Seismic Structure of Tanaga and Takawangha Volcanoes, Tanaga Island, Alaska

Date: 4/26/2019

Time: 06:00 PM

Room: Fifth Avenue

Tanaga Island is located in the Central Aleutian Islands and includes four stratovolcanoes: Sajaka, Tanaga, and East Tanaga in the north, and Takawangha in the central part of the island. Of these volcanoes, only Tanaga has reported eruptive activity in 1914. Over 6000 earthquakes have been recorded beneath the island and the surrounding offshore region since the six-station seismic network was emplaced in 2003. 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 area 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. In 2008, a M6.6 earthquake was recorded 2.5 km east of Tanaga Island, associated with bookshelf faulting and block rotation in the overriding plate. 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 swarm episodes in the Tanaga area. From 2006 – 2017, we identify activity along the Delarof Block (Geist et al., 1988), one of five forarc crustal blocks, including a small swarm in 2010. In 2013 and 2014, heightened seismicity suggests brittle rupture possibly accompanied by fluid movement due to the observations of hybrid events 8 km south of Takawangha. In addition, shallow zones of brittle rupture were discovered 6 km south of Takawangha 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.

Presenting Author: Kevin F. Lally

Authors