocks. We use the aftershocks to illuminate the faults that ruptured in the sequence, and show that the tip of the initial rupture plane is within ∼200 m of active injection wells and within ∼1 km of the surface; 30% of early aftershocks occur within the sedimentary section. Subsurface data indicate that fluid was injected into effectively sealed compartments, and we interpret that a net fluid volume increase after 18 yr of injection lowered effective stress on reservoir-bounding faults. Significantly, this case indicates that decades-long lags between the commencement of fluid injection and the onset of induced earthquakes are possible, and modifies our common criteria for fluid-induced events. The progressive rupture of three fault planes in this sequence suggests that stress changes from the initial rupture triggered the successive earthquakes, including one larger than the first.

Sharp increase in central Oklahoma seismicity since 2008 induced by massive wastewater injection

Wastewater disposal linked to earthquakes The number of earthquakes is increasing in regions with active unconventional oil and gas wells, where water pumped at high pressure breaks open rock containing natural gas, leaving behind wastewater in need of disposing. Keranen et al. show that the steep rise in earthquakes in Oklahoma, USA, is likely caused by fluid migration from wastewater disposal wells. Twenty percent of the earthquakes in the central United States could be attributed to just four of the wells. Injected fluids in high-volume wells triggered earthquakes over 30 km away. Science, this issue p. 448 The recent surge in central U.S. seismicity is likely attributable to injection of wastewater at a small number of wells. Unconventional oil and gas production provides a rapidly growing energy source; however, high-production states in the United States, such as Oklahoma, face sharply rising numbers of earthquakes. Subsurface pressure data required to unequivocally link earthquakes to wastewater injection are rarely accessible. Here we use seismicity and hydrogeological models to show that fluid migration from high-rate disposal wells in Oklahoma is potentially responsible for the largest swarm. Earthquake hypocenters occur within disposal formations and upper basement, between 2- and 5-kilometer depth. The modeled fluid pressure perturbation propagates throughout the same depth range and tracks earthquakes to distances of 35 kilometers, with a triggering threshold of ~0.07 megapascals. Although thousands of disposal wells operate aseismically, four of the highest-rate wells are capable of inducing 20% of 2008 to 2013 central U.S. seismicity.

Three-dimensional seismic imaging of a protoridge axis in the Main Ethiopian rift

Models of continental breakup remain uncertain because of a lack of knowledge of strain accommodation immediately before breakup. Our new three-dimensional seismic velocity model from the Main Ethiopian rift clearly images mid-crustal intrusions in this active, transitional rift setting, supporting breakup models based on dike intrusion and magma supply. The most striking features of our velocity model are anomalously fast, elongate bodies (velocity, V p ∼6.5–6.8 km/s) extending along the rift axis, interpreted as cooled mafic intrusions. These 20-km-wide and 50-km-long bodies are separated and laterally offset from one another in a right-stepping en echelon pattern, approximately mimicking surface segmentation of Quaternary volcanic centers. Our crustal velocity model, combined with results from geologic studies, indicates that below a depth of ∼7 km extension is controlled by magmatic intrusion in a ductile middle to lower crust, whereas normal faulting and dike intrusion in a narrow zone in the center of the rift valley control extension in the brittle upper crust. This zone is inferred to be the protoridge axis for future seafloor spreading.

Enhanced remote earthquake triggering at fluid-injection sites in the midwestern United States

Movers and Shakers We tend to view earthquakes as unpredictable phenomena caused by naturally shifting stresses in Earths crust. In reality, however, a range of human activity can also induce earthquakes. Ellsworth (p. 10.1126/science.1225942) reviews the current understanding of the causes and mechanics of earthquakes caused by human activity and the means to decrease their associated risk. Notable examples include injection of wastewater into deep formations and emerging technologies related to oil and gas recovery, including hydraulic fracturing. In addition to directly causing increased local seismic activity, activities such as deep fluid injection may have other ramifications related to earthquake occurrence. Van der Elst et al. (p. 164; see the news story by Kerr) demonstrate that in the midwestern United States, some areas with increased human-induced seismicity are also more prone to further earthquakes triggered by the seismic waves from large, remote earthquakes. Improved seismic monitoring and injection data near deep disposal sites will help to identify regions prone to remote triggering and, more broadly, suggest times when activities should, at least temporarily, be put on hold. Wastewater injected deep underground can make some faults more susceptible to triggering by large remote earthquakes. A recent dramatic increase in seismicity in the midwestern United States may be related to increases in deep wastewater injection. Here, we demonstrate that areas with suspected anthropogenic earthquakes are also more susceptible to earthquake-triggering from natural transient stresses generated by the seismic waves of large remote earthquakes. Enhanced triggering susceptibility suggests the presence of critically loaded faults and potentially high fluid pressures. Sensitivity to remote triggering is most clearly seen in sites with a long delay between the start of injection and the onset of seismicity and in regions that went on to host moderate magnitude earthquakes within 6 to 20 months. Triggering in induced seismic zones could therefore be an indicator that fluid injection has brought the fault system to a critical state.

