Seismicity in Nebraska and adjacent states: The historical perspective and current trends

A sudden spike in earthquake events has been observed in central Nebraska. Since April 2018, 26 earthquakes with equivalent moment magnitudes from 2.7 to 4.1 occurred, clustered tightly in Custer County. A similar cluster of 24 earthquakes with equivalent moment magnitudes from 2.6 to 3.7 occurred in Jewell County in northern Kansas in 2017. We have compiled an earthquake database for Nebraska and parts of adjacent states from different sources to determine whether these recent earthquake spikes are consistent with historic seismicity. We identified two historic earthquake clusters occurring in our study area. The first contained 32 events and was active in Red Willow County in southwestern Nebraska from 1977 to 1982. As it coincides spatially with the Sleepy Hollow oil field, it may be related to enhanced oil recovery from that field, although it is also located at the edge of the Chadron-Cambridge Arch. The second historical earthquake cluster is located in Pawnee and Richardson counties in southwestern Nebraska and includes eight earthquakes with equivalent moment magnitudes of 2.3 to 2.8 that occurred in a period from 1982 to 1989 over the Nemaha uplift and appear to be related to the Humboldt fault. We note an increase in both maximum magnitude, as well as in the cumulative seismic moment per cluster with time. We have also used gravity and magnetic fields to map potential basement faults in the study area. Our analysis shows that the two recent earthquake spikes are aligned with the proposed basement faults. Despite this correlation, the cause of this sudden spike in seismicity is not well understood, as the stresses that might reactivate these basement faults are unknown. In addition, both recent clusters are distant from oil and gas operations. More seismic stations are necessary in central Nebraska in order to better detect focal depths and faulting style in the ongoing cluster of earthquakes and investigate possible causes

Joint and Lineament Patterns across the Midcontinent Indicate Repeated Reactivation of Basement-Involved Faults

Joint networks hosted in successively younger rocks, developing as a result of forced (trishear) folding of a rock mass above a deep-seated fault, can be used to infer the reactivation history of that deep-seated fault. This study aims to use joint networks in Pennsylvanian, Permian and Cretaceous rocks to document evidence of reactivation on basement faults during the Paleozoic and Mesozoic of Nebraska and Kansas. The most prominent basement features in southeast Nebraska and northeast Kansas are oriented NE-SW, likely related to the Midcontinent Rift System and Nemaha Uplift, and oriented NW-SE, likely related to fabrics from the Central Plains Orogeny. These features are well defined in the potential fields data. Joint patterns in the study area show an E-W oriented trend, as well as clearly discernable NE-SW and subsidiary N-S and NW-SE trends. The E-W trend is interpreted to be related to far-field stresses from Laramide and Ancestral Rocky Mountain orogenic events, whilst the NE-SW trend is interpreted to be related to subtle reactivation on the Mid-continent rift and related faults, observed in basement data. These movements produced stresses of sufficient magnitude to produce joints in the post-rift rock units, but not sufficient to generate shear fractures. Similarly, the ~N-S and NW-SE joint trends are taken as evidence of subtle reactivation on the Nemaha Uplift and Central Plains Orogeny systems, generating joints by the formation of forced folds. This contribution therefore provides a convincing case study of the value of coupled potential fields and surface feature studies in discerning buried tectonic trends and subtle reactivation thereon

The High Resolution Imaging Science Experiment (HiRISE) during MRO’s Primary Science Phase (PSP)

The High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO) acquired 8 terapixels of data in 9137 images of Mars between October 2006 and December 2008, covering ~0.55% of the surface. Images are typically 5–6 km wide with 3-color coverage over the central 20% of the swath, and their scales usually range from 25 to 60 cm/pixel. Nine hundred and sixty stereo pairs were acquired and more than 50 digital terrain models (DTMs) completed; these data have led to some of the most significant science results. New methods to measure and correct distortions due to pointing jitter facilitate topographic and change-detection studies at sub-meter scales. Recent results address Noachian bedrock stratigraphy, fluvially deposited fans in craters and in or near Valles Marineris, groundwater flow in fractures and porous media, quasi-periodic layering in polar and non-polar deposits, tectonic history of west Candor Chasma, geometry of clay-rich deposits near and within Mawrth Vallis, dynamics of flood lavas in the Cerberus Palus region, evidence for pyroclastic deposits, columnar jointing in lava flows, recent collapse pits, evidence for water in well-preserved impact craters, newly discovered large rayed craters, and glacial and periglacial processes. Of particular interest are ongoing processes such as those driven by the wind, impact cratering, avalanches of dust and/or frost, relatively bright deposits on steep gullied slopes, and the dynamic seasonal processes over polar regions. HiRISE has acquired hundreds of large images of past, present and potential future landing sites and has contributed to scientific and engineering studies of those sites. Warming the focal-plane electronics prior to imaging has mitigated an instrument anomaly that produces bad data under cold operating conditions