We use SESL’09 technique for a unified estimation of seismicity level in a given space-time regions. The application of the distribution function of radiated seismic energy permits the formalization of qualitative description of seismicity. We present the results from statistical estimations of seismicity level before the 2000- 2012 Bezymianny Volcano eruptions. The state of seismicity was describes by the monogram, constructed according the SESL’09 technique. By processing the Klyuchevskoy volcano group seismic catalog, we estimated variations of seismicity and detected background level. Statistically significant variations of the seismicity level preceded the eruptions were detected and discussed.

The article presents a technique for formalized separation of volcano-tectonic earthquakes and low-frequency seismic events which occurred on volcanoes. The technique was used as a case study for four episodes of volcanic activity in Kamchatka based on data from seismic stations located near volcanoes: explosive eruption of Kizimen Volcano (2010-2011), activization of Gorely Volcano (2009-2011), activization of Koriaksky Volcano (2008-2009), long-term intense hydrothermal activity of Mutnovsky Volcano. For visualization of hidden correlation in waveform spectral characteristics we use the triangle diagram. We have targeted a trend in volcano investigation which supposes study of possible relation between types of volcano activity and distribution of spectral components in seismic records.

This paper reports a study of the Tolud earthquake sequence; the sequence was a burst of shallow
seismicity between November 28 and December 7, 2012; it accompanied the initial phase in the Tolbachik
Fissure Eruption of 2012‒2013. The largest earthquake (the Tolud earthquake of November 30, 2012, to be
referred to as the Tolud Earthquake in what follows, with KS = 11.3, ML = 4.9, MС = 5.4, and MW = 4.8) is
one of the five larger seismic events that have been recorded at depths shallower than 10 km beneath the entire Klyuchevskoi Volcanic Cluster in 1961‒2015. It was found that the Tolud earthquake sequence was the foreshock–aftershock process of the Tolud Earthquake. This is one of the larger seismicity episodes ever to have
occurred in the volcanic areas of Kamchatka. Data of the Kamchatka seismic stations were used to compute
some parameters for the Tolud Earthquake and its largest (МL = 4.3) aftershock; the parameters include the
source parameters and mechanisms, and the moment magnitudes, since no information on these is available
at the world seismological data centers. The focal mechanisms for the Tolud Earthquake and for its aftershock are consistent with seismic ruptures at a tension fault in the rift zone. Instrumental data were used to estimate the intensity of shaking due to the Tolud Earthquake. We discuss the sequence of events that was a signature of the time-dependent seismic and volcanic activity that took place in the Tolbachik zone in late November 2012 and terminated in the Tolud burst of seismicity. Based on the current ideas of the tectonics and magma sources for the Tolbachik volcanic zone, we discuss possible causes of these earthquakes.

The seismicity that accompanied the Tolbachik Fissure Eruption was recorded by additional seismic stations that were installed in the southern Klyuchevskoi Volcanic Cluster area in January to October 2013. We used broadband (0.033–50 Hz) three-component digital Guralp CMG-6TD seismometers. This temporary network provided seismicity data at a lower energy level than can be done using the regional seismograph network of Kamchatka. The processing of the resulting digital records supplied data for compiling a catalog of over 700 ML=0–3.5 (КS=1.5–8.5) earthquakes, which is an order of magnitude greater than the number of events located by the regional network for the same period of time. The seismicity in the area of Ploskii Tolbachik Volcano was found to concentrate mostly in spatially isolated areas during the eruption. The main isolated clusters of earthquakes were identified both in the eruption area itself and along the periphery of Ploskii Tolbachik Volcano, in the area of the Zimina volcanic massif, and in the Tolud epicenter zone; the eruption zone was not dominant in the seismicity. The region of a shallow seismicity increase beneath Ploskii Tolbachik before the eruption was not found to exhibit any increased activity during the time the temporary seismograph network was operated, which means that a seismicity inversion took place at the beginning of the eruption. We discuss the question of what the earthquake-generating features are that we have identified.

