Structure of the Upper Crust beneath the Klyuchevskoy Group of Volcanoes Revealed from Ambient Noise Tomography

Abstract

––The Klyuchevskoy group of volcanoes (KGV) located in the central part of Kamchatka is a unique complex that demonstrates exceptional variety and intensity of volcanic manifestations. These features of the eruptive activity of the KGV are determined by a complex system of magmatic sources in the crust and mantle. While the structure of deep anomalies is quite reliably determined by tomography techniques based on body waves, the structure of the upper crust can only be determined using ambient noise tomography. We present the results of processing data from the KISS temporary network. This network consisted of more than 100 seismic stations that were installed from 2015 to 2016 over a large area covering the Klyuchevskoy group of volcanoes and its surroundings. To retrieve Rayleigh surface waves, cross-correlation of continuous seismic noise records from pairs of stations was used. We obtained the dispersion curves of the group velocities of these Rayleigh surface waves using frequency–time analysis (FTAN) of the calculated correlograms. These curves served as input data for performing ambient noise tomography. Tomography was performed in two stages: (1) computation of two-dimensional group velocity maps for different frequencies and (2) calculation of a three-dimensional model of the shear wave velocity to a depth of about 8 km based on the inversion of local dispersion curves obtained from these maps. The resulting models revealed the structural features of individual volcanic systems of the KGV. High velocities were observed at shallow depths beneath the large basaltic edifices of the Ushkovsky and Tolbachik volcanoes. At greater depths, while the velocity structure beneath Ushkovsky remained unchanged, we detected low velocities beneath Tolbachik. This fact illustrates the difference between dormant and active magmatic systems. Velocity anomalies of a complex shape are observed beneath the Klyuchevskoy, Kamen, and Bezymianny volcanoes, varying both laterally and with depth. Absolute velocities in vertical sections show that the edifices of these volcanoes are relatively low-velocity bodies located on a horizontal high-velocity basement. A low-velocity anomaly was discovered under the Bezymianny Volcano at a depth of 6 km, which is presumably associated with a shallow magma reservoir. An intense low-velocity anomaly was found beneath the Udina Volcano. It was interpreted as an image of a magma reservoir experiencing strong seismic unrest that began in December 2017 and continues to this day.

Mantle and Crustal Sources of Magmatic Activity of Klyuchevskoy and Surrounding Volcanoes in Kamchatka Inferred From Earthquake Tomography

“…In the crust, the model is generally consistent with the previous study by Koulakov et al (2017), because at shallow depths, the tomography is mostly controlled by the same subsets as used in the previous case. We also observe some correspondence with the new results of the ambient noise tomography (Green et al, 2020, and especially with Egorushkin et al, 2020).…”

“…To close this gap, a large-scale experiment named KISS (Klyuchevskoy Investigation-Seismic Structure of an Extraordinary Volcanic System) was initiated in 2015 to cover the entire area of the NGV with more than 100 stations operating simultaneously . Recently, the data of the KISS network were used to investigate the upper crustal structure beneath the KVG using the ambient noise tomography in two independent studies (Egorushkin et al, 2020;Green et al, 2020) In this study, we present the first results of body wave tomography based on the KISS data that allowed us to enhance the resolution of the structures in the entire crust and mantle beneath NGV. In particular, this work provides a fundamentally new model of the mantle wedge that sheds light on the deep sources of the NGV volcanism.…”

Mapping the Pacific Slab Edge and Toroidal Mantle Flow Beneath Kamchatka

“…Since the first 3-D seismic velocity model was obtained for the Kamchatka subduction zone (Slavina & Pivovarova, 1992), many studies have been made to clarify the detailed isotropic velocity structure of this region, with the accumulation of seismic data and the improvement of tomographic methods (e.g., Egorushkin et al, 2021;Fan & Zhao, 2021;Gorbatov et al, 1999Gorbatov et al, , 2000Gorbatov et al, , 2001Green et al, 2020;Jiang et al, 2009;Koulakov et al, 2011Koulakov et al, , 2017Koulakov et al, , 2019Koulakov et al, , 2020Lees et al, 2007;Levin, Shapiro, et al, 2002;Nizkous et al, 2017). The most notable feature revealed by these previous studies is that the subducting Pacific slab is imaged clearly as a high-velocity zone in the upper mantle on the south of the Shiveluch volcano, but the slab is absent on the north of the volcano.…”