The Mw 5.1 Fairview, Oklahoma, earthquake on 13 February 2016 and its associated seismicity produced the largest moment release in the central and eastern United States since the 2011 Mw 5.7 Prague, Oklahoma, earthquake sequence and is one of the largest earthquakes potentially linked to wastewater injection. This energetic sequence has produced five earthquakes with Mw 4.4 or larger. Almost all of these earthquakes occur in Precambrian basement on a partially unmapped 14 km long fault. Regional injection into the Arbuckle Group increased approximately sevenfold in the 36 months prior to the start of the sequence (January 2015). We suggest far‐field pressurization from clustered, high‐rate wells greater than 12 km from this sequence induced these earthquakes. As compared to the Fairview sequence, seismicity is diffuse near high‐rate wells, where pressure changes are expected to be largest. This points to the critical role that preexisting faults play in the occurrence of large induced earthquakes.

The 3 September 2016, Mw 5.8 Pawnee earthquake was the largest recorded earthquake in the state of Oklahoma. Seismic and geodetic observations of the Pawnee sequence, including precise hypocenter locations and moment tensor modeling, shows that the Pawnee earthquake occurred on a previously unknown left‐lateral strike‐slip basement fault that intersects the mapped right‐lateral Labette fault zone. The Pawnee earthquake is part of an unprecedented increase in the earthquake rate in Oklahoma that is largely considered the result of the deep injection of waste fluids from oil and gas production. If this is, indeed, the case for the M5.8 Pawnee earthquake, then this would be the largest event to have been induced by fluid injection. Since 2015, Oklahoma has undergone wide‐scale mitigation efforts primarily aimed at reducing injection volumes. Thus far in 2016, the rate of M3 and greater earthquakes has decreased as compared to 2015, while the cumulative moment—or energy released from earthquakes—has increased. This highlights the difficulty in earthquake hazard mitigation efforts given the poorly understood long‐term diffusive effects of wastewater injection and their connection to seismicity.

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Basement-cored uplifts are observed globally and remain an enigmatic feature of plate tectonics due to the fact that, in many cases, they occur distant from a plate boundary. The Laramide Bighorn Arch in Wyoming is an archetypal basement-involved foreland arch and provides an excellent setting for the investigation of such structures. Previous studies proposed diverse arch formation models; each of which predicts a unique crustal geometry. We use high-resolution crustal imaging from teleseismic P wave receiver functions to test these models. We obtained our data from 239 three-component seismometers deployed as part of the Bighorns Arch Seismic Experiment as well as coeval regional Transportable Array stations. A sequential, two-layer thickness V P /V S (H-κ) stacking algorithm constrains sediment and crustal structure. Receiver function Common Conversion Point stacking results in 2-D transect images across the arch. Our results define an upwarp of the crust beneath the central and northern arch that extends into the Powder River Basin, north-northeast of the arch. The lack of Moho-cutting faults or a Moho geometry mirroring the arch rules out most shortening models except a crustal detachment model where shortening was accomplished by fault-propagation folding on a thrust splay ramping off a midcrustal detachment fault. The mismatch between gentle, symmetric Moho and asymmetric Laramide arch geometries and their trends suggests a pre-Laramide origin for at least a part of the Moho high. This high, perhaps in combination with a lesser degree of Laramide lithospheric buckling, may have caused emergent Laramide thrusting and thus nucleated the Bighorn Arch. Our results suggest that midcrustal detachment can form basement-involved foreland arches and suggest the hypothesis that preexisting undulations in the Moho may have nucleated individual arches.

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The crustal structure of north-central Wyoming records a history of complex lithospheric evolution from Precambrian accretion to Cretaceous-Paleogene Laramide shortening. We present two active source P wave velocity model profiles collected as part of the Bighorn Arch Seismic Experiment in 2010. Analyses of these velocity models and single-fold reflection data, together with potential field modeling of regional gravity and magnetic signals, constrain crustal structure and thickness of the Bighorn region. We image a west dipping reflection boundary and model a sharp magnetic contact east of the Bighorn Arch that together may delineate a previously undetected Precambrian suture zone. Localized patches of a high-velocity, high-density lower crustal layer (the "7.× layer") occur across the study area but are largely absent beneath the Bighorn Arch culmination. Moho topography is relatively smooth with no large-scale offsets, with depths ranging from~50 to 37 km, and is largely decoupled from Laramide basement topography. These observations suggest that (1) the edge of the Archean Wyoming craton lies just east of the Bighorn Mountains, approximately 300 km west of previous interpretations, and (2) Laramide deformation localized in an area with thin or absent 7.× layer, due to its relatively weak lower crust, leading to detachment faulting. Our findings show that Precambrian tectonics in northern Wyoming may be more complicated than previously determined and subsequent Laramide deformation may have been critically dependent on laterally heterogeneous crustal structure that can be linked to Precambrian origins.The potential role of Archean (>2.5 Ga) and Proterozoic (0.541-2.5 Ga) structures in the development of the Laramide orogeny (~75 Ma to~40 Ma) has been suggested by numerous studies that note contrasting structural patterns in the southern and northern Rocky Mountain provinces [Brown, 1993;Erslev and Koenig, 2009;Stone, 1986]. Deformation, probably driven by flat-slab subduction [e.g., Bird, 1988;Coney and Reynolds, 1977;Dickinson and Snyder, 1978], led to basement-involved thick-skinned shortening during the Laramide orogeny (~75 Ma to~40 Ma) [Dickinson et al., 1988], forming basement-cored foreland arches. In the Proterozoic basement of Colorado, Laramide arches typically strike north-northwest, are separated by narrow basins, and form a well-defined, en echelon deformation front on their eastern margin. In the large Archean basement of Wyoming, Laramide structures exhibit a larger variety of strikes and are separated by wide basins, with no well-defined deformation front. These contrasting geometries of thick-skinned deformation have been attributed to the greater diversity of structural fabrics in the Archean Wyoming basement [Brown, 1993;Erslev and Koenig, 2009] and different crustal architectures. Most notably, the high-velocity, high-density lower crustal WORTHINGTON ET AL.CRUSTAL STRUCTURE OF THE BIGHORN ARCH 208

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[1] Peat deposits contain on the order of 1/6 of the Earth's terrestrial fixed carbon (C), but uncertainty in peat depth precludes precise estimates of peat C storage. To assess peat C in the Northern Highlands Lake District (NHLD), a $7000 km 2 region in northern Wisconsin, United States, with 20% peatland by area, we sampled 21 peatlands. In each peatland, peat depth (including basal organic lake sediment, where present) was measured on a grid and interpolated to calculate mean depth. Our study addressed three questions: (1) How spatially variable is peat depth? (2) To what degree can mean peat depth be predicted from other field measurements (water chemistry, water table depth, vegetation cover, slope) and/or remotely sensed spatial data? (3) How much C is stored in NHLD peatlands? Site mean peat depth ranged from 0.1 to 5.1 m. Most of the peatlands had been formed by the in-filling of small lake basins (terrestrialization), and depths up to 15 m were observed. Mean peat depth for small peat basins could be best predicted from basin edge slope at the peatland/upland interface, either measured in the field or calculated from digital elevation (DEM) data (Adj. R 2 = 0.70). Upscaling using the DEM-based regression gave a regional mean peat depth of 2.1 ± 0.2 m (including $0.1-0.4 m of organic lake sediment) and 144 ± 21 Tg-C in total. As DEM data are widely available, this technique has the potential to improve C storage estimates in regions with peatlands formed primarily by terrestrialization.