TRANSTENSIONAL DEFORMATION KINEMATICS IN THE SEISMIC PARABOLA AROUND THE EASTERN SNAKE RIVER PLAIN, IDAHO, MONTANA, WYOMING, AND UTAH

Elevated earthquake rates relative to surrounding regions define a “seismic parabola” about the northern Eastern Snake River Plain (ESRP). Based on analysis of the GPS velocity field, the seismic parabola is associated with localized deformation that accommodates 1-3 mm/yr of southwest translation of the ESRP as a quasi-rigid lithospheric block relative to stable North America. The southeastern arm of the parabola is the reach of Intermountain Seismic Belt (ISB) between Salt Lake City and Yellowstone, and it accommodates left-oblique motion of the ESRP relative to central Wyoming. Kinematic inversions of earthquake focal mechanisms from the ISB for components of a reduced incremental strain tensor indicate transtensional deformation accommodated by mixed strike-slip and normal faulting. The major active normal faults of the ISB exhibit a right-stepping en echelon geometry relative to the direction of macroscopic sinistral shear bordering the southeastern ESRP. Similarly, seismicity and deformation kinematics in the Centennial Tectonic Belt bordering the ESRP to the northwest primarily accommodate distributed right-oblique motion of the ESRP relative to western Montana and northern Idaho. Deformation in the Yellowstone plateau, which is at the apex of the seismic parabola, accommodates NE-SW extension parallel to ESRP-NA motion. In detail, there is good agreement between the orientations of the principal strains derived from analysis of the GPS data, inversions of small earthquake focal mechanisms, and extension/contraction-axes from larger events such as the 2020 M6.5 Stanley, Idaho earthquake. Together, these data indicate that both small and large earthquakes are relaxing a component of the elastic strain accumulating around the margins of the northern ESRP. The very low rate of seismicity within the ESRP is consistent with its quasi-rigid, block-like motion.