Seismotectonics and stress field of the Yellowstone volcanic plateau from earthquake first‐motions and other indicators

Abstract

We have found spatial variations in seismic stress indicators at the Yellowstone volcanic field, Wyoming, by examining source mechanisms of 25 years of network‐recorded earthquakes, 1973–1998. Yellowstone seismicity is characterized by swarms of earthquakes ( M C < 3) within the 0.64 Ma Yellowstone caldera and between the caldera and the eastern end of the 44‐km‐long rupture of the 1959 M S 7.5 Hebgen Lake earthquake. We relocated more than 12,000 earthquake hypocenters using three‐dimensional velocity models. Focal mechanisms calculated for 364 earthquakes, carefully selected for location accuracy, reveal predominantly normal faulting; however, fault orientations vary across the Yellowstone caldera. Specifically, focal mechanism T axes trend N‐S in the vicinity of the Hebgen Lake earthquake fault zone NW of the Yellowstone caldera and rotate to ENE‐WSW 35 km east of there. This rotation of the T axis trends occurs in the area of densest seismicity north of the caldera. Stress inversions performed using earthquake first‐motion data reveal a similar pattern in the minimum principal stress orientations. The extension directions derived from the focal mechanisms and stress inversions are generally consistent with extension directions determined from geodetic measurements, extension inferred from alignments of volcanic vents within the caldera, and extension directions determined from regional normal faults. The N‐S trending Gallatin normal fault north of the caldera is a notable exception; we find extension to be perpendicular to the direction of past extension on the Gallatin fault in the area immediately south of it. We interpret this N‐S extension north of the caldera to be related to postseismic viscoelastic relaxation in the upper mantle and lower crust following the Hebgen Lake earthquake. The dominantly extensional tectonic regime at Yellowstone inferred from these results demonstrates the influence of NE‐SW Basin and Range extension in this area.