Seismicity in the Challis, Idaho, Region, January 2014–May 2017: Late Aftershocks of the 1983 Ms 7.3 Borah Peak Earthquake

In April 2014, after about 20 yrs of relatively low seismicity, an energetic earthquake sequence (maximum ML 4.8) began 25–30 km northwest of the 1983 Ms 7.3 Borah Peak earthquake rupture area near the town of Challis, Idaho. This sequence ended in the fall of 2014, but in January 2015, a second energetic sequence (maximum ML 5.0) began about 20 km to the southeast. Modest seismicity has continued in both regions with ∼1000 earthquakes detected and located through May 2017. To better characterize the seismicity in the area, we deployed a seven-station local seismometer network during April–October 2014; one of the stations remained active until July 2015. Here, we report updated locations for earthquakes in the Challis area for 1 January 2014–31 May 2017. Using a combination of absolute and differential arrival times, we generated a catalog of high-accuracy relocations. The earthquakes clustered into four primary groups, three of them with strikes similar to the Lost River fault—the fault responsible for the 1983 Borah Peak event. We used a modified cut-and-paste method to determine moment tensors for 15 of the largest events. All of the moment tensors showed normal faulting with nodal plane strikes consistent with the trend of the relocated seismicity and the regional stress field. We suggest that the recent seismicity near Challis is best interpreted as a continuation of the 1983 Ms 7.3 Borah Peak aftershock sequence, which is unusually long compared to plate boundary aftershock sequences because of the lower regional strain rate.