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Abstract—This paper offers a general approach to the modelling of the wellbore stress state in the Bazhenov formation. In modelling, strata are presented as transversely isotropic objects with a set direction of the anisotropy axis that does not coincide with the vertical line. The paper also presents results of the laboratory Bazhenov formation core analysis. The analysis of core materials yielded correlation dependences for reconstructing stress-strain parameters of a transversely isotropic medium in selected wells. Parameters of the rock anisotropy were analyzed on the basis of standard and non-standard laboratory tests. The solution of the stress state problem in a deformation bounded medium was used to suggest an algorithm of calculating the components of the regional stress state along the well path. RN-SIGMA software tools were used to construct borehole stability models for wells penetrating anisotropic rocks. The effect of the anisotropy of elasticity on a safe drilling mud density window is demonstrated. The results of the calculations show that the anisotropy effects are crucial for calculating a safe drilling window. Examples are given to demonstrate potential effects of the anisotropy in image interpretation as well as in selecting horizontal wellbore drilling directions. [ABSTRACT FROM AUTHOR]

Seismic bursts in Southern California are sequences of small earthquakes strongly clustered in space and time and include seismic swarms and aftershock sequences. A readily observable property of these events, the radius of gyration (RG), allows us to connect the bursts to the temporal occurrence of the largest M ≥ 7 earthquakes in California since 1984. In the Southern California earthquake catalog, we identify hundreds of these potentially coherent space‐time structures in a region defined by a circle of radius 600 km around Los Angeles. We compute RG for each cluster then filter them to identify those bursts with large numbers of events closely clustered in space, which we call "compact" bursts. Our basic assumption is that these compact bursts reflect the dynamics associated with large earthquakes. Once we have filtered the burst catalog, we apply an exponential moving average to construct a time series for the Southern California region. We observe that the RG of these bursts systematically decreases prior to large earthquakes, in a process that we might term "radial localization." The RG then rapidly increases during an aftershock sequence, and a new cycle of "radial localization" then begins. These time series display cycles of recharge and discharge reminiscent of seismic stress accumulation and release in the elastic rebound process. The complex burst dynamics we observe are evidently a property of the region as a whole, rather than being associated with individual faults. This new method allows us to improve earthquake nowcasting, which is a technique to evaluate the current state of hazard in a seismically active region. Plain Language Summary: Earthquake nowcasting is a method to evaluate the current state of seismic hazard from large earthquakes. In this paper, we connect the temporal occurrence of the largest and most potentially destructive earthquakes in California since 1984 with a readily observable property of small earthquake seismicity in the region. Our method involves the calculation of the time history of the average radius (horizontal size or extent) of "bursts" of small earthquakes, in the time leading up to and following major earthquakes in the region. We observe that the radius systematically and gradually decreases leading up to major earthquakes, increasing suddenly and discontinuously following the event. This observable pattern resembles the long‐hypothesized cycle of regional tectonic stress buildup and release, or elastic rebound, associated with large destructive earthquakes. We propose that the radius of these bursts might be considered to be a proxy variable for the changing state of regional stress in Southern California. Key Points: A new nowcasting method using earthquake swarms and aftershocks reveals the hazard from large M ≥ 7 earthquakes in Southern CaliforniaMachine learning defines a time series of bursts using an exponential moving average filtered and optimized to reveal temporal structureCycles of recharge and discharge associated with M ≥ 7 earthquakes are revealed, reminiscent of stress accumulation and release [ABSTRACT FROM AUTHOR]

The Southern Patagonian Icefield (SPI) lies above an area of slow convergence between Antarctic and South-America plates, where limited seismicity is recorded by global and regional seismic networks. To understand the seismic behavior of this zone, we analyze two years of continuous broad-band data recorded by 27 seismometers, deployed around the SPI. Substantial ice loss coupled with the unusually low viscosity of the underlying mantle is causing a rapid uplift. Our findings indicate that most of the seismicity occurs in the upper crust , likely associated with the (re)activation of regional compressive structures. However, earthquakes immediately beneath the SPI generally are shallower and show normal or strike-slip faulting. We suggest that this activity is promoted as a response to the crustal relaxation after rapid ice removal of SPI. The almost complete absence of interplate and intraslab events is consistent with a locked megathrust fault interface, highlighting the similarity of this region with the Cascadia subduction zone. • Shallow seismicity is observed in the majority in the area of southern Patagonia Icefield. • The inferred stress suggests the solid earth response to mass loss from the icefield as the major cause. • The almost complete absence of interplate earthquakes suggests that the megathrust may be locked. [ABSTRACT FROM AUTHOR]

