N. S. Stovbun

The paper presents the results of rock deformation measurements conducted in 2024–2025 on the Shikotan Island (Lesser Kuril Ridge). The measurements were conducted using strainmeters installed in the adit of the former Shikotan geophysical observatory, now the Shikotan seismic station of the Sakhalin Branch of the Geophysical Survey. The research was conducted by means of an instrumental complex developed specially for monitoring rock deformation. The goal of the work is to provide a comprehensive analysis of the variations of deformations of rocks depending on various natural processes. The first results of the analysis showed the correctness of the employed deformation measurement technique. An important finding was the relationship between deformation rate changes and the local seismic regime. There has been a reaction from both components (alternating impulse) to strong remote earthquakes on July 20 and 30, 2025, in Kamchatka. It was shown that both components of soil deformation consistently reflect the passage of cyclones, which is expressed in short-term variations of an impulsive nature.

В работе исследуются вариации вертикальной компоненты электротеллурического поля (ЭТП) до и после землетрясений, произошедших в период 2024-2025 г. Анализировались данные с двух станций, расположенных на юге о. Сахалин в центральной («Южно-Сахалинск») и западной («Колхозное») части. В качестве датчиков напряженности поля использовались заглубленные металлические пластины площадью 0.25 м2. На станции «Колхозное» для измерений также применялись свинцовые электроды цилиндрической формы. Запись вертикального ЭТП в частотном диапазоне до 10 Гц проводилась в квазиреальном режиме времени с января 2024 г по настоящее время. Цель исследования – проверка гипотезы о возможном использовании вариаций вертикальной компоненты ЭТП как одного из предвестников землетрясения. Конкретной задачей данной работы является исследование вариаций вертикальной компоненты ЭТП на двух станциях в южной части о. Сахалин и анализ взаимосвязи с локальной сейсмичностью. По результатам измерений ЭТП установлено наличие периодов с повышенным уровнем напряженности вертикального поля, не связанные с сезонностью. Сейсмические события с магнитудами M≥2, зарегистрированные на юге Сахалина, происходят преимущественно в эти периоды. Полученные результаты могут свидетельствовать, что вариации вертикальной ЭТП отражают локальные изменения сейсмического режима. This study investigates variations of the vertical component of the electrotelluric field (ETF) before and after earthquakes that occurred during 2024-2025. Data from two stations located in the south of Sakhalin Island in the central («Yuzhno-Sakhalinsk») and western («Kolhoznoe») parts were analyzed. Buried metal plates with an area of 0.25 m2 were used as field strength sensors. At the «Kolhoznoe»station, cylindrical lead electrodes were also employed for measurements. Recording of the vertical ETF in the frequency range up to 10 Hz was conducted in quasi-real-time mode from January 2024 to the present. The aim of the study is to test the hypothesis regarding the possible use of variations of the vertical component of the ETF as earthquake precursors. The specific task of this work is to investigate variations of the vertical component of the ETF at two stations in the southern part of Sakhalin Island and to analyze the relationship with local seismicity. Based on the ETF measurements, periods with high levels of vertical field strength, not associated with seasonality, have been identified. Seismic events with magnitudes M≥2, recorded in the south of Sakhalin, occur predominantly during these periods. The obtained results may indicate that variations in the vertical ETF reflect local changes in the seismic regime.

