Kinematic evidence for the effect of changing plate boundary conditions on the tectonics of the northern U.S. Rockies

Abstract

We derive surface velocities from GPS sites in the interior Northwest U.S. relative to a fixed North American reference frame to investigate surface tectonic kinematics from the Snake River Plain (SRP) to the Canadian border. The Centennial Tectonic Belt (CTB) on the northern margin of the SRP exhibits west directed extensional velocity gradients and strain distributions similar to the main Basin and Range Province (BRP) suggesting that the CTB is part of the BRP. North of the CTB, however, the vergence of velocities relative to North America switches from westward to eastward along with a concomitant rotation of the principal stress axes based on available seismic focal mechanisms, revealing paired extension in the northern Rockies and shortening across the Rocky Mountain Front. This change in orientation of surface velocities suggests that the change in the boundary conditions on the western margin of North America influences the direction of gravitational collapse of Laramide thickened crust. Throughout the study region, fault slip rate estimates calculated from the new geodetic velocity field are consistently larger than previously reported fault slip rates determined from limited geomorphic and paleoseismic studies.

The 2017–2018 Maple Creek Earthquake Sequence in Yellowstone National Park, USA

“…Using the methodology of Minson and Dreger (), and assuming a deviatoric source mechanism, we obtained an M w 4.4 oblique‐normal solution (Figure S4) with one nodal plane dipping 62° to the northeast and striking to the northwest at 327°, which is subparallel to the 1959 M w 7.2 Hebgen Lake fault scarp (Figure ). The northeast‐southwest oriented T axis is consistent with the GPS‐determined regional strain field as well as historic earthquakes (Payne et al, ; Puskas et al, ; Schmeelk et al, ). The best fitting moment centroid depth is 15 km, ~6 km deeper than the focal depth derived from arrival times.…”

Afterslip Enhanced Aftershock Activity During the 2017 Earthquake Sequence Near Sulphur Peak, Idaho

“…Although no GPS or strain meter data are available to test this idea directly, the 2017 Sulphur Peak aftershock migration rates are similar to those of other seismic sequences with confirmed afterslip (e.g., Canitano et al, 2018), as well as creep events in California (e.g., Linde et al, 1996;Lohman & McGuire, 2007). The combination of afterslip in the 2017 Sulphur Peak sequence and the cyclic/repeating nature of seismicity in this area-as indicated by the previous energetic, co-located sequences in 1960 and 1982-suggests that southeastern Idaho might be a region with slow-slip or creep (Peng & Gomberg, 2010), a style of deformation that is consistent with the relatively high strain rates (Payne et al, 2012;Schmeelk et al, 2017) and high heat flow (Blackwell et al, 2011) in the region.…”

Spatial Scales in Topography and Strain Rate Magnitude in the Western United States

“…For the topographic analysis, we directly calculate power spectra from the digital elevation model (DEM) GTOPO30 with a 30‐arc sec (approximately 1 km) grid spacing (Figure ) resampled from the most current U.S. Geological Survey 3‐arc sec DEMs (https://lta.cr.usgs.gov/GTOPO30). For the geodetic analysis, regularized strain rate magnitude is calculated from a compilation of Global Positioning System (GPS) horizontal velocities reported from several different experimental deployments, including Earthscope/PBO (http://www.earthscope.org), MAGNET (http://geodesy.unr.edu/magnet.php), SCEC (http://pfostrain.ucsd.edu/scecgps/), and other regional and subregional arrays (e.g., McCaffrey et al, ; Schmeelk et al, ) combined with earthquake focal mechanisms from the global centroid moment tensor catalog (Dziewonski et al, ; Ekström et al, ) as described in the following section.…”