Observations of static Coulomb stress triggering of the November 2011 M5.7 Oklahoma earthquake sequence

In November 2011, a M5.0 earthquake occurred less than a day before a M5.7 earthquake near Prague, Oklahoma, which may have promoted failure of the mainshock and thousands of aftershocks along the Wilzetta fault, including a M5.0 aftershock. The M5.0 foreshock occurred in close proximity to active fluid injection wells; fluid injection can cause a buildup of pore fluid pressure, decrease the fault strength, and may induce earthquakes. Keranen et al. [2013] links the M5.0 foreshock with fluid injection, but the relationship between the foreshock and successive events has not been investigated. Here we examine the role of coseismic Coulomb stress transfer on earthquakes that follow the M5.0 foreshock, including the M5.7 mainshock. We resolve the static Coulomb stress change onto the focal mechanism nodal plane that is most consistent with the rupture geometry of the three M ≥ 5.0 earthquakes, as well as specified receiver fault planes that reflect the regional stress orientation. We find that Coulomb stress is increased, e.g., fault failure is promoted, on the nodal planes of ~60% of the events that have focal mechanism solutions, and more specifically, that the M5.0 foreshock promoted failure on the rupture plane of the M5.7 mainshock. We test our results over a range of effective coefficient of friction values. Hence, we argue that the M5.0 foreshock, induced by fluid injection, potentially triggered a cascading failure of earthquakes along the complex Wilzetta fault system.

Crustal structure of the northern Main Ethiopian Rift from the EAGLE controlled-source survey; a snapshot of incipient lithospheric break-up

Abstract The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) was undertaken to provide a snapshot of lithospheric break-up above a mantle upwelling at the transition between continental and oceanic rifting. The focus of the project was the northern Main Ethiopian Rift (NMER) cutting across the uplifted Ethiopian plateau comprising the Eocene-Oligocene Afar flood basalt province. A major component of EAGLE was a controlled-source seismic survey involving one rift-axial and one cross-rift c. 400 km profile, and a c. 100 km diameter 2D array to provide a 3D subsurface image beneath the profiles’ intersection. The resulting seismic data are interpreted in terms of a crustal and sub-Moho P-wave seismic velocity model. We identify four main results: (1) the velocity within the mid- and upper crust varies from 6.1 km s−1 beneath the rift flanks to 6.6 km s−1 beneath overlying Quaternary axial magmatic segments, interpreted in terms of the presence of cooled gabbroic bodies arranged en echelon along the axis of the rift; (2) the existence of a high-velocity body (Vp 7.4 km s−1) in the lower crust beneath the northwestern rift flank, interpreted in terms of about 15 km-thick, mafic under-plated/intruded layer at the base of the crust (we suggest this was emplaced during the eruption of Oligocene flood basalts and modified by more recent mafic melt during rifting); (3) the variation in crustal thickness along the NMER axis from c. 40 km in the SW to c. 26 km in the NE beneath Afar. This variation is interpreted in terms of the transition from near-continental rifting in the south to a crust in the north that could be almost entirely composed of mantle-derived mafic melt; and (4) the presence of a possibly continuous mantle reflector at a depth of about 15–25 km below the base of the crust beneath both linear profiles. We suggest this results from a compositional or structural boundary, its depth apparently correlated with the amount of extension.

Advancing techniques to constrain the geometry of the seismic rupture plane on subduction interfaces a priori: Higher‐order functional fits

[1] Ongoing developments in earthquake source inversions incorporate nonplanar fault geometries as inputs to the inversion process, improving previous approaches that relied solely on planar fault surfaces. This evolution motivates advancing the existing framework for constraining fault geometry, particularly in subduction zones where plate boundary surfaces that host highly hazardous earthquakes are clearly nonplanar. Here, we improve upon the existing framework for the constraint of the seismic rupture plane of subduction interfaces by incorporating active seismic and seafloor sediment thickness data with existing independent data sets and inverting for the most probable nonplanar subduction geometry. Constraining the rupture interface a priori with independent geological and seismological information reduces the uncertainty in the derived earthquake source inversion parameters over models that rely on simpler assumptions, such as the moment tensor inferred fault plane. Examples are shown for a number of wellconstrained global locations. We expand the coverage of previous analyses to a more uniform global data set and show that even in areas of sparse data this approach is able to accurately constrain the approximate subduction geometry, particularly when aided with the addition of data from local active seismic surveys. In addition, we show an example of the integration of many two-dimensional profiles into a threedimensional surface for the Sunda subduction zone and introduce the development of a new global threedimensional subduction interface model: Slab1.0. Components: 8122 words, 8 figures, 1 table.

