Moment tensor catalogue of microearthquakes in West Bohemia from 2008 to 2018

We present a unique catalogue of full moment tensors (MTs) of microearthquakes that occurred in West Bohemia, Czech Republic, in the period from 2008 to 2018. The catalogue is exceptional in several aspects: (1) it represents an extraordinary extensive dataset of more than 5.000 MTs, (2) it covers a long period of seismicity in the studied area, during which several prominent earthquake swarms took place, (3) the locations and retrieved MTs of microearthquakes are of a high accuracy. Additionally, we provide three-component records at the West Bohemia (WEBNET) seismic stations, the velocity model in the region, and the technical specification of the stations. The dataset is ideal for being utilized by a large community of researchers for various seismological purposes, e.g., for studies of (1) the migration of foci and the spatiotemporal evolution of seismicity, (2) redistribution of stress during periods of intense seismicity, (3) the interaction of faults, (4) the Coulomb stress along the faults and local stress anomalies connected to fault irregularities, (5) diffusivity of fluids along the activated faults, or (6) the time-dependent seismic risk due to the migration of seismicity in the region. In addition, the dataset is optimum for developing and testing new inversions for MTs and for tectonic stress. Since most of the earthquakes are non-shear, the dataset can contribute to studies of non-double-couple components of MTs and their relation to shear-tensile fracturing and/or seismic anisotropy in the focal zone.

In-plane and out-of-plane shear fracture toughness of rocks

<p><span>Rocks in the subsurface are exposed to high amount of confinement which can potentially suppress the formation or the development of tensile-based cracks and thus, give rise to shear-based fracture growth. However, measuring the shear fracture toughness of rocks have been studied less in the literature, as providing the required confinement to force the shear fracturing precede tensile fracturing is not an easy task. In the current study, two new tests namely the double-edge notched Brazilian disk (DNBD) and the axially double-edge notched Brazilian disk (ANBD) are proposed to measure the in-plane (true mode II) and the out-of-plane (true mode III) shear fracture toughness of rocks, </span><span>K</span><sub><span>IIc </span></sub><span>and </span><span>K</span><sub><span>IIIc</span></sub><span>, respectively. We use the term </span><span>true </span><span>to emphasis that not only sustains the crack shear loading, but also the type of fracturing is shear-based. Finite element method is used to study the variations of stress field around the crack tip in these tests and to prove the applicability of the tests in providing mode II and mode III loading conditions. It is argued that both tests are straightforward and have several advantages compared to the existing ones. The effectiveness of the tests is empirically corroborated by conducting some experiments on Bedretto Granite. The pulverized surface of fracture in both the tests denotes the existence of friction which indicate the shear-based nature of fracture. Finally, the measured values of </span><span>K</span><sub><span>IIc </span></sub><span>and </span><span>K</span><sub><span>IIIc </span></sub><span>for Bedretto granite are compared to each other and to the reported values of </span><span>K</span><sub><span>Ic </span></sub><span>in the literature. It is shown that </span><span>K</span><span>IIc </span><span>and </span><span>K</span><span>IIIc </span><span>values are close to each other while both are more than two times greater than </span><span>K</span><span>Ic</span><span>.</span></p>

Tensile and Shear Mechanical Characteristics of Longmaxi Shale Laminae Dependent on the Mineral Composition and Morphology

Laminae are well developed in shale and generally influence fracture propagation during hydraulic fracturing. Hence, comprehensively understanding the tension and shear behaviors of shale laminae is crucial. There have been limited systematic studies thus far on the tensile and shear strength as well as fracture morphology of shale laminae. In this study, the Lower Silurian Longmaxi Shale (China) was investigated via Brazilian tensile and angle-varied plate shear tests. Five lamina types were tested, i.e., calcite (Cal), pyrite (Py), organic-enriched (Oc), the interface between Cal and Oc (Cal-Oc), and the interface between Py and Oc (Py-Oc) laminae. Results showed that the tensile strength was in the range 0.43–8.22 MPa, mainly in the order of Cal > Py > Cal-Oc > Py-Oc > Oc. The modes of fracture morphology were highly related to the occurrence, continuity, and mineralogy fillings of laminae. Shear strength parameters were within the range 22.50–29.64 MPa for cohesion and 37.29–43.60° for internal friction angle. Fracture surface roughness was strongly related to its cohesion. Calcite laminae considerably influenced the tensile fracturing of shale, suggesting that the geometry and properties of calcite lamina should receive more attention during the design of shale gas exploration.

Damage indicators and failure prediction in Focal Mechanism solutions

<p>Acoustic Emissions (AE), the laboratory analogue to seismic events, recorded during conventional triaxial deformation tests allow for an unprecedented amount of information on the evolution of fractured media within a controlled environment. This study presents the results of a new and robust derivation of first motion polarity focal mechanism solutions (FMS). 4 x 10 cm cylindrical samples of Alzo Granite (AG) and Darley Dale Sandstone (DDS) underwent systematic triaxial deformation testing (5, 10, 20 and 40 MPa) in order to investigate the relationships between increasing confining pressure, deformation and failure mode and role of pre-existing microstructure. With an average of 11 of 12 waveforms picked using a neural network for each AE, high resolution datasets are obtained that can track the evolution of deformation structure through time. Focal mechanisms are solved using a least squares minimisation of the fit between projected polarity measurements and the deviatoric stress field induced by tensile, shearing and collapse/closing type sources. Results reveal a surprisingly limited dependency on the distribution of shear fracturing in the lead up to dynamic failure. Instead, deformation is driven by the competition between the opening and closure of fractures that is strongly related to the coupling of local stress fields with pre-existing damage.Spatio-temporal trends in mechanism type and AE amplitude allow for clear identification of: a) Fracture Enucleation. This phase is characterised by broadly distributed tensile fracturing that becomes preferentially aligned as confining pressure increases; b) Fracture Growth. The onset is characterized by a discrete increase in low amplitude shearing events and cyclic fracture development that evolves from a dominance of collapse to shearing followed by tensile fracturing which then returns to collapse type. Influences in mechanism dominance due to rock type are highlighted by increased tensile fracturing in AG, which is replaced by shearing in DDS. A reduction in low amplitude tensile events at 10 MPa in both rock types further reveals a switch from axial splitting to planar localisation as confinement increases; c) Crack Coalescence. The cyclic fracture growth prior to dynamic failure and the amount of strain of this phase share a positive log-linear relationship with confining pressure, allowing to identify the potential for real-time failure prediction; d) Dynamic Failure: High amplitude events characterize the propagation of fractures. Taken together results highlight that failure of the studied samples is the result of the complex interaction between distinct regions of dilatant and compactant deformation. Although planar localisation and preferentially aligned flaws play a more significant role at higher confining pressures, it is the initial heterogeneity or patchiness of the regions undergoing damage that control dynamic failure occurrence and the eventual fracture plane features.</p>