The paper presents the results of shallow-depth electrical sounding carried out in 2021–2022 near the Central Sakhalin fault at the geophysical test site of the IMGG FEB RAS (Petropavlovskoye village, Aniva district). Electrical soundings were carried out using a pulse voltage generator developed at the IMGG FEB RAS. The aim of the experiment was to evaluate the response of the medium in the near-fault zone to the impact of current pulses, the amplitude of which is significantly greater than that for resistivity prospecting or seismoelectric prospecting methods but many times less than that for high-power electrical sounding in Central Asia with the aid of geophysical MHD generators or electric pulse installations. The response of the medium was determined based on the records obtained from the seismic instruments located at the Petropavlovskoye test site – a broadband seismometer CME-6111, molecular-electron hydrophone, – as well as on the records of seismic noise at the "Ozhidayevo" and "Kholmsk" stations of the of the SB FRS GS RAS and on short-period seismometers Zetlab 7152-N and SPV-3K installed near the temporary sites in the test area. The results of processing of the seismoacoustic data obtained by a hydrophone for four experimental stages (13 days of serial sounding using unipolar current pulses) showed an increase in the noise amplitude in the near-surface layer during electrical soundings in the Central Sakhalin fault zone. This may indicate the influence of electrical soundings on the sources of seismoacoustic signals, i.e. on the medium destruction at the electrical impact site. Waveform recordings from all seismometers revealed the changes in seismic noise levels that correlate with the passage of cyclones causing high wind loads and changes in atmospheric pressure. Autumn-spring cyclones, frequent in Sakhalin, occurred at all stages of the experiment, which is why responses to electrical impact in seismic noise could only be recorded by the CME-6111 seismometer, located near the sounding source. This research determined the direction for improving electric current sources, increasing their power for shallow and deep sounding of the earth’s crust.

In 2024, the Institute of Marine Geology and Geophysics of the Far Eastern Branch of the Russian Academy of Sciences started continuous measurements of the vertical component of the electrotelluric field (ETF), which were integrated into the previously organized monitoring of the horizontal components of the ETF. The vertical component of the ETF is measured by the potential difference between metal plates located in the ground, one above the other, at a depth of about 2 m. Two systems of plates were installed at the Yuzhno-Sakhalinsk geophysical test site (on the territory of IMGG FEB RAS) and were connected to an analog-to-digital converter (ADC). The aim of the study is to identify and analyze variations in the vertical ETF that may be associated with the geological deformation and variations in seismicity. In the first year of measurements of the vertical component of the ETF, 4 periods with an abnormally high level of field strength were identified compared to the average value for the observation period: maximum by 35 times, minimum by 5 times. In total, these periods occupy about half of the entire registration time; it was during these periods that 80% of all earthquakes in the southern part of Sakhalin Island occurred over 11 months of 2024 (24 events out of 30). For the horizontal components of the ETF, no such significant anomalies were observed as for the vertical one. Obtained results indicate that changes in the vertical component of the ETF may reflect variations in seismic activity in the near field. These changes can presumably be considered a sign of earthquake activity near the measurement point.

В работе представлены результаты анализа данных электротеллурических потенциалов, полученных в пункте измерений, развернутом в июне 2023 года на территории ИМГиГ ДВО РАН в г. Южно-Сахалинск. Уже в первые месяцы работы были обнаружены серии квазипериодических всплесков (импульсов) в ночное время. Сигналы длительностью 4-5 секунд и с периодом следования 130-150 секунд имеют разную форму, которая является производной от некоего оригинального синусоподобного сигнала. Серии отмечались с 20 июля по 11 сентября 2023 г., а их средняя длительность колеблется в районе 8-9 часов. С 12 сентября 2023 года по 10 февраля 2024 года серии не обнаружены. Максимальная интенсивность сигналов и серий в целом наблюдается в период с 5 по 10 августа. В этот же период (9 августа 2023 года) рядом с точкой измерений (в круге с радиусом 0,25 градуса с центром на полигоне) произошло землетрясение умеренной силы с M = 3,8, причём за весь период наблюдений оно было самым сильным из 2 событий с M > 3, произошедших в данном районе. Перед вторым, более слабым землетрясением, подобных серий совсем не было обнаружено. Появление обнаруженных серий квазипериодических импульсов может быть связано с подготовкой очага землетрясения, однако для подтверждения этой гипотезы потребуются дополнительные наблюдения The work represents the results of analysis of electro-telluric potentials data obtained at the Yuzhno-Sakhalinsk test site (deployed in June 2023on the territory of the IMGG FEB RAS). Unexpectedly, a new kind of signals – series of quasiperiodic spikes (pulses) in night times were found in first few months after start of recording. Signals of 4-5 s length and of 130-150 s repetition period have a various waveform, which is derived from some primary quasi-sinusoidal signal. Such series were recorded from July 20 to September 11, 2023, and their average duration was nearly 8-9 hours. No episodes were found after September 12, up to December 20. The maximal intensity of the signals and the series as a whole was revealed in the period from 5 to 10 August. During this period the moderate earthquake M=3.8 occurred on 9 August, 2003 in the vicinity of measurement point (within a circular zone of 0.25 degrees radius around the test site) It was the strongest event from pair of that occurred in the given zone, the magnitudes were being M = 3.8 (08/09/2023) and M = 3.1 (09/19/2023). No similar series were observed before the second earthquake, being the weaker. Origination of quasiperiodic pulses series could be related to the preparedness of earthquake source – site. However extra surveys are required to proof this hypothesis.

