Geologic aspects (faults, landslides and volcanic dikes) of the Nateekin River shallow earthquake swarm of 15 June 2020 near Makushin Volcano, Unalaska Island, Alaska, U.S.A.-siftdesk

Abstract

Starting 15 June 2020, a shallow earthquake swarm occurred in the Nateekin River valley just east of Makushin Volcano of Unalaska Island, Alaska, U.S.A. Focal mechanism determinations were made on the larger earthquake events. These correlated with four known fault groups originally recognized during 1980-85 field work in this region. These are (a) numerous N 54° W ± striking normal faults, (b) prominent N 74° W ± striking normal faults, (c) N 50° E ± strike-slip faults, and (d) a N 36° E ± striking reverse faults. These fault groups correlate with known dikes for the region, which suggest magmatic intrusions are associated with this earthquake swarm. These faults reflect a predictable fault structure for a regional maximum horizontal tectonic stress direction of N 54° W ± with a localized spreading rift zone associated with Makushin Volcano that is striking N 74° W ±. This rift zone has a reactivated landslide and numerous nearby small landslides caused by this June/July earthquake swarm. This swarm sheds insight into the faulting, volcanic and geothermal processes of the region. Key words: Shallow earthquake swarm in a volcanic rift zone; four different seismic focal (fault) mechanisms recognized; associated active faults and landslides observed; related Holocene dikes and volcanic activity; regional maximum horizontal tectonic stress and magmatic processes; extensive geothermal resources.