[1] Seismicity of the Yellowstone volcanic field, northwest Wyoming, is characterized by swarms of earthquakes (M C < 3) within the 0.64-Myr-old, 70 km by 40 km Yellowstone caldera and between the caldera and the eastern end of the 44-km-long rupture of the M S 7.5 1959 Hebgen Lake, Montana, earthquake. Over 3000 earthquakes with M C < 5 were recorded during the largest historic swarm that spanned >3 months beginning in October 1985. The swarm had unusual characteristics indicative of interaction between seismicity and hydrothermal/magmatic activity: (1) the swarm followed the reversal of caldera-wide uplift of up to 1 m from 1923 to 1984 to subsidence; (2) swarm hypocenters occupied a nearly vertical northwest trending zone, and during the first month of activity, the pattern of epicenters migrated laterally away from the caldera at an average rate of 150 m/d; (3) the dominant focal mechanisms of the swarm were oblique-normal to strike-slip contrasting with the normal-faulting mechanisms typical of the region; and (4) the maximum principal stress axis averaged for the swarm events was rotated 90°from that of the normal background seismicity, from vertical to horizontal with a trend 30°from the strike of the plane defined by the swarm. We examined analytic models that best fit the focal mechanisms and the orientation of the plane defined by the swarm and found that the temporal shift of earthquake activity could be explained by the migration of hydrothermal fluids radially outward from the Yellowstone caldera following rupture of a sealed hydrothermal system within the caldera.

[1] We recorded 126 calving and iceberg breakup events from the terminus of the Bering Glacier during five days in August 2008 using seismometers and three smallaperture arrays of infrasound sensors. The seismic signals were typically emergent, narrow-band, and lower-frequency, similar to records at other glaciers. The acoustic records were characterized by shorter-duration, higher-frequency signals with more impulsive onsets. We demonstrate that triangular infrasound arrays permit improved locations of calving events over seismic arrivals that rely on a relatively complicated, poorly known, velocity model. Twenty-six of 35 well-located events occurred on icebergs in Vitus Lake, rather than the glacier face. While our data do not permit a complete description of the source process, the distinctive frequency contents and durations in the seismic and infrasound data suggest that the two data types record different aspects of the same process. Citation: Richardson,

[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.

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