Magma‐Fluid Interactions Beneath Akutan Volcano in the Aleutian Arc Based on the Results of Local Earthquake Tomography

Abstract

Volcanoes in the Aleutian arc and Alaska are located in low-populated areas; nevertheless, they represent serious hazard to the local population and infrastructure, as well as to the air traffic along dense aviation routes in the northern Pacific (Casadevall, 1993). To enable fast detection of ongoing volcanic eruptions, most active volcanoes in these regions are equipped with telemetered seismic networks that continuously transmit the records to the offices of the Alaska Volcano Observatory (AVO), where they are processed in real time (Dixon et al., 2019). Besides the hazard assessment, the data recorded by these networks are used for studying fundamental aspects of functioning magma plumbing systems, which still remain poorly understood due to their complexity. Mechanical, chemical, and thermal interactions of materials in the magmatic systems require multidisciplinary approaches, in which geophysical imaging of structures beneath volcanoes is one of the essential elements. In this study, we investigate the upper-and middle-crustal structure beneath Akutan Island located in the eastern part of the Aleutian arc (Figure 1a). This island hosts Akutan volcano, which is one of the most active in the Aleutian arc, with dozens of documented eruptions starting from the eighteenth century (Siebert & Simkin, 2013). These eruptions were mostly weak or moderate; however, the existence of a Holocene caldera with a diameter ∼2 km and thick scoria-bearing, lapilli tephra (Akutan tephra) widely spreading over the entire island surface (Waythomas, 1999) indicates that Akutan has a serious potential for explosive caldera-forming eruptions. During the past century, Akutan demonstrated variable styles of volcanic activity, such as lava flows, gas emissions, and strombolian explosive eruptions ejecting ash plumes (e.g., Miller et al., 1998). The most recent eruption of Akutan occurred in 1992. However, after this, in March 1996, a remarkable episode of seismic unrest occurred, which did not lead to a volcanic eruption, but caused earthquakes with magnitudes of up to M5.3 and triggered strong ground deformations (Lu et al., 2000, 2005). These and other aspects of Akutan's activity are discussed in more detail in the next section.

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

“…The derived d Vp , d Vs , and, especially, Vp / Vs -ratio anomalies provided important information about the magma plumbing processes beneath the Semisopochnoi Island volcanoes. Based on experimental studies 29 and several previous tomographic studies of other volcanoes 8 – 11 , 25 , 30 , 31 , some common relationships between seismic parameters and rock properties can be proposed. For example, P -wave velocity is mostly sensitive to rock composition, whereas S -wave velocity is mostly affected by the presence of a liquid phase (e.g., melt and volatiles).…”

“…Therefore, semi-molten magma stores and conduits are usually associated with anomalies with higher d Vp , lower d Vs , and a very high Vp / Vs ratio. For the most active volcanoes worldwide, where P - and S -wave velocities are available, such relationships are very helpful for imaging magma reservoirs and conduits 8 – 11 , 25 , 30 , 31 .…”

“…The structure obtained for Semisopochnoi Island in this study is consistent with a number of other tomography models derived from other Aleutian and Alaskan volcanic islands. For example, Koulakov et al’s 11 modeling of the structure beneath Akutan Volcano indicated a single deep conduit up to 5 km below the surface that branches at shallower depths to form a number of local conduits connected to volcanic vents and centers of geothermal activity. Koulakov et al’s 9 , 31 tomography modeling beneath Mount Spurr indicated a columnar high Vp / Vs -ratio anomaly from 4 to 5 km below the surface, which, at shallower depths, is replaced by a zone of low Vp / Vs .…”

“…The P -wave velocities at each level were determined after several runs of the full inversion procedure by adjusting the average velocity according to the results of the previous model. The initial velocity model in the first run was taken from a case of Akutan 11 , where the tomography model was previously constructed for similar geological settings. The values of the P velocities in the final 1D reference model are presented in Table 1 .…”

“…These stations have been in operation for decades, so they provide useful information on the internal processes and structures of active volcanoes. Data from these networks have been used to create seismic tomography models for several Aleutian and Alaskan volcanoes, which has helped better understand the evolution of the magma sources 6 – 11 .

Figure 1 Location and general structural units of Semisopochnoi Island.

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