Recording of Local Earthquakes under the Udina Volcanoes (Kamchatka) by Regional and Temporary Seismic Stations. Refinement of the Velocity Model by Reversing the Travel-Time Curve from a Deep Source

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Currently, the Udina volcanoes (Kamchatka, Klyuchevskaya Volcano Group) are considered extinct. However, at the end of 2017, seismic activation was recorded for the first time since instrumental observations beneath them have been carried out at the middle−upper crustal level. To improve the accuracy in determining the coordinates of hypocenters in the vicinity Udina volcanoes, work was carried out to refine the local velocity model for this area. The velocity structure of the setting was refined using proprietary approaches to determining the coordinates of earthquake hypocenters and reversing the travel-time curves from deep sources. Seismic wave propagation velocities were obtained from data on earthquakes of the Udina seismic activation 2017−2019. The results of the Kamchatka regional seismic network and four additional temporary stations installed specifically to study the Udina seismogenic region were used. A one-dimensional velocity model was obtained for depths up to 40 km: the set of layers in which the longitudinal wave velocity varies linearly at a constant ratio VP/VS = 1.85. The model differs significantly from the generalized baseline curve obtained for the northern part of the Klyuchevskaya Volcano Group based on DSS data DSS. The largest differences in velocities are observed in the upper crust (at a depth of up to 4 km) and in the region of the Conrad discontinuity at a depth of ∼20 km In the depth range of 20−25 km, a reduced-velocity layer was revealed.