Earthquake swarm and b-value characterization of the Yellowstone volcano-tectonic system

Farrell, Jamie; Husen, Stephan; Smith, Robert

The Yellowstone volcanic field, Yellowstone National Park, is one of the most seismically active areas of thewestern U.S., experiencing the deadly 1959 M7.5 Hebgen Lake, MT, earthquake adjacent to the 0.64-Macaldera, as well as more than 30,000 earthquakes from 1973 to 2007. This well-recorded seismic activityoffers the opportunity to study the temporal and spatial occurrence of earthquakes and extensive earthquakeswarms and how they relate to active volcanic and tectonic processes. We characterize the distribution ofearthquakes by analyzing the rate of occurrence characterized by the b-value. To accurately determine bvalues,the earthquake catalog was filtered to identify statistically time- and spatially-dependent relatedevents, defined as swarms, from independent single main and aftershocks. An algorithm was employed thatidentified 69 swarms for 1984–2006 based on inter-event times and spatial clustering. The swarms varied induration from 1 to 46 days with the number of events varying from 30 to 722 with magnitudes of −1.2 to4.8. All of the swarm events as well as the 597 events triggered by the 2002 Denali fault, AK, earthquake wereremoved from the catalog for analysis. The catalog data were then filtered for a magnitude of completeness(MCOMP) of 1.5 and the b-value distribution for the Yellowstone region was determined with the deswarmeddata. b-values ranged from 0.6±0.1 to 1.5±0.05 with the highest values associated with theyouthful 150,000-year old Mallard Lake resurgent dome. These variations are interpreted to be related tovariations in stresses accompanying the migration of magmatic and hydrothermal fluids. An area of high bvalues(up to 1.3±0.1) associated with the Hebgen Lake fault zone west of the Yellowstone caldera could berelated to the transport of magmatic fluids out of the Yellowstone volcanic system or could be indicative of arelative low stress regime resulting from the stress release by the Hebgen Lake earthquake. An area of low bvalues(0.6±0.1) south of the Yellowstone caldera is interpreted as evidence of a relatively higher stressregime associated with an area of dominantly extensional stress. This seismicity was associated with a nearly90° change in the principal stress axes direction to northeast-southwest, compared to east-west within theYellowstone caldera, and may be influenced by buoyancy loading by the Yellowstone hotspot.

Earthquake swarm and b-value characterization of the Yellowstone volcano-tectonic system

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