Monthly Luncheon With Guest Speaker Will Levandowski

04/12/2018 @ 11:30 am – 1:00 pm

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Date(s) - 04/12/2018
11:30 am - 1:00 pm

The Wright Room @ Appaloosa Grill

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Please Join Us April 12, 2018

With Guest Speaker Will Levandowski

A revised seismotectonic view of Colorado and the central U.S.


Located At Appaloosa Grill


535 16th Street, Suite 110,
Denver, CO 80202


Understanding the crustal stress field, its origins, and its spatial variations is key to numerous industrial, academic, and hazard-related applications. Over the past decade, advances in nationwide seismic networks and increases in earthquake rates in several parts of midplate North America have provided a nearly 10-fold growth in coseismic stress indicators. Using these new constraints, this lecture will integrate the results of several recent studies to argue for a causal link among crustal structure (and tectonic history), stress, and both natural and potentially induced seismicity. This proposed connection implies that intraplate stress and strain may be focused by local factors rather than controlled by processes at distant plate boundaries.


Gravity-derived stress: Ancient tectonic events can leave behind lateral variations in density in the form of crustal thickening/thinning, igneous intrusions, or metamorphism. This density heterogeneity creates pressure variations in the subsurface that are, inherently, localized sources of gravity-derived stress. A joint inversion of surface wave dispersion, receiver functions, gravity, and surface topography for lithospheric density reveals anomalously buoyant zones in the lower crust beneath the Las Animas Arch (southeast Colorado) and the Cheyenne Belt/Hartville Uplift (southeast Wyoming). These same zones host high modern earthquake rates relative to the surrounding Great Plains and geomorphic markers of Quaternary deformation, including the Holocene-active Cheraw Fault. Finite-element modeling shows that the buoyant crustal bodies focus tensile stress—and therefore strain—in the overlying seismogenic crust. Both in these zones and across the central US as a whole, orientations of gravity-derived stress alone largely reproduce the stress directions documented in wellbore breakouts and earthquake focal mechanisms, suggesting a limited role of long-wavelength tectonic stress. Moreover, nearly all natural earthquakes >M4 in the central US have occurred in areas of higher-than-median gravity-derived stress magnitude, reinforcing the idea that local, rather than tectonic, stresses dominate the interior of North America. Finally, recent potentially induced earthquakes—defined as events that are spatiotemporally associated with injection wells—are more prevalent in these areas of elevated stress, suggesting that both natural and induced earthquakes are ultimately driven by the same stresses.

Updated stress map of the continental US: A recent compilation of earthquake focal mechanisms contains nearly 10 times as many data as the current World Stress Map and roughly 100 times as many as early catalogs. Coverage is now sufficient to perform formal stress inversions of focal mechanisms in most of the continental US, which allows full quantification of the stress field rather than inference from individual mechanisms or control only on stress directions as from wellbore breakouts. Nearly all of the plate-boundary tractions applied to North America are compressive, so it is not surprising that contractile strain dominates the edges of the continent. The new stress map, however, shows that contraction gives way to an immense zone of extension that subsumes more than half the continent and is not restricted to Cenozoic-active portions of the western US. This extension is in opposition to the stress imparted at plate boundaries and by asthenospheric flow, requiring a local origin. Additionally, extension directions are broadly uniform across large physiographic provinces of the North American interior but reorient by as much as 90 degrees over less than 30 km at the boundaries between provinces, including near but east of the Rocky Mountain Front. Models of crustal surface wave attenuation show coincident sharp boundaries. The agreement among physiography, stress, and crustal attenuation strongly supports a local, crustal (e.g., gravity-derived) control on seismotectonics in the North American interior.

About The Speaker:

After accidentally signing up for a geology class during the first semester of college and then getting thoroughly hooked, Will graduated summa cum laude from Princeton University in 2007 and earned a PhD in geophysics from the University of Colorado in 2014. From 2014 until 2017, he was a postdoctoral fellow at the USGS Hazards Science Center in Golden working on prospective models of seismicity rates in the central and eastern US. Currently, he is a visiting professor of Geology at Colorado College. Will’s research ranges from imaging lithospheric structure to seismic source studies and attenuation. He lives in Boulder with his wife, 1.5 children, and two dogs.


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