Laboratory loading experiments, in general, are used to estimate the mechanical properties of rocks. The application of acoustic emission (AE), a proxy for very small-scale earthquakes, offers insights into rock fracturing, especially when advanced seismological methods are applied. The uniaxial compressive test, the most conventional test within the scope of laboratory rock mechanics, estimates strength, elastic constants, or microcracking thresholds. Obviously, rock structure and grain size heterogeneity are the crucial parameters controlling these mechanical properties. Here, four granites with a different structure were uniaxially loaded, while AE was monitored, in order to observe the manner of fracturing. AE data were utilized at three different quality levels (AE recognition; AE localization; and the AE source mechanism). However, our focus was to study AE source mechanisms via the application of a shear-tensile crack source model. During a uniaxial compressive test, common (structurally independent) AE features led to the general characterization of microfracturing. On the other hand, sample related differences in AE results are discussed with respect to the role of granite heterogeneity. The comprehensiveness of the AE application demonstrated the validity of various levels of AE experiments.

The focal mechanism is a parameter of an earthquake that typically demands observations surrounding the focus. If such observations are not available, only limited information can be retrieved regarding the process of rupturing (e.g., geometry, and the orientation of the fault plane and slip), not its shear versus non-shear nature. In extreme cases of monitoring that incorporate a single station, a mechanism can only be estimated if data from additional seismic phases are available. However, such is not the case for weak seismic events, in which a station often only records direct P and S waves. Under such conditions, information is severely limited, and standard synthetics-to-data-matching does not allow even the simplest and most constrained seismological source model, namely a double-couple focal mechanism, to be determined. During data review, we detected an internal ambiguity for solutions of the inverse task using single-station geometry. Nevertheless, we found that at least some important features of a focal mechanism may be retrieved. Such scenarios occur for particular configurations of focal mechanism orientation and source-station direction, as follows: (1) If a station lies in a direction within the quadrant of compressions, the T axis can be retrieved with reasonable error. However, the P axis remains ambiguous. (2) Conversely, if a station lies in a direction within the quadrant of dilatations, reasonable resolution of the P axis is possible, while the T axis remains ambiguous. (3) If the data are noisy, apart from extreme cases of contamination, the possibility of estimating the direction of the T/P axis remains intact. The closer source-station direction is to the center of the quadrant of compressions or dilatations, the better the resolution of the T/P axis. Our method was applied to selected weak earthquakes recorded by the three-component borehole seismic station MDBI, located near the Dead Sea in Israel.

<p><span lang="en-US">The seismic moment tensor (MT) is a universal tool for describing earthquake source mechanisms which enables a description of both a shear-slip and an opening or closing of a fracture. A linear relationship between data, which are the displacement amplitudes of the direct P- and SH waves, and the source parameters makes the MT the most favorable description of the source mechanism a priori. Observation geometry plays a key role in resolution of the mechanism. For success, observations should be available from many different directions surrounding the focus. This is the case of intense earthquake-swarm activity on the Reykjanes Peninsula in 2020 to 2021 preceding the Fagradalsfjall volcanic eruption (March 2021), as a local network REYKJANET complemented the network of </span><span lang="en-GB">the </span><span lang="en-GB">University of Cambridge</span><span lang="en-US"> and by the </span><span lang="en-US">n</span><span lang="en-US">ear</span><span lang="en-US"> stations of the regional SIL network covered well the whole area of the peninsula; a total of 33 stations are used. For a sufficient resolution of the source mechanism, availability of accurate enough response of the medium – the Green’s function (GF) – is demanded. For this purpose we used </span><span lang="en-GB">a 1-D</span><span lang="en-US"> velocity model </span><span lang="en-GB">derived by Vogfjörd et al.(2002) for the Reykjanes</span><span lang="en-US"> Peninsula. For the GF construction, we applied the ray theory (the ANRAY package), where especially in the task of two-point ray-tracing a smoothness of the velocity model is essential; hence the </span><span lang="en-GB">Vogfjörd et al. (2002) model</span><span lang="en-US"> was smoothed. Apart from the smoothness, we paid attention to the topmost P-wave velocity to match the synthetic incidence angle and the observed one. In the result, the topmost velocity was modified (both up and down) for most of the stations used. Substantial effort has been devoted to facilitate a batch processing of the events, first of all it means to allow the ANRAY to work in a semi-automatic regime, which is not its inherent mode of work. As a result, we have a comprehensive tool for retrieval of moment tensors on the Reykjanes Peninsula, particularly in the in 2020/2021 earthquake-swarm activity. We demonstrate its usefulness by treating a set of several tens of events. Source mechanisms of some of them are compared to those obtained by ISOLA, a standard software for processing of regional earthquakes, which however applies different data and different approach</span><span lang="en-GB">. Our set of MTs retrieved provides a deeper insight into the mechanism of faulting due to dyke propagation </span><span lang="en-US">in the region in question from the beginning of 2020 to March of 2021.</span></p>

