Volcanic and Tectonic Sources of Seismicity Near the Tanaga Volcanic Cluster, Alaska

Tanaga Island in the Central Aleutian Islands includes four stratovolcanoes: Sajaka, Tanaga, and East Tanaga in the northwest, and Takawangha in the central part of the island. Of these volcanoes, only Tanaga has a confirmed record of historical eruptive activity. We use double-difference methods to relocate Tanaga Island earthquakes from the period 2003-2017 to gain insight into volcanic and tectonic activity in the region. High precision relative relocations show subsurface structures in the Tanaga region related to volcanism and crustal tectonics. In 2005, a swarm of similar to 600 volcano-tectonic events located below the NW portion of the island culminated with an episode of volcanic tremor. Although there was no known eruption associated with this swarm, we suggest that this activity is associated with fluids beneath Takawangha volcano. Overall, relatively little seismicity appears to be associated with volcanism: of 5,680 earthquakes relocated in this study, only similar to 700 took place within 10 km of the volcanoes themselves. Other regions appear to experience primarily, if not exclusively, tectonic seismicity. Two regions on Tanaga Island became seismically active following a M6.6 tectonic earthquake east of the island on 2 May 2008, suggesting the triggering of tectonic earthquakes by the M6.6. Seismic activity recorded offshore and below the southern part of the island is interpreted as due to the clockwise rotation of the Delarof Block in the forearc, and bookshelf faulting along the volcanic arc. This suggests a complex pattern of earthquake hypocenters governed by both volcanic and tectonic processes surrounding Tanaga and Takawangha volcanoes. Plain Language Summary Since 2003, when a seismic network was first deployed in the area, earthquakes have been frequently recorded near Tanaga Island, Alaska, in the Western Aleutian Islands. The cause of these earthquakes, however, has not been clear. Tanaga Island is home to four volcanic features: Tanaga volcano in the west, with associated cones East Tanaga and Sajaka, and Takawangha volcano in the center of the Island. Previous studies have proposed that Tanaga and Sajaka are the youngest and most recently active features on the island. Although Takawangha has no known historical eruptions, there is evidence of volcanic activity in the Holocene (Jicha et al., 2012; https://doi.org/10.1130/B30472.1), and analysis of Tanaga Island earthquakes suggests that in fact Takawangha hosts more earthquakes than Tanaga. A significant cluster of earthquakes in 2005 may have been associated with magmatic or hydrothermal activity beneath Takawangha; these observations suggest that Takawangha may be a more active volcano than was previously known. Other earthquakes in the Tanaga region appear to be caused by regional plate tectonics and are not thought to be associated with volcanism.