Tomographic Images of Magma Chambers Beneath the Avacha and Koryaksky Volcanoes in Kamchatka

Abstract

A new tomographic model (Vp, Vs, and Vp/Vs ratio) was built for two neighboring active volcanoes, Avacha and Koryaksky, which represent a serious hazard to the population and infrastructure of Petropavlovsk‐Kamchatsky, the main city of Kamchatka, Russia. Arrival times of seismic P and S waves from almost 5,000 local events, recorded by a permanent network of seismic stations during 2009–2018, were used for tomography. The resolution of the derived models was carefully tested by a series of synthetic simulations. Prominent anomalies with extremely high Vp/Vs ratios (up to 2.4) were retrieved directly beneath both volcanoes and interpreted as magma reservoirs containing high degrees of partial melt and/or fluids. Beneath Avacha, the upper limit of the anomaly is located at the depth of ~2 km below the surface. The reservoir appears to be connected to the surface by a neck‐shaped anomaly of high Vp/Vs ratio associated with active seismicity, which is interpreted as a magma and fluid conduit. Beneath Koryaksky, the magma related anomaly is deeper: Its upper limit is located at a depth of ~7 km below the surface. This anomaly is connected with the volcanic cone and is associated with a vertical seismicity cluster, which possibly marks the pathway of fluid ascent and degassing. Between the volcanoes, a 2‐ to 3‐km thick layer of very low Vp and Vs is interpreted as deposits of volcanoclastic sediments. Generally low Vp/Vs ratios in the area between the volcanoes show that the magma reservoirs in the upper crust are not interconnected.

Anatomy of the Bezymianny volcano merely before an explosive eruption on 20.12.2017

“…At greater depths of ~ 3 km, located slightly off-centered beneath the southeastern flank of Bezymianny, the coincidence of high Vp and low Vs results in an anomaly of high Vp/Vs ratio (anomaly 2 in Fig. 4 ): a typical feature of many active volcanoes usually interpreted as a magma reservoir 33 , 44 . In contrast, we observe the opposite pattern beneath the northwestern flank of Bezymianny (anomaly 3 in Fig.…”

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

“…(2013) has revealed almost identical columnar structure of high Vp/Vs ratio. The other cases of coexistence of high Vp , low Vs , and high Vp/Vs beneath active volcanoes are: Colima Volcanic Complex (Sychev et al., 2019), Avacha Volcano Group (Bushenkova et al., 2019), Gorely (Kuznetsov et al., 2017), Nevado del Ruiz (Vargas et al., 2017), and many others.…”

“…Based on consideration of a series of previous tomography studies having similar distributions of stations and earthquakes (e.g., Bushenkova et al., 2019; Kasatkina et al., 2014; Koulakov et al., 2019, 2011), we propose that the available data in the case of Akutan may enable higher resolution than reported by Syracuse et al. (2015).…”

“…Based on consideration of a series of previous tomography studies having similar distributions of stations and earthquakes (e.g., Bushenkova et al, 2019;Kasatkina et al, 2014;Koulakov et al, 2019Koulakov et al, , 2011, we propose that the available data in the case of Akutan may enable higher resolution than reported by Syracuse et al (2015). The main purpose of this study is to revisit the updated Akutan seismological data and to obtain new details of the magmatic system beneath this volcano.…”

“…To perform the tomographic inversion based on the local earthquake data, we use the LOTOS code (Koulakov, 2009), which was previously used for studying several volcanoes having similar geometry of stations and seismicity, such as Avacha volcano (Bushenkova et al., 2019) and Mount Spurr (Koulakov et al., 2013). As input information, this code uses the arrival times of the P and S waves from seismic events located inside or slightly outside the recording network.…”