Mining, water-reservoir impoundment, underground gas storage, geothermal energy exploitation and hydrocarbon extraction have the potential to cause rock deformation and earthquakes, which may be hazardous for people, infrastructure and the environment. Restricted access to data constitutes a barrier to assessing and mitigating the associated hazards. Thematic Core Service Anthropogenic Hazards (TCS AH) of the European Plate Observing System (EPOS) provides a novel e-research infrastructure. The core of this infrastructure, the IS-EPOS Platform (tcs.ah-epos.eu) connected to international data storage nodes offers open access to large grouped datasets (here termed episodes), comprising geoscientific and associated data from industrial activity along with a large set of embedded applications for their efficient data processing, analysis and visualization. The novel team-working features of the IS-EPOS Platform facilitate collaborative and interdisciplinary scientific research, public understanding of science, citizen science applications, knowledge dissemination, data-informed policy-making and the teaching of anthropogenic hazards related to georesource exploitation. TCS AH is one of 10 thematic core services forming EPOS, a solid earth science European Research Infrastructure Consortium (ERIC) (www.epos-ip.org.).

During inversion for the source mechanisms of laboratory acoustic emission events, relatively high misfit values (expressed as the RMS of the inverted equations) were observed. Our experiment was performed on Westerly Granite. A processed set of data consisting of 2405 acoustic emission events was used and a semi-homogeneous velocity model was considered. A correction for sensor radiation patterns and individual sensor constants was taken into account, and an acausal attenuation model was assumed. Source mechanisms were inverted for the moment tensor. The application of a more sophisticated medium model improved inversion quality only for some events. Introducing the source directivity, a standard approach for earthquakes with magnitudes larger than approximately 4, increased the number of successfully inverted events. Directivity was introduced using a Haskell source model; optionally unilateral and bilateral versions of the source were considered. Lower values of RMS for the Haskell source model were considered to justify the directivity approach. This formalism enables us to select the preferable fault from the two nodal planes within the fault plane solution. The rupture directions were observed to tend to the dip direction of the preferred fault. They were found to be preferably subparallel to slip directions for acoustic emissions with a significant DC component. The source time functions retrieved from the seismograms are in agreement with finite source theory.

Traditionally in seismology and in acoustic emission (AE), full moment tensor (MT) is applied as a default model of the mechanism. We present an alternative - an application of shear tensile crack (STC) source model to AEs generated by uniaxial compression loading of Westerly granite. The advantages of STC over the conventional MT are as follows: (i) contrary to the MT, the STC is physical source since it describes straight and simple fracture modes anticipated inside a loaded sample, namely the shear-slip and both of opening and closing tensile cracks; and (ii) the STC is simpler because it is described by fewer parameters (five instead of six required for an unconstrained MT), an essential feature for stabilizing the inverse problem.Better suitability of STC over MT is demonstrated by three exemplary AEs (tensile, shear, and combined). The obtained results were confirmed using a statistical analysis of 1630 reliably determined source mechanisms. The STC, as compared to the MT, provides smaller confidence regions for orientation and even smaller regions for decomposition parameters. Thus, the STC solution appeared to be substantially more useful than the MT, namely for mechanisms with a high content of non-double-couple (non-DC) component, as it allowed better distinction between tensile and shear AEs.Grain scale cracks of tension and combined source type, localized within the middle-height circumferential portion of the specimen, dominated fracturing. Azimuthal distribution of fault planes was found to be approximately uniform for all three source types. The fault dip increased with increasing content of the non-DC component. The average values were 16°, 21°, and 26° for the tensile, combined and shear source types, respectively. The specimen failed by flaking in areas of a high AE activity. AE locations and failure mechanisms indicate a perfect confinement end-boundary conditions between the tested specimen and the loading platens.

