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Abstract:We used satellite interferometric synthetic-aperture radar (InSAR) data to document ground deformation across North America suspected to be caused by human activities. We showed that anthropogenic deformation can be measured from space across the continent and thus satellite observations should be collected routinely to characterize this deformation. We included results from the literature as well as new analysis of more than 5000 interferograms from the European Remote Sensing (ERS) satellite, Envisat, the Advanced Land Observing Satellite (ALOS), and other satellites, collectively spanning the period 1992-2015. This compilation, while not complete in terms of spatial or temporal coverage nor uniform in quality over the region, contains 263 different areas of likely anthropogenic ground deformation, including 65 that were previously unreported. The sources can be attributed to groundwater extraction (50%), geothermal sites (6%), hydrocarbon production (20%), mining (21%), and other sources (3%) such as lake level changes driven by human activities and tunneling. In a few areas, the source of deformation is ambiguous. We found at least 80 global positioning system (GPS) stations within 20 km of of these areas that could be contaminated by the anthropogenic deformation. At sites where we performed a full time series analysis, we found a mix of steady and time-variable deformation rates. For example, at the East Mesa Geothermal Field in California, we found an area that changed from subsidence to uplift around 2006, even though publicly available records of pumping and injection showed no change during that time. We illustrate selected non-detections from wastewater injection in Oklahoma and eastern Texas, where we found that the detection threshold with available data is >0.5 cm/yr. This places into doubt previous results claiming detection below this threshold in eastern Texas. However, we found likely injection-induced uplift in a different area of eastern Texas at rates in excess of −2 cm/yr. We encourage others to expand the database in space and time in the supplemental material.

Recent seismic observations challenge the conventional view that tectonic plates are driven by slab pull and ridge push forces. The observations indicate that the asthenosphere flows faster and in a different direction than the plate above. Previous mantle convection models argued that pressure‐driven flow, in combination with a non‐Newtonian upper mantle, can account for these observations. We expand those models by introducing a formulation that allows for the development of weak plate margins. Weak plate margins lead to an increase in the ratio of slab‐driven to pressure‐driven asthenosphere flow, but pressure‐driven flow remains active and increases with plate margin strength. Locally, the asthenosphere can drive plates and can change flow direction with depth. Furthermore, a non‐Newtonian upper mantle allows for a hysteresis effect where, depending on initial conditions, single‐plate and plate tectonic modes can exist at the same parameter conditions.

Summary Previous studies have shown that a low viscosity upper mantle can impact the wavelength of mantle flow and the balance of plate driving to resisting forces. Those studies assumed that mantle viscosity is independent of mantle flow. We explore the potential that mantle flow is not only influenced by viscosity but can also feedback and alter mantle viscosity structure owing to a non-Newtonian upper mantle rheology. Our results indicate that the average viscosity of the upper mantle, and viscosity variations within it, are affected by the depth to which a non-Newtonian rheology holds. Changes in the wavelength of mantle flow, that occur when upper mantle viscosity drops below a critical value, alter flow velocities which, in turn, alter mantle viscosity. Those changes also affect flow profiles in the mantle and the degree to which mantle flow drives the motion of a plate analog above it. Enhanced upper mantle flow, due to an increasing degree of non-Newtonian behavior, decreases the ratio of upper to lower mantle viscosity. Whole layer mantle convection is maintained but upper and lower mantle flow take on different dynamic forms: fast and concentrated upper mantle flow; slow and diffuse lower mantle flow. Collectively, mantle viscosity, mantle flow wavelengths, upper to lower mantle velocities, and the degree to which the mantle can drive plate motions become connected to one another through coupled feedback loops. Under this view of mantle dynamics, depth-variable mantle viscosity is an emergent flow feature that both affects and is affected by the configuration of mantle and plate flow.