With the use of the method of low-frequency microseismic sounding, the configuration of the magmatic feeding system of the Tolbachinsky Dol—a regional zone of areal basaltic volcanism in the southern part of the Klyuchevskoy volcano group in Kamchatka—is studied. The initial data are obtained by a step-by-step recording of the background microseismic noise in 2010–2015 within a thoroughly marked-out survey area covering the zones of fissure eruptions in 1975–1976 and 2012–2013 and, partly, the edifice of thePloskii (flat) Tolbachik volcano. The depth sections ref lecting the distributions of the relative velocities of seismic waves in the Earth’s crust are constructed. For a more reliable interpretation of the revealed deep anomalies, the results of independent geological and geophysical studies are used. The ascertained low-velocity structures are closely correlated to the manifestations of present-day volcanism. It is shown that the feeding structure of the Tolbachinsky Dol is spatially heterogeneous, incorporating subvertical and lateral pipe-shaped magma conduits, closely spaced magma feeding channels, and shallow magma reservoirs. A long-lived local transcrustal magma conducting zone is revealed, and regularities in the deep structure of the feeding systems of fissure eruptions are identified. The configuration of the established subvertical magma conduits permits basalts moving to rise to the surface by different paths, which, inter alia, explains the contrasting magma compositions observed during a single eruption. Thus, based on the instrumental data, it is shown that the magmatic feeding structure of the Tolbachinsky Dol has a number of specific peculiarities and is significantly more complicated than has been previously thought about the areal volcanic fields.

По данным микросейсмического зондирования построена глубинная модель среды под районом Узон-Гейзерной вулкано-тектонической депрессии и прилежащего к ней вулканического массива Кихпиныч (Камчатка) до глубины 30 км. Для этого была осуществлена регистрация естественного фонового микросейсмического поля переносными широкополосными сейсмометрами Guralp CMG-6TD в точках специально спланированной наблюдательной сети: в 60-ти точках вдоль трех профилей общей длиной около 28 км. Выявленные структурные неоднородности проинтерпретированы с учетом известных ранее результатов геологических, геолого-морфологических и петрологических исследований. Идентифицирована и пространственно локализована область малоглубинного закристаллизовавшегося магматического очага под депрессией. Выявлены области предположительной концентрации базальтовых расплавов, с которыми может быть связана наблюдающаяся в последние ~ 15 лет локальная геодинамическая активизация исследуемого района: периферический магматический очаг вулканического массива Кихпиныч на глубине 5-12 км, более глубокая (15-20 км) магматическая камера. Получено согласие геометрии обнаруженных глубинных структур с локальной микросейсмичностью и моделью современного магматического внедрения в верхние горизонты коры, разработанной по данным спутниковой интерферометрии.

Abstract—This paper is concerned with ground deformation and seismicity prior to the Tolbachik Fissure Eruption, which began in Kamchatka on November 27, 2012. Seismic and GPS data were analyzed to reveal synchronous precursory anomalies in crustal deformation and seismicity that lasted approximately 4 months (August to November 2012). The seismic anomaly was a statistically significant increase of seismicity with low energy (mostly KS = 4–6) beneath the Ploskii Tolbachik Volcano edifice at depths of less than 5 km. The rates of seismicity and seismic energy release were exceeded by factors of approximately 40 compared with the
2000–2011 mean values during the 2 to 3 weeks immediately before the eruption. The strain anomalies were observed as movements in the middle of the Klyuchevskoy volcanic group: a radial (relative to the eruption) compression and an extension in the tangential direction. The strain had reached ~ 10^–7 by the beginning of the eruption. The durations of the seismic and strain anomalies were comparable in value (~ 4 months before the eruption), thus providing evidence of a common origin. We can classify them as belonging to the same time scale of precursors (the intermediate�term in the accepted terminology).