Resolving the Magmatic and Tectonic Structure of the Tanaga Volcanic Cluster, Aleutian Arc, Alaska, Using Double Difference…

Located in the Central Aleutians along the Pacific-North American convergent margin, Tanaga Island features four tightly clustered stratovolcanoes: Sajaka, Tanaga, East Tanaga, and Takawangha, collectively referred to as the Tanaga Volcanic Cluster (TVC). During periods of unrest in 2005 and 2023, temporal patterns in seismicity suggest that volcanoes in the cluster may be interconnected in the subsurface and geochemical data suggests they may share a parent magma source. Here, we use 21 years (2003 - 2024) of locally recorded seismic events cataloged by the Alaska Volcano Observatory and the double-difference seismic tomography algorithm tomoDD, to simultaneously relocate earthquake hypocenters and generate a 3-D P-wave velocity structure beneath Tanaga Island. We find that high P-wave velocity anomalies in the shallow (0-5 km below sea level) subsurface likely represent buried basaltic lava flows and a low P-wave velocity anomaly south of the volcanic cluster beneath Tanaga Bay likely represents hydrothermally altered material. A low P-wave velocity anomaly begins at ~7.5 km and extends to ~18 km and is centrally located beneath the volcanic cluster which we interpret as common magma storage for the entire TVC. Rigidity contrasts between materials likely controls the location of tectonic faulting beneath the island. We find that seismicity occurs primarily along a right-lateral, east-west striking transform fault system and is controlled by regional tectonics. Oblique subduction of the Pacific Plate causes strain to be partitioned on the overriding North American Plate which is accommodated by clockwise rotating bookshelf faults and their conjugates proximal to the volcanic system. In 2023, right-lateral motion on the east-west transform fault caused dilatation beneath the western TVC and allowed for the ascent of magmatic fluids and potential dike emplacement. We suggest that tectonic and volcanic processes exist in concert in the TVC system.