TO STUDY THE POSSIBILITIES OF USING THE SEISMOLOGICAL NETWORK OF THE ALTAI-SAYAN REGION FOR REGIME VIBRO-SEISMIC OBSERVATIONS

The results of experiments on the study of the radiation characteristics of powerful vibrators and reception conditions at a number of seismic stations in the Altai-Sayan region in the range of distances of 50-450 kilometers in the problem of selecting points for active vibro-seismic monitoring are presented. It is shown that the real effective range of the CV-40 vibrator can be significantly increased by using multiple and semi-multiple harmonics. A method for assessing the applicability of stations for vibration monitoring using special programs for assessing the seismic background and seismic resonances within the boundaries of the location of seismic stations is proposed and tested.

TO THE PROBLEM OF DEVELOPING NEW METHODS FOR ESTIMATING THE MECHANISM OF FORMATION OF FOCAL ZONES OF CATASTROPHIC EVENTS IN CARBON BEDS

The article shows that the problem of safe mining is largely related to ideas about the mechanisms of interaction of nonlinear geomechanical and physicochemical mass-gas exchange processes in multiphase coal-rock massifs. For the first time, a complex of generalizing analytical, experimental laboratory and field studies at the regional level, carried out by the authors, made it possible to establish that in modern conditions, with the development of deep horizons, the role of rock pressure, temperature and geostructural factors, as well as the presence of an increasing influence of the seismic background from earthquakes and technological explosions. A numerical simulation method and an original experimental system for determining the regularity of the decay of this two-phase flow were developed by Chinese and Russian specialists taking into account the influence of the gas expansion energy on the propagation characteristics of a mixture of pulverized coal and gas. The analytical basis for describing the established deterministic relationships between the existing nonlinear geomechanical and physicochemical processes in stressed coal-bearing massifs was the establishment of an operator correspondence between the physicochemical equation of I. Langmuir and the kinematic equation of V.N. Oparin for pendulum waves.

Permanent, seasonal, and episodic seismic sources around Vatnajökull, Iceland from the analyses of correlograms

<p>This study aims at characterizing different seismic sources in the region of the Vatnajökull glacier using seismic interferometry. Vatnajökull is the largest Icelandic icecap, covering 4active volcanic systems. The seismic context is therefore very complex with glacial and volcanic events occurring simultaneously and a classification between the two can become cumbersome. </p><p>We used seismic interferometry or cross-correlation of seismic noise on seismic data from 2011 to 2019). Being based on continuous records, this passive monitoring method is not relying on earthquakes to locate seismic sources. We computed the cross-correlation functions between every pair of seismic stations using MSNoise for different frequency bands, from 0.5 to 8 Hz. The first step towards the location of seismic sources was to calculate the propagation velocities for each frequency range. The total range of velocities is between 1.39 km/s and 3.92 km/s. Then, we used two different location methods based on the calculated propagation velocities. The first method is based on hyperbole’s geometry and provides the location of seismic sources as the intersection between several hyperboles, while the second one, the Ballmer’s method (Ballmer et al. 2013), is based on the calculation of theoretical differential times and provides location probabilities for the seismic sources. We located and characterized persistent oceanic seismic noise located along the southern shoreline of Iceland potentially associated with waves activity and geometry of the shore, as well as a seasonal glacial tremor around outlet glaciers in the west part of the Vatnajökull icecap, potentially linked to glacial processes inside the glacier or in the glacial rivers. The uncertainty of a few kilometers is observed. Some limitations exist for these methods. For example, The Ballmer’s method (Ballmer et al. 2013) is reliable for seismic sources inside the seismic network but can only give an azimuthal direction for seismic sources located outside of it. When using hyperboles, slightly different propagation velocities between pairs of stations can affect the precision of the intersection. Therefore, the association of the two methods is important to diminish the impact of these limitations. </p><p>These results provide a better understanding of the seismic background of this region and will be compared and validated with other localization methods in the future.</p>

The Finnish National Seismic Network: Toward Fully Automated Analysis of Low-Magnitude Seismic Events

We present an overview of the seismic networks, products, and services in Finland, northern Europe, and the challenges and opportunities associated with the unique combination of prevailing crystalline bedrock, low natural intraplate seismic background activity, and a high level of anthropogenic seismicity. We introduce national and local seismic networks, explain the databases, analysis tools, and data management concepts, outline the Finnish macroseismic service, and showcase data from the 2017 M 3.3 Liminka earthquake in Ostrobothnia, Finland.

