GPS Imaging of Vertical and Horizontal Crustal Motion Across the Centennial Tectonic Belt and Eastern Snake River Plain of Central Idaho, western United States

Abstract:

We use a new compilation of GPS velocities to assess the distribution and source of three-component crustal motion in and around the Centennial Tectonic Belt (CTB) and Eastern Snake River Plain (ESRP) regions of central Idaho. The GPS velocity field is a compilation assembled from the MIDAS velocities obtained for continuous stations processed by the Nevada Geodetic Laboratory, and from a newly updated GPS velocity solution that includes campaign stations from the USGS, Idaho National Laboratory, and other Pacific Northwest networks. The velocity fields are aligned to the same North America fixed reference frame, filtered and interpolated using the robust GPS Imaging technique to derive gridded horizontal and vertical fields.

The results show uplift and strain rate anomalies centered over the epicenters of the region's largest 20th century earthquakes, namely the 1983 M6.9 Borah Peak and 1959 M7.2 Hebgen Lake events. We model these anomalies as effects of postseismic viscoelastic relaxation with the VISCO-1D (v3) software, searching for upper mantle viscosity and lithospheric thickness that best explain them, subtracting the model predictions from the uplift and strain rate fields, to reveal patterns of uplift attributable to long term crustal motion. The field corrected for postseismic relaxation indicates broad subsidence of the ESRP with respect to CTB at a rate of ∼0.5 mm/yr, near the level of the data uncertainties. This motion is consistent with a model where ESRP subsidence drives crustal flexure and normal faulting on a collection of discontinuous northeast-southwest striking faults observed near the CTB/ESRP boundary.

Consistent with earlier studies, we find a concentration of active strain rate along the Intermountain Seismic Belt. However, unlike earlier studies we also find a concentration of strain rate north of the CTB. While the extent of the anomaly is difficult to resolve owing to the sparsity of GPS stations in north-central Idaho, its location coincides with that of the March 31, 2020 Stanley, Idaho M6.5 earthquake. The event's strike-slip seismic P- and T-axes align with the principal contractional and extensional axes of GPS-measured shear strain, supporting the assertion that the concentration of strain rate north of the CTB is representative of the distribution of hazard from earthquakes.