The laboratory approach brings a significant simplification compared to the actual conditions in situ. Laboratory experiments represent the only chance to control the physical conditions under which the investigated physical phenomena occur. Acoustic emission (AE) is the process accompanying the brittle fracturing of solid body and simultaneously an indispensable tool for its study. Laboratory experiments under controlled loading conditions make it possible to differentiate the effect of important factors like material structure, stress field, crack occurrence, etc. on fracture initiation and development, and allow simulate the nature in situ. Microearthquakes detected during AE can be analyzed by methods developed in earthquake seismology. We apply the shear-tensile crack (STC) to describe the source mechanism of the AE events with the aim to detect the mode of rock fracturing, in particular to distinguish between a shear slip and tensile crack, the latter both in the phase of its opening and closing. The benefit of discerning between shear and tensile fracturing is an insight into changes of the permeability of the rock massif both in space and time. By contrast to natural seismology, tens of thousands AE events occur in laboratory during the experiment. Expecting to process large volumes of data, an urgent demand was to make the non-linear STC search together with the estimate of the errors involved as fast as possible. To assess the reliability of the STC solution, the confidence regions of source model parameters are constructed. The misfit function is converted into the probability density function which is integrated over a trial volume of low misfit until requested probability content is achieved. For individual microearthquakes, we display confidence regions both for the mechanism orientation and its decomposition. Aiming to process a large bulk of AE data, a method of assessing of these zones needs to be proposed, which describes them by estimates of their extreme size. This allows us select for subsequent interpretation from all solutions only those that are stable and reliable. We have applied this approach to the experimental data obtained from a couple of uniaxial loading tests performed on a Westerly Granite and Liberec Granite specimen using a 14 channel AE monitoring system.

In recent decades, the earthquake mechanism, regardless of scale, has commonly come to be described by the moment tensor (MT).

Here, we present an experimental study of the fracturing of anisotropic migmatite with plane-parallel structure (foliation). Four specimens, with different dips of foliation, including subhorizontal (13°), subvertical (86°), and oblique (47°, 67°), were uniaxially loaded up to failure. Acoustic emission monitoring and ultrasonic sounding were applied for fracturing characterization. In case of subhorizontal and subvertical foliation, the tensile source type played an important role before reaching nucleation stress when shearing became dominant. The dominance of non-tension microcracking was characteristic for oblique foliation. The uniaxial loading of migmatite resulted in triaxial stress state regardless of the foliation dip. The minimum stress axis was in the subhorizontal direction of the foliation dip. The triaxial stress state caused the preferential orientation of induced shear and tension microcracks that had the same azimuthal orientation as the foliation. Preferential microcracking resulted in preferential orientation of the failure plane. Based on the anisotropic behavior of migmatite’s characteristic mechanical properties, as well as its mechanism of failure that is typical for anisotropic rocks, the obtained conclusions may be generalized for other types of metamorphic anisotropic rocks with a plane-parallel structure.

Here a shear‐tensile crack (STC) model is presented as suitable for acoustic emission (AE) events. Experimental data were obtained from a uniaxial compression test performed on a Westerly Granite specimen using a 14‐channel AE monitoring system. The advantages of the STC versus a traditional MT (moment tensor) approach are as follows: (i) it is a physical source, contrary to the MT, since the STC describes the straight and simple fracture modes anticipated inside a loaded sample, namely, the shear slip and both the opening and closing tensile cracks; and (ii) it is simpler because it is described by fewer parameters (five instead of the six required for an unconstrained MT), which is essential for solving the inverse problem. The presented STC procedure was tested on 38 AE events selected over a range of 50–98% for the uniaxial compressive strength. As compared to the MT model, the STC model displayed a similar fit for input data while providing far smaller confidence regions. The results indicate a more certain determination for the mechanism of orientation and improved reliability for the decomposition components. In addition, use of STC model allowed better distinction between tension and shear type for AE events, which may be crucial for recognizing an approaching failure. For our experiment, application of the STC model proved to be useful for recognizing the threshold of unstable microcracking and indicative for determining the failure plain orientation.

