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

Abstract

[1] We have found spatial variations in seismic stress indicators at the Yellowstone volcanic field, Wyoming, by examining source mechanisms of 25 years of networkrecorded 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 firstmotion 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.

Models of lithosphere and asthenosphere anisotropic structure of the Yellowstone hot spot from shear wave splitting

“…A similar model is proposed for the average ϕ at station Y100. Waite and Smith [2004] did not calculate the stress field around Y100 as there are few earthquakes in that area, but it is likely to be consistent with the rotation observed west of that area.…”

“…The directions of maximum extensional strain (ɛ max ) estimated from GPS measurements [ Puskas et al , 2002], and minimum principal stress (σ 3 ) estimated from focal mechanisms [ Waite and Smith , 2004] at Yellowstone support this interpretation of stress‐induced crack opening. The directions of ɛ max and σ 3 rotate from roughly N‐S NW of the Yellowstone caldera to NE‐SW within the central part of the caldera (e.g., near LKWY).…”

Crustal deformation and source models of the Yellowstone volcanic field from geodetic data

“…The faults associated with the resurgent domes and the northeast‐trending faults along the top of Elephant Back Mountain are some of the youngest normal faults in the caldera and offset glacial and alluvial material [ Smith and Braile , 1994]. Furthermore, contemporary seismicity supports the notion that the volcanic vents southeast of Norris Geyser Basin are related to an active fault system that may continue southeasterly beneath the central caldera [ Smith and Braile , 1994; Waite and Smith , 2004; Husen et al , 2004]. The eastern edge of the first of the three superimposed calderas (caldera I) roughly coincides with this fault system and the line of volcanic vents [ Smith and Braile , 1994].…”

Dynamics of the Yellowstone hydrothermal system

“…In contrast, hydrothermal activity in the Mammoth Hot Springs area is not associated with abundant seismicity. Within the caldera, most earthquakes are shallower than ~6 km, but deeper events occur between Norris Geyser Basin and the epicenter of the M −7.3 Hebgen Lake, Montana earthquake [ Waite and Smith , ; Farrell et al ., ]. A majority of the small earthquakes occur in swarms, meaning that they are clustered in space and time [ Waite and Smith , ; Farrell et al ., , ; Shelly et al ., ].…”