Effects of geologic outcrops on long-term geomorphic trends : New Madrid, MO, to Hickman, KY

The Mississippi River between New Madrid, MO, and Hickman, KY, is of particular interest because of divergent trends in water surface profiles at the upstream and downstream ends of the reach. This report documents the investigation of the bathymetry, geology, and hydraulics of this segment of the river. The report shows that the area near River Mile 901 above Head of Passes strongly affects the river stages at low flows. This part of the river can experience high shear stresses when flows fall below 200,000 cfs, as opposed to most other locations where shear stress increases with flow. One-dimensional hydraulic modeling was also used to demonstrate that an increase of depth at a single scour hole, such as the one downstream from Hickman near River Mile 925, is unlikely to cause reach-wide degradation.

Investigating the relationship between seismic sequences and hydrological loading in the Azores

<p>Hydrological loads can be either surface loads induced by precipitation, changes in water levels at crater volcanic lakes, or subsurface loads created by seasonal changes in groundwater levels. These may contribute to strain and stress transients that trigger small earthquake swarms at faults that are already near failure. This work focusses on how annual and multi-annual stress changes of hydrological origin may affect the generation of seismic sequences on several tectonic settings, such as the New Madrid Seismic Zone and the Azores. The New Madrid seismic Zone is used as a benchmark test study region, while the Azores has been chosen for its intense seismic activity of both tectonic and volcanic origin. The magnitude of the hydrologically derived variations in stress is small compared with the long-term tectonic stresses, so we look for seasonal and inter-annual modulations of the earthquake occurrence rate. This requires the manipulation of seismic catalogues and the use of statistical methods to check if the seasonal and inter-annual variations are statistically significant, and not the result of extreme climatic events. The impact of hydrologic loads on faults is addressed using high-quality time series of seismic sequences, rainfall and other loads produced by variations in water levels, methods of decomposition and reconstruction of geophysical time series (SSA and wavelet transform) to identify modes of oscillation, and correlation analysis to recognize common patterns in seismicity and water loads. The results provide the first assessment of cyclic variations in seismicity and its relationship with atmospheric disturbances and hydrologically-driven load in the Azores region, and contributes to improve our understanding of the physics of earthquake triggering processes. The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL. This is a contribution to the RESTLESS project PTDC/CTA-GEF/6674/2020.</p>

Closing a scale-gap in Earth observation using regional-scale airborne geophysics in the lower Mississippi Valley

<p>Critical groundwater resources and hidden seismic hazards underly much of the Mississippi Alluvial Plain. Spanning nearly 100,000 square kilometers across seven states, this region hosts one of the most prolific shallow aquifer systems in the United States that supports a $12 billion agricultural economy amidst chronic groundwater decline. Further, underlying fault structures of the Reelfoot Rift and New Madrid Seismic Zone represent an important and poorly understood hazard with a complex pattern of historical impacts. Despite its societal and economic importance, mapping of shallow subsurface architecture with spatial resolution needed for effective management is insufficient. Here, we report the results of 40,000 flight-line-kilometers of electromagnetic, magnetic, and radiometric data collectively providing a system-scale snapshot of an entire aquifer system, the first such effort in the United States. This survey enables new understanding of the regional hydrogeology while also revealing previously unseen large vertical displacements (exceeding 50 m) in the uppermost Tertiary units within the New Madrid Seismic Zone.</p>