SEISMIC STRUCTURE OF TANAGA ISLAND, ALASKA

Tanaga Island is located in the western Aleutian Islands of Alaska, and comprises four active stratovolcanoes: the three volcanoes of the Tanaga cluster in the north, and Takawangha volcano in the central part of the island. Earthquakes occur beneath Tanaga Island on a daily basis, with >6000 earthquakes recorded since the seismic network was installed in 2003. While many of these earthquakes are likely associated with volcanic processes, others may have a tectonic origin: the oblique subduction of the Pacific Plate in the central and western Aleutian Arc results in the clock-wise rotation of five fore-arc crustal blocks on the overriding North American Plate. The rotation of crustal blocks in the fore-arc is responsible for much of the local stress orientations and many large magnitude strike-slip earthquakes (Ruppert et al., 2012). This study examines the seismicity nearby Tanaga Island to explore the relationship between volcanism, subduction, and crustal block tectonics. Double-difference earthquake relocations take both catalog phase picks and adjusted arrival time picks after cross-correlation to optimally relocate earthquake hypocenters. Three distinct seismic swarms (ranging from 400 to 800 events each) were observed and analyzed in detail to identify subsurface structures and the sources of of seismicity associated with them. Source processes were examined by characterizing event waveforms based on spectral content. In late 2005, a swarm of deep (10-15 km) volcanic events occurred below the Tanaga cluster; however, towards the end of this swarm, events were observed much shallower (0-10 km) with hypocenter locations shifted toward Takawangha. Later in 2008 and 2017, offshore swarms occurred in three different locations east of the island near both Takawangha and the crustal block margin. Background seismicity in the area is mainly focused beneath the volcanic centers and offshore, south of Tanaga Island. There are also many deep (>60 km) tectonic events recorded in this area, and thought to be related to the subduction of the Pacific Plate. High resolution earthquake relocations and the interpreted seismic source processes are utilized to examine the level of seismic activity at the four volcanoes on Tanaga Island as well as the nearby crustal block margins.