Semisopochnoi Island belongs to the Rat Island Group in the western part of the Aleutian arc. Morphologically, Semisopochnoi Island is the largest subaerial volcano in the western Aleutian Arc. The 19 x 17 km island is almost circular (Fig. 1b). The island’s relief is dominated by the 6–7 km diameter circular caldera and several stratovolcano cones, which give the island its name (semisopochnoi means Seven Hills in Russian). As for most of the Aleutian Islands, Semisopochnoi Island volcanism is associated with the ongoing oblique subduction of the Pacific Plate at 74 mm/yr[16]. Semisopochnoi Island is located on the southern extent of the Bower Ridge, a relict subduction zone, which may determine a special character of volcanism in this area[17].
The Pleistocene-aged basement of the island is a large predominantly basaltic shield volcano[12]. The exact age of the volcano is unknown, but it is estimated to be older than several hundred thousand years. The island’s highest point is Anvil Peak, a late-Pleistocene double-headed cone that occupies most of the northern part of the island. Two other peaks on the eastern part of the island, Ragged Top and Pochnoi, are extinct Pleistocene-aged volcanoes that are thought to have not been active during the Holocene[12].
During the Holocene, the composition of the volcanic activity became strongly variable, ranging from basaltic to dacitic. A strong explosive eruption that occurred 5,000 to 6,980 years ago[12] destroyed the central part of the island and formed the caldera. The ignimbrites associated with this eruption created a layer across the entire island, ranging in thickness from several meters up to dozens of meters[12]. The edge of the caldera is well expressed (dotted line in Fig. 1b), except to the southwest, where it is covered by post-caldera lava flows. The oldest lavas on the floor of the caldera are andesitic to dacitic, but they are mostly covered by younger, predominantly basaltic to basalt-andesitic composition lavas and tephras.
The post-caldera volcanic centers developed independently of the caldera geometry; they were controlled by subduction-related tectonic processes and relatively fast magma ascent through a fractured lithosphere[18]. Therefore, the most recently active vents are located both inside and outside the caldera and are apparently not associated with the caldera’s borders. Inside the caldera, the main volcanic activity is represented by the three peaks of Mount Cerberus, all with well-expressed craters. Voluminous, mostly basalt-andesitic composition lava flows from these vents cover most of the caldera surface, with some reaching the sea (green area in Fig. 1b). On the northeastern flank of Mount Cerberus, small fresh lava flows that are dozens of years old have been found. Windy and Lakeshore, monogenic cones located in the northeastern part of the caldera in the vicinity of Fenner Lake (Fig. 1b), represent two other intra-caldera vents that have produced relatively small lava flows[12].
Outside the caldera, most of the Holocene activity, which was basaltic, occurred at Sugarloaf Peak on the southernmost point of the island[12]. Its young satellite peak on the southern flank, Sugarloaf Head, and associated cinder cones produced large basaltic lava flows that propagated to offshore areas (red area in Fig. 1b). Two smaller Holocene-aged volcanic vents, Ringworm crater and Threequarter Cone, are located in the northwestern part of the island. These monogenic cones have identical andesitic compositions and similar stratigraphic positions[12, 13].
Close to the Windy and Lakeshore Cones, a few relatively weak hydrothermal 21°–24°C springs were found[12], which appear to be the only hydrothermal activity on the island. The water from these springs is of meteoric origin; however, Evans et al.[19] found traces of dissolved magmatic CO2.
Because of its remoteness and lack of population, historical volcanic eruptions on Semisopochnoi Island are not well documented. Some volcanic activity was mentioned in historical records dating from 1772, 1790, 1792, 1830, and 1873, but no details of these events were described. Since the 1980s, when satellite images became available, it has been possible to observe the volcanic activity more regularly. The first instrumentally observed eruption occurred on April 13, 1987. A 90-km long ash plume was detected in satellite images[20, 21]. AVO reports recorded minor ash explosions in September and October 2018; between July 16 and August 24, 2019; from December 2019 through to mid-March 2020; and from February to June 2021.
The background seismicity beneath Semisopochnoi Island is relatively weak[5, 22]. However, strong seismic unrest took place beneath Semisopochnoi Island in June 2014[23]. Several thousand recorded volcano-tectonic events may have marked magmatic intrusion; however, none resulted in a magmatic eruption. Interferometric synthetic aperture radar (InSAR) observations were used to evaluate ground deformations during this unrest, and DeGrandpre et al.[24] reported more than 25 cm of total inflation during 2003 and up to June 26, 2014, of which more than half occurred in the ten days between June 15 and 26, 2014. In June, the inflation rate reached 6 mm per day[24], with inflation occurring as a simple radial pattern centered inside the caldera with a slight northeast–southwest oblique skew. After June 26, 2014, the ground deformation almost ceased until resuming during another episode of seismic activity from January to May 2015[23, 24]. Although the total number of events was considerably lower compared to 2014, some of the events had relatively large magnitudes between M 2.0 and 2.8, with depths of up to 10 km[22]. Based on the observed InSAR measurements and relocated seismicity, DeGrandpre et al.’s[24] inverse modeling estimated the locations of the deformation sources for both episodes. They associated the observed ground deformations with magma influx from a depth of ~ 8 km directly below the caldera. Until now, these results, together with the seismicity distribution, were the only geophysical data that provided information on the structure beneath Semisopochnoi Island.