Seismic Structure of Tanaga Island, Alaska

Tanaga Island is located in the Central Aleutian Islands and 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 record of historical eruptive activity (in 1914). Over 3,000 earthquakes have been recorded beneath the island and the surrounding offshore region since the six-station seismic network was emplaced in 2003. The origin of these earthquakes is not completely understood, and to arrive at this understanding, more accurate hypocenter locations and power spectra need to be determined. A better analyses including improved locations of earthquake hypocenters can provide useful constraints on volcanic and tectonic activity in the region. We use cross-correlation and double-difference methods to relocate Tanaga Island earthquakes from the period 2003-2017. High precision relative relocations show multiple subsurface faults in the Tanaga region related to volcanism and crustal tectonics. In 2005, a large volcanic swarm of nearly 600 volcano-tectonic (VT) events located below the NW portion of the island culminated with a several minute long episode of volcanic tremor. Although there was no verified eruption associated with this swarm, we suggest that this activity is associated with Takawangha volcano due to the shallowing of events towards Takawangha and the inferred location of the tremor signal. In 2008, a M6.6 earthquake was recorded 2.5 km east of Tanaga Island, associated with bookshelf faulting and the northwest translation of the Andreanof region. Shortly after the M6.6 event, two regions on Tanaga Island, 20 and 30 km west of the epicenter became seismically active, suggesting triggering by the M6.6. We also examine a variety of smaller swarms in the Tanaga area. From 2006-2017, we identify activity accommodating the clock-wise rotation of the Delarof Block (Geist et al., 1988), one of five forearc crustal blocks, including a small swarm in 2010. In 2006 heightened seismicity suggests brittle rupture possibly accompanied by fluid movement inferred from the observations of hybrid events 4 km south of Takawangha. In addition, shallow zones of brittle rupture were discovered 8 km south and 25 km southeast of Takawangha. This analysis suggests a complex pattern of earthquake hypocenters that is governed by both volcanic and tectonic processes surrounding Tanaga and Takawangha Volcanoes.