As a result of a review of methods for the identification of group events, it is argued that to date there is no universal algorithm for detecting formations of a physical nature in the structure of seismicity. Therefore, the method of catalog declustering should be selected both for the existing structure of the source data and for a specific task. It is proposed to use the declustering of earthquake catalogs with great caution, since such a procedure can significantly distort the seismic hazard assessment. Information on earthquakes in the Yano-Indigirskaya lowland in northern Yakutia, was collected which seismicity is relatively poorly studied. An analysis of the data of 43 seismological agencies resulted in a catalog of 3161 earthquakes from 1920 to 2020 that has been compiled with a representative magnitude of M-W = 3.3 to distinguish the grouped events. The catalog was declustered using different methods, and the seismicity has also been divided into concentrated (group) and scattered (background) components. It is shown that these components differ significantly in recurrence plots and M-max estimates, which significantly affects the seismic hazard assessment.

Deep long-period (DLP) earthquakes observed beneath active volcanoes are sometimes considered as precursors to eruptions. Their origin remains, however, unclear. Here, we present a possible DLP generating mechanism related to the rapid growth of gas bubbles in response to the slow decompression of over-saturated magma. For certain values of the gas and bubble content, the elastic deformation of surrounding rocks forced by the expanding bubbly magma can be fast enough to generate seismic waves. We show that amplitudes and frequencies of DLP earthquakes observed beneath the Klyuchevskoy volcano (Kamchatka, Russia) can be predicted by our model when considering pressure changes of ~10 7 Pa in a volume of ~10 3 –10 4 m 3 and realistic magma compositions. Our results show importance of the deep degassing in the generation of volcanic seismicity and suggest that the DLP swarms beneath active volcanoes might be related to the pulses of volatile-rich basaltic magmas rising from the mantle.

Длиннопериодные землетрясения и треморы, наравне с вулкано-тектоническими землетрясениями, являются одним из двух основных классов вулкано-сейсмической активности.Считается, что длиннопериодная вулканическая сейсмичность связана с колебаниями давления в магматической и гидротермальной системах под вулканами и поэтому может быть использована в качестве предвестника готовящихся извержений.В тоже время, физический механизм длиннопериодной сейсмичности остаётся не полностью понятым.В данной работе мы исследовали длиннопериодные землетрясения, происходящие на границе кора-мантия под Ключевской группой вулканов на Камчатке, с целью установить их закон повторяемости: связь между магнитудой и частотой событий.Данный тип землетрясений наиболее многочислен в изучаемом районе и характеризует состояние глубинного магматического резервуара, находящегося на границе кора-мантия.Изменения сейсмического режима в этой части магматической системы могут быть одним из ранних предвестников извержений.Для более полной характеризации закона повторяемости мы создали новый каталог глубоких длиннопериодных землетрясений на основе обработки непрерывных сейсмограмм, записанных сетью станций КФ ФИЦ ЕГС РАН в 2011-2012 годах, по методу согласованного фильтра.Также мы применили метод определения магнитуд этих землетрясений, приближенный к моментной шкале

Deep Long Period (DLP) earthquakes have been observed in many volcanic regions and are often considered as one of the important precursors to volcanic eruptions. At the same time, the physics of the source of these earthquakes remains unclear. We focus our study on Klyuchevskoy group of volcanoes in Kamchatka, Russia, one of the World’s most active volcanic system. The DLP earthquakes in this region occur at the limit between the lower crust and the upper mantle at depths of 30-35 km where ductile flow is expected to dominate rock deformation. Their occurrence also appears to correlate with the eruptive activity. Therefore, this is natural to consider that their generating mechanism is not related to brittle mechanism but rather to pressure fluctuations in the magmatic system as often suggest for the LP seismicity in general. We suggest a possible generating mechanism related to the rapid pressure changes caused by nucleation and growth of gas bubbles in response to the slow decompression of over-saturated magma. The pressure variation is simulated using the mathematical model of bubble nucleation and growth accounting for multiple dissolved volatiles (H2O-CO2) and diffusive gas transfer from magma into growing bubbles. Results of simulations show that fast pressure increase followed by its relaxation almost to its initial level is not very sensitive to the assumptions on the values of governing parameters. Typical pressure changes of a few tens of MPa in a volume of 3500 m3 occurring on time scales of fractions of a second to a second following bubble nucleation and growth can generate seismic waves with amplitudes similar to those recorded by seismographs in the vicinity of the Klyuchevskoy volcano.

The paper investigates the effect of different methods for preprocessing earthquake catalogs (declustering, i.e., removal of dependent events from them, and selection of the magnitude of completeness) on seismic hazard assessment. Seismic catalogs of the Kamchatka and Caucasus regions have been used for the analysis, because synthetic catalogs do not always reflect the real features of regional seismicity. Test sites were selected for these regions. Three declustering methods are considered that leave different numbers of events in the catalogs. The plotted seismic hazard curves indicate a complex interaction of catalog declustering and selection of the magnitude of completeness. Since both methods affect the b -value, it cannot be predicted in advance, which will lead to an increase or decrease in the estimate for b . After it is applied, the declustering method leaves the largest number of events in the catalog, but does not always give the highest seismic hazard rating. Therefore, it is necessary to be extremely careful and attentive when declustering a catalog. At the very least, it should be borne in mind that the end result may be unpredictable.