Abstract

Coincident with the arrival of low-frequency, large-amplitude surface waves of the Mw 7.9 Denali fault earthquake (dfe), an abrupt increase in seismicity was observed in the Yellowstone National Park region, despite the large epicentral distance of 3100 km. Within the first 24 hr following the dfe mainshock, we located more than 250 earthquakes, which occurred throughout the entire Yellowstone National Park region. The elevated seismicity rate continued for about 30 days and followed a modified Omori law decay with a P value of 1.02 ± 0.07. For a declustered earthquake catalog, the seismicity following the 2002 dfe uniquely stands out with a significance of 30σ. The increase in seismicity occurred over all magnitude bands. In general, we observed that seismicity following the dfe outlined the spatial pattern of past seismicity routinely observed in the Yellowstone National Park region. However, we found significant differences in triggered seismicity inside and outside the caldera. Earthquakes inside the Yellowstone caldera occurred preferentially as clusters close to major hydrothermal systems, were of larger magnitude, and seismicity decayed more rapidly. This suggests that either different trigger mechanisms were operating inside and outside the caldera or that the crust responded differently to the same trigger mechanism depending on its different mechanical state. Compared with other sites that experienced remote earthquake triggering following the 2002 dfe, Yellowstone showed the most vigorous earthquake activity. We attribute this to strong directivity effects of the dfe, which caused relatively large peak dynamic stresses (0.16–0.22 MPa) in Yellowstone, and to the volcanic nature of Yellowstone.

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