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Seismic waves from the 2002, Mw 7.9 Denali, Alaska, earthquake triggered earthquakes thousands of kilometers away, particularly along the southeasterly direction of fault rupture towards which the amplitudes of seismic waves were enhanced. As the waves spread past Bozeman, Montana, 3000 km from the source faulting, they stressed local faults, causing them to fail in the form of tiny earthquakes. Signals from these local earthquakes (indicated by arrows in the middle panel) are apparent when the muchlower-frequency waves from the Denali event are removed from the seismogram by filtering. A closer look at one of these small events (lower panel) confirms that they are indeed from local earthquakes triggered by the Denali event waves. The reason why these local earthquakes persist long after the Denali seismic waves have passed remains an unsolved mystery. These data provide a means of determining the stress changes that drive frictional instabilities of earthquake faulting. (Image courtesy of M. Manga and E. Brodsky.)
Date Taken: February 18, 2009 Photographer / Contributor: M. Manga and E. Brodsky