This paper deals with the seismic stability of the survey areas of potential sites for the deep geological repository of the spent nuclear fuel in the Czech Republic. The basic source of data for historical earthquakes up to 1990 was the seismic website [10]. The most intense earthquake described occurred on September 15, 1590 in theNiederroesterreich region (Austria) in the historical period; its reported intensity is Io = 8-9. The source of the contemporary seismic data for the period since 1991 to the end of 2014 was the website [11]. It may be stated based on the databases and literature review that in the period from 1900, no earthquake exceeding magnitude 5.1 originated in the territory of the Czech Republic.In order to evaluate seismicity and to assess the impact of seismic effects at depths of hypothetical deep geological repository for the next time period, the neo-deterministic method was selected as an extension of the probabilistic method. Each one out of the seven survey areas were assessed by the neo-deterministic evaluation of the seismic wave-field excited by selected individual events and determining the maximum loading.Results of seismological databases studies and neo-deterministic analysis of Cihadlo locality are presented.

Shear-tensile crack is a model for an earthquake mechanism that is more constrained than the moment tensor but that can still describe a non-shear focus. As such, the shear-tensile crack model is more robust than the moment tensor model and yields more reliable estimates for the earthquake mechanism. Such an advantage verifies the credibility of the non-double-couple component found for some events of the 1997 West Bohemia-Vogtland earthquake swarm. As expected, in several cases, a significantly resolved non-double-couple component was obtained where the moment tensor approach failed. Additionally, for non-shear sources, the shear-tensile crack model offers optimization of the Poisson number within the focus, concurrently with retrieval of the mechanism. However, results obtained for the joint inversion of the 1997 swarm indicate that resolution is low. A series of synthetic experiments indicated that limited observations during 1997 were not the cause. Rather, hypothetical experiments of both very good and extremely poor network configurations similarly yielded a low resolution for the Poisson number. Applying this method to data for recent swarms is irrelevant because the small non-double-couple components detected within the inversion are spurious and, thus, the events are pure double-couple phenomena.

The resolution of a source mechanism is investigated in terms of three differently constrained source models: the moment tensor, the shear-tensile crack source model, and the double couple source model. The moment tensor (MT) is an unconstrained description of a general dipole source; the shear-tensile crack (STC) represents a slip along a fault with an off-plane component and the double couple (DC) corresponds to a simple shear slip along a fault. The inversion of body wave amplitudes is applied on microseismic events located in the vicinity of underground gas storage Háje (Czech Republic) where volume changes in the source can be expected. The orientation of the simple shear fracture component is resolved almost always well, independently of the source model used. On the other hand, the non-shear components differ largely among the source models considered, from both the model definition and robustness of the inversion. A comparison of the inversion results for the three alternative source models permits an assessment of the reliability of the non-shear components retrieved. Application of the STC model to all events appears to be the most appropriate. The analysis confirms a shear slip for three events and a tensile fracturing for other three events.

The resolution of event mechanism is investigated in terms of the unconstrained moment tensor (MT) source model and the shear-tensile crack (STC) source model representing a slip along the fault with an off-plane component. Data are simulated as recorded by the actual seismic array installed at Ocnele Mari (Romania), where sensors are placed in shallow boreholes. Noise is included as superimposed on synthetic data, and the analysis explores how the results are influenced (i) by data recorded by the complete seismic array compared to that provided by the subarray of surface sensors, (ii) by using three- or one-component sensors and (iii) by inverting P- and S-wave amplitudes versus P-wave amplitudes only. The orientation of the pure shear fracture component is resolved almost always well. On the other hand, the noise increase distorts the non-double-couple components (non-DC) of the MT unless a high-quality data set is available. The STC source model yields considerably less spurious non-shear fracture components. Incorporating recordings at deeper sensors in addition to those obtained from the surface ones allows for the processing of noisier data. Performance of the network equipped with three-component sensors is only slightly better than that with uniaxial sensors. Inverting both P- and S-wave amplitudes compared to the inversion of P-wave amplitudes only markedly improves the resolution of the orientation of the source mechanism. Comparison of the inversion results for the two alternative source models permits the assessment of the reliability of non-shear components retrieved. As example, the approach is investigated on three microseismic events occurred at Ocnele Mari, where both large and small non-DC components were found. The analysis confirms a tensile fracturing for two of these events, and a shear slip for the third.