Re-examination of the 1958 Huslia earthquake sequence and regional tectonics of the northwestern Koyukuk Basin, Alaska

In 1958 a M_w 6.8 (M_S = 7.3) mainshock and two M_w ∼ 5.9 aftershocks occurred near the village of Huslia in the remote Koyukuk Basin of northern interior Alaska, with no surface faulting recognized afterward. This region lies in a transitional zone between compressional tectonics associated with the collision of the Pacific and Yakutat plates with North America and the extrusion of western Alaska. Since 1969 several earthquake swarms with M_w4–5 events have occurred ∼50 km northwest of Huslia in the western Purcell Mountains, including a swarm in 2019–2022 with over 9500 events. Previous waveform modeling studies conducted 30 years ago suggest the 1958 mainshock occurred on a northeast-trending normal fault. This sense of rupture is inconsistent with focal mechanisms and trends of post-1973 seismicity in the 1958 epicentral region that indicate strike-slip motion along northwest or southeast trending faults. A recently mapped east-west striking fault scarp along the southern front of the Purcell Mountains is also inconsistent with northeast-striking normal faulting. To better characterize the 1958 sequence and its relation to the regional tectonics, we measured scarp profiles across the Purcell Mountains scarp, relocated pre-1974 seismicity, analyzed first motions of pre-1974 events, and modeled potential fields (gravity, magnetics) geophysical data. Our results suggest complex regional deformation occurs along east-west, north-south and northwest-southeast trending strike-slip faults and northwest-southeast trending normal faults. The Purcell Mountains swarm may be related to hydrothermal activity as several hot springs surround the region.