The 2014 Challis, Idaho Earthquake Swarm

In late March 2014 an energetic sequence of earthquakes began near Challis, ID, about 20-30 km northwest of the portion of the Lost River Fault that produced the M6.9 1983 Borah Peak earthquake. Many events were, and continue to be, felt by local residents. During the first month locations of more than 100 events (1.5 < Md < 4.7) outlined a quasi-linear NW-SE trend similar to the strike of the Lost River fault. As of early August 2014 many Md > 2 and occasional Md > 3 earthquakes continue to occur. In mid-April 2014 the University of Utah Seismograph Stations, in cooperation with the U.S. Geological Survey, installed a temporary seismic array consisting of four 6-channel (3 broadband plus 3 accelerometer) instruments and one 3-channel accelerometer near the swarm. Two additional 6-channel instruments were added in early July. All data are being archived at the IRIS DMC, with six of the seven instruments streaming data in near real time. The temporary array decreased the distance from the swarm to the nearest station from more than 70 to less than 12 km for nearly all events, providing better-constrained depths than could be provide by the regional network. For larger events, first-motion focal mechanisms show oblique normal and strike-slip faulting consistent with regional tectonics and mechanisms observed for the Borah Peak aftershocks. However, a moment tensor solution for a 3.8 Mw event indicates normal faulting on a NW-striking plane. Using events identified by the local array as templates, we will use a subspace detector to completely identify the low-magnitude events and relocate all of the seismicity. The March-August Challis sequence is curious because of its proximity to the Lost River fault zone and its swarm-like character. There are no known fluid injection sources in the region.