Structure of the magma plumbing system beneath Semisopochnoi Island (Aleutian Arc) inferred from seismic tomography

Abstract

Semisopochnoi Island is a remote and little-studied volcanic island in the western part of the Aleutian Arc. The existence of several active volcanic centers and a 5000–7000-year-old large caldera makes this island an important site for volcanic hazard assessment in the Northern Pacific. Based on local seismicity data recorded by six permanent seismic stations, we created the seismic tomography model, including the 3D distributions of Vp, Vs, and Vp/Vs ratios to a depth of 10 km. This model provides the first geophysical insight into the interior structure of Semisopochnoi Island and sheds light on the processes in the magma plumbing system beneath all volcanic centers on the island. At depths of 5–10 km, we observed a columnar-shaped high Vp/Vs-ratio anomaly below the caldera in the central part of the island, which likely represents the steady magma conduit. This conduit is headed by a prominent high Vp/Vs-ratio anomaly located 3–5 km directly below the caldera, which represents the magma reservoir feeding Cerberus and other Holocene-aged volcanic centers on Semisopochnoi Island.

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“…The stronger P-wave anomaly may be indicative of a porous structure of the material, which could behave as a sponge and has low bulk elastic modulus (Takei, 2002). In this case, the Vp/Vs shows strongly negative anomaly, which is quite typical for caldera-filling rocks (see, e.g., a similar feature in the Semisopochnoi Island in the tomography model by Yaroshenko et al, 2022). In the vertical section (Figure 10), we can see that this anomaly of low Vp/Vs ratio is rather thin and hardly reaches 1 km.…”