CONTINUING SEISMICITY NEAR CHALLIS, IDAHO

The 1983 M 6.9 Borah Peak, Idaho earthquake ruptured northwestward along 36 km of the Lost River Fault (LRF). Short discontinuous scarps formed near the northwestward extent of surface rupture along the Warm Springs segment of the LRF and the antithetic Lone Pine Fault. During the last 33 years, significant seismicity has propagated northwestward from these surface ruptures. The latest episode of seismicity, centered 12 km northwest of Challis, began Mar 24, 2014 with a swarm-like sequence that included 18 earthquakes M ≥ 3 and a maximum M 4.8 quake on Apr 13. Seismicity from this source area continued at a declining rate for six months. On Dec 22, 2014, an M 4.0 earthquake initiated a vigorous seismic sequence centered 8 km southeast of Challis that included 37 M ≥ 3 earthquakes and a maximum M 4.9 quake on Jan 3, 2015. Both sequences included foreshock-like progressions of 6 or 7 M ≥ 3 earthquakes beginning about two weeks before the largest events when seismic activity peaked. Low-level seismicity continues to the present in both source areas. Epicenters of both sequences formed NW-SE elongate clusters about 8 km long by 3 km wide, west of the northern end of the Challis segment of the LRF. A focal mechanism for the Apr 13 earthquake indicates strike-slip faulting with a small component of normal slip. The Jan 3 mechanism indicates roughly equal amounts of strike-slip and normal-slip. Both mechanisms have N10°W-trending nodal planes that more closely match the trend of nearby Quaternary faults and epicenter elongations than do the east-west nodal planes. However, the N-trending nodal plane dips are not compatible with down-dip projections of the LRF or the Lone Pine Fault. Focal mechanisms for five smaller earthquakes (M 3.7 – 4.4) indicate strike-slip and normal faulting. The variety of mechanisms suggests that no single fault is responsible for all earthquakes. All mechanisms have nearly horizontal T-axes with an average trend of N44°E. Most recently, beginning on Dec 8, 2015 and lasting for five days, 48 earthquakes including an M 3.3 event were centered about 7 km northwest of Challis. This latest sequence occurred at the southeastern tip of the Mar-Sep 2014 cluster and extended the epicentral zone towards the town of Challis, into the previously quiescent area between the two active zones.