Seismic Monitoring in Gujarat, India, during 2020 Coronavirus Lockdown and Lessons Learned

The Gujarat region, situated in the westernmost part of India, experienced a deadly intraplate 2001 Mw 7.6 Bhuj earthquake. In the aftermath of the disaster, the Institute of Seismological Research established the Gujarat (India) seismic network in 2006. The network is being operated in online and offline modes, whereas, seismicity monitoring is being done in near-real-time, using data received from the online seismic stations. The Coronavirus disease-19 lockdown provided an opportunity to assess the network reliability in a difficult and challenging scenario. The positive aspect of the lockdown is reflected in signal-to-noise ratio, which improved significantly at all the sites during the lockdown, with more prominent being at sites located on top of the Quaternary sediments due to the absence of high-frequency anthropogenic noise. A sharp fall in the seismic background noise is noticed at most of the stations during the lockdown period, with respect to the prelockdown period. We used the lockdown data to identify other natural sources of noise, besides anthropogenic. The lockdown helped in solving the enigma of seismicity in certain pockets, which turned out to be related to quarry blasts.

Influence of water content on seismic activity of rocks mass in apatite mining in Khibiny

The authors analyze connection between the rock mass water content due to surface water (precipitation, snow melting and water bodies) and rock bursts in mines. The seismological monitoring performed from 2001 at the surroundings of the Apatite Circus and Rasvumchorr Plateau apatite deposits identified fluctuation of the general seismic background from year to year and seasonably. For these deposits, increased water content of rocks is seen as a natural trigger for rock mass seismicity during intensive rains and snow melting. The 15 year-long seismic monitoring of the apatite deposits under study has revealed regularity in a seasonal growth of seismicity during snow melting and precipitations, mainly in spring months. Particularities of geodynamical event occurrence allow a conclusion on the ultimately stress state of the upper earth’s crust in whole in the deposits’ vicinities. Reactivation of tectonic disturbances and weakened rock mass zones occurring due to relief of normal compression during infiltration can be a potential principal mechanism of seismic activation of the rock mass during the increased water content periods. The origin of the events observed is linked with the dynamic release of ultimate rock mass stresses when geomechanical conditions change under the action of hydrostatic pressure on tectonically disturbed zones. The authors make a conclusion on necessity to apply the revealed regularities of impact of natural factors on rockburst hazard when planning mining operations and performing geomechanical monitoring.

Mapping Seismic Tonal Noise in the Contiguous United States

A subset of seismic signals generated in industrial environments displays spectral peaks organized in sequences of fundamental frequencies with multiple overtones, which we refer to as tonal noise (TN). Using one year of data from each of 1732 stations in the USArray Transportable Array, we detected around 1.5 million TN sequences in the contiguous United States, which corresponds (on average) to around 2.4 detections per day (869 detections per year) at each station. TN across the continent is clustered around specific regions and frequencies. The majority (>70%) of stations in the 90th percentile of total detection numbers (more than 2100 detections per year) are concentrated in the Interior Plains, Canadian Shield, and Appalachian Highlands. We found that the fundamental frequencies of all TN detections are concentrated in six spectral bands with value ranges of 0.9–0.95, 1.8–1.85, 2.5–2.55, 3.3–3.35, 5–5.05, and 5.45–5.5 Hz with around 104, 37, 46, 37, 62, and 45 thousand detections, respectively. Detections in these bands account for around 22% of all detections. We suggest that large regions with similar TN are related to noise from industrial activities driven by physiographic characteristics such as favorable winds or abundance of water (wind and hydroelectric power generators). The presence of TN and other spectrally discrete components in the seismic wavefield is a ubiquitous feature in the seismic background. This type of noise has the potential to affect subsurface imaging efforts by introducing potentially static and continuous sources of noise. The effects of TN can be especially significant as near-surface imaging studies move toward utilizing higher frequency (>1 Hz) for ambient seismic noise.

Characterization of seasonal glacial seismicity from a single-station on-ice record at Holtedahlfonna, Svalbard

ABSTRACTGlacial seismicity provides important insights into glacier dynamic processes. We study the temporal distribution of cryogenic seismic signals (icequakes) at Holtedahlfonna, Svalbard, between April and August 2016 using a single three-component sensor. We investigate sources of observed icequakes using polarization analysis and waveform modeling. Processes responsible for five icequake categories are suggested, incorporating observations of previous studies into our interpretation. We infer that the most dominant icequake type is generated by surface crevasse opening through hydrofracturing. Secondly, bursts of high-frequency signals are presumably caused by repeated near-surface crevassing due to high strain rates during glacier fast-flow episodes. Furthermore, signals related to resonance in water-filled cracks, fracturing or settling events in dry firn or snow before the melt season, and processes at the glacier bed are observed. Amplitude of seismic background noise is clearly related to glacier runoff. We process ambient seismic noise to invert horizontal-to-vertical spectral ratios for a sub-surface seismic velocity model used to model icequake signals. Our study shows that a single seismic sensor provides useful information about seasonal ice dynamics in case deployment of a network is not feasible.