Downdip variations in seismic reflection character: Implications for fault structure and seismogenic behavior in the Alaska subduction zone

Seismic reflection data collected offshore of Alaska Peninsula across the western edge of the Semidi segment show distinctive variations in reflection characteristics of the megathrust fault with depth, suggesting changes in structure that may relate to seismic behavior. From the trench to ~40 km landward, two parallel reflections are observed, which we interpret as the top and bottom of the subducted sediment section. From ~50-95 km from the trench, the plate interface appears as a thin (<400 ms) reflection band. Deeper and farther landward, the plate interface transitions to a thicker (1–1.5 s) package of reflections, where it appears to intersect the forearc mantle wedge based on our preferred interpretation of the continental Moho. Synthetic waveform modeling suggests that the thin reflection band is best explained by a single ~100- to 250-m-thick low velocity zone, whereas the thick reflection band requires a 3- to 5-km-thick zone of thin layers. The thin reflection band is located at the center of the 1938 Mw 8.2 Semidi earthquake rupture zone that now experiences little interplate seismicity. The thick reflection band starts at the downdip edge of the rupture zone and correlates with a dipping band of seismicity and projects to the location of tremor at greater depth. We interpret the thin reflection band as a compacted sediment layer and/or localized shear zone. The thick reflection band could be caused by a wide deformation zone with branching faults and/or fluid-rich layers, representing a broad transition from stick–slip sliding to slow slip and tremor.

Influence of the Amlia fracture zone on the evolution of the Aleutian Terrace forearc basin, central Aleutian subduction zone

During Pliocene to Quaternary time, the central Aleutian forearc basin evolved in response to a combination of tectonic and climatic factors. Initially, along-trench transport of sediment and accretion of a frontal prism created the accommodation space to allow forearc basin deposition. Transport of sufficient sediment to overtop the bathymetrically high Amlia fracture zone and reach the central Aleutian arc began with glaciation of continental Alaska in the Pliocene. As the obliquely subducting Amlia fracture zone swept along the central Aleutian arc, it further affected the structural evolution of the forearc basins. The subduction of the Amlia fracture zone resulted in basin inversion and loss of accommodation space east of the migrating fracture zone. Conversely, west of Amlia fracture zone, accommodation space increased arcward of a large outer-arc high that formed, in part, by a thickening of arc basement. This difference in deformation is interpreted to be the result of a variation in interplate coupling across the Amlia fracture zone that was facilitated by increasing subduction obliquity, a change in orientation of the subducting Amlia fracture zone, and late Quaternary intensification of glaciation. The change in coupling is manifested by a possible tear in the subducting slab along the Amlia fracture zone. Differences in coupling across the Amlia fracture zone have important implications for the location of maximum slip during future great earthquakes. In addition, shaking during a great earthquake could trigger large mass failures of the summit platform, as evidenced by the presence of thick mass transport deposits of primarily Quaternary age that are found in the forearc basin west of the Amlia fracture zone.

Low stress drops observed for aftershocks of the 2011 MW 5.7 Prague, Oklahoma earthquake

In November 2011, three MW ≥ 4.8 earthquakes and thousands of aftershocks occurred along the structurally-complex Wilzetta fault system near Prague, Oklahoma. Previous studies suggest that wastewater injection induced a MW4.8 foreshock [Keranen et al., 2013], which subsequently triggered a MW5.7 mainshock [Sumy et al., 2014]. We examine source properties of aftershocks with a standard Brune-type spectral model, and jointly solve for seismic moment (M0), corner frequency (f0), and kappa (κ) with an iterative Gauss-Newton global downhill optimization method [Neighbors et al., 2016]. We examine 934 earthquakes with initial moment magnitudes (MW) between 0.33-4.99 based on the pseudo-spectral acceleration, and recover reasonable M0, f0, and κ for 87 earthquakes with MW 1.83-3.51 determined by spectral fit. We use M0 and f0 to estimate the Brune-type stress drop, assuming a circular fault and shear-wave velocity at the hypocentral depth of the event. Our observations suggest that stress drops range between 0.005-4.8 MPa with a median of 0.2 MPa (0.03-26.4 MPa with a median of 1.1 MPa for Madariaga-type), which is significantly lower than typical eastern United States intraplate events (>10 MPa). We find that stress drops correlate weakly with hypocentral depth and magnitude. Additionally, we find the stress drops increase with time after the mainshock, although temporal variation in stress drop is difficult to separate from spatial heterogeneity and changing event locations. The overall low median stress drop suggests that the fault segments may have been primed to fail as a result of high pore fluid pressures, likely related to nearby wastewater injection.