The paper considers topical issues of geophysics related to the possible influence of disturbances in the magnetosphere on seismicity. The study was conducted based on a detailed catalog of seismic events in southern Sakhalin for the period from 2003 to 2023. The paper aims to test the assumption that such an influence can manifest itself in individual seismogenic zones during their proximity to discharge. The testing was carried out in randomly selected segments of the West Sakhalin (WSF) and Central Sakhalin (CSF) faults. The coincidence of the moments of some seismic events (with M > 2.7) and magnetic storms with a high index (G1 and higher) was revealed in these segments. The LURR (load-unload response ratio) method was used to identify periods when fault segments were in a subcritical stress-strain state. It was shown that the main part of the coincidences occurred during the periods of increased abnormal activity of the LURR parameter.

The results of the analysis of changes in electrotelluric potentials (ETP) during the observation of intense solar flare events and intense magnetic storms on Sakhalin are presented. The data were studied in the period from July 20 to October 12, 2023. The absence of characteristic changes in the ETP (integral amplification or attenuation of noise in the low-frequency region) depending on the presence or absence of a solar flare event is shown. At the same time, in some cases, the strongest flashes were found to coincide with the appearance of signals of the GUV type (Geyser type ULF Variation). For almost three months of observations, five cases of quasi-periodic GUV series have been identified, four of which coincide completely or partially with the times of solar flares and magnetic storms. It should be noted that earlier in the literature, the appearance of these signals was not correlated with any physical process. At the same time, the identification of such patterns is an integral part of extensive work on identifying predictive signs of earthquake preparation in the ETP.

The results of experiments on electrical sounding of the near-surface layer of the Earth's crust in the fault zone, which have involved a recording of seismoacoustic and seismic noise in the close zone near the source (the primary dipole source), are represented. The experiments were carried out in 2021-2022 in the southern part of the Central Sakhalin fault with the use of the generator of electric pulses developed at IMGG FEB RAS, output electric power being up to 3 kW. The aim was to reveal seismoacoustic signatures of the medium reaction to the soundings with current pulses of 5–13 A. The generator provided significantly higher current in the dipole than its typical characteristics in the case of soundings for electrical exploration by resistance methods, as well as in the case of conventional seismic and electrical exploration. At the same time, the range of current amplitudes was much smaller in comparison with the case of a deep sounding based on application of geophysical MHD generators or other extra high-power electric pulses units. Up to now, the inverse seismoelectric effect has remained practically unexplored at currents in the “intermediate” range of ~10 A and scale lengths of the order of few hundreds of meters. The presence or absence of the medium reaction to electrical soundings was distinguished by the records of molecular-electronic sensors developed by R-sensors LLC: the CME-6111 broadband seismometer and the hydrophone, installed at a distance of about 50 m from one of the poles of the electric dipole source. An increase in the average level of seismoacoustic noise during electrical soundings was revealed, which is essentially a variety of the inverse seismoelectric effect of the second kind (excitation of elastic waves during an electric current run in a two-phase medium). Previously, no similar signature of medium reaction to the current pulses was noted in the close zone adjacent to one of the dipole electrodes. The noise level increase occurs almost without delay after the start of electrical soundings, and this is in accordance with the previously obtained results on the responses of seismic acoustic emission to powerful current pulses, which were used for a deep sounding in the Northern Tien Shan.