Magma transfer in the upper crust including emplacement at the surface of the Earth, have been regarded as largely controlled by the regional tectonic activity. While this relationship is well highlighted for systems like monogenetic, mostly basaltic, volcanic fields, the tectonic control on the transfer and emplacement of magmas in complex, polygenetic, volcanic systems, remains poorly documented in evolving tectonic settings. The Mont-Dore Plio-Quaternary volcanic complex (4.7 to 0.3 Ma) is one of such polygenetic volcanic fields as it is formed of three successive eruptive cycles: The caldera unit (3.3 to 2.2 Ma); The Aiguiller complex (2.5 to 1.5 Ma) and the Sancy stratovolcano unit (1.5 to 0.3 Ma). Analyzing fractures, fault kinematics and regional lineaments in the Variscan basement, together with attitude of dikes and volcanic centers alignments of the three volcanic units, we document the influence of the brittle structuration of the underlying basement on the volcanism distribution over time while regional stress changed. The late-Variscan N02E0, and secondary N160, striking crustal faults were reactivated since the Oligocene and channeled at depth the magma, while similarly oriented faults and fractures triggered the final storage in the shallow crust and spatial distribution of the volcanic vents. This fault system continued to act from the Miocene to the present-day uplift, associated with new N020E striking, flat-lying, fractures. An exception arises during the upper Miocene while N060E and N135E striking faults enabled the development of the caldera unit in a NW-SE extensional context. The emplacement of the three units from the mid-Pliocene to the end of the Pleistocene while a change in regional stress field conditions emerged at about 2.5 Ma exemplify the control of the brittle Variscan basement on the magmatic activity of the Mont-Dore Plio-Quaternary volcanic complex. • We studied the control of the regional tectonics on the emplacement of the Mont-Dore polygenetic volcanic complex (France). • We analyzed fractures kinematics and regional lineaments together with attitude of dikes and volcanic centers alignments. • We document the influence of the brittle structuration on the volcanism distribution over time while regional stress changed. • We highlight the relationship between reactivated brittle structures of the basement and the final emplacement of the magmas. [ABSTRACT FROM AUTHOR]

Eastern Ohio is an area of North America where a significant increase in seismicity rate was noted in the early 2010s. This increase has been associated with intensification of unconventional gas extraction performed in the Appalachian Basin and has been directly linked to two processes: hydraulic fracturing and disposal of the associated wastewater. In this paper, we review the recent seismicity in the Appalachian Basin including various episodes of induced seismicity that were temporally and spatially linked to operational activity, and we have performed some comparable analyses on the most recent sequences. The activities have not been as pervasive as other areas of North America, such that the cases are typically isolated and provide opportunities to study the seismogenic process in detail. The observed seismicity is concentrated in a narrow corridor that extends north-south in eastern Ohio and into central West Virginia, perhaps due to differences in operational targets and geologic variations. Ohio appears to have a higher prevalence of seismicity induced by wastewater disposal than surrounding states, but this is based on limited number of cases. Ohio also has an order of magnitude higher prevalence of seismicity induced by hydraulic fracturing than surrounding states, and prior work has suggested this is due to the targeting of the deeper Utica-Point Pleasant formation in Ohio that is closer to basement rocks than the Marcellus formation in West Virginia or Pennsylvania. In areas where hydraulic fracturing has induced seismicity, the percentage of stimulated wells that produce detectable seismicity is approximately 10-33%. Detailed studies of induced seismicity via double-difference relocation and focal mechanism analysis have revealed a series of linear fault segments, none of which correspond to previously mapped faults. Yet, the remarkable coherence in their orientation suggests these were preexisting, optimally oriented, and critically stressed. These fault orientations reveal a consistent regional stress field that only varies over a narrow azimuthal range from ~ 50° to 74°. The strongest observed seismic events in Ohio appear to occur in the Precambrian basement and indicate that these rocks have the maturity needed to produce M > 2 earthquakes and hence the greater potential hazard. [ABSTRACT FROM AUTHOR]

Abstract We present a geochemical and mineralogical study of the satellite scoria cones at Llaima volcano, Chile, providing insights on magmatic processes and how these are affected by the local and regional tectonics. At Llaima, we identify two different and distinctive groups of satellite scoria cones with a common magmatic source but different petrological evolutions. Cones from group 1 (the "glomeroporphyritic group"), predominantly occur in the NE flank of the volcano, are characterized by basaltic to basaltic andesitic compositions (51–55 wt% SiO 2) and have glomeroporphyritic textures. We have interpreted these cones as the product of magma replenishment, followed by an unimpeded propagation of their feeder dikes controlled mainly by the regional stress. As a result, dikes are emplaced parallel to the regional σ 1 , and perpendicular to the axis of local crustal extension. Cones from the group 2 (the "pilotaxitic group"), appear at the NE, NW and SW flanks of the volcano, they have more evolved compositions (55–60 wt% SiO 2) and textures lacking glomerocrysts. Their location and morphometric parameters suggest that their feeder dike emplacement is mainly controlled by the stress exerted by the load of the volcanic edifice, where the compressional stress hinders magma ascent, allowing fractionation and volatile overpressure buildup. The characteristic texture of these cones is interpreted as fractionation, followed by a single crystallization event upon eruption. The lack of scoria cones at the SE flank is attributed to the local horizontal compressional stress caused by the local tectonic deformation associated to the Liquiñe-Ofqui Fault Zone. Highlights • The scoria cones at Llaima represent two distinct groups, based in their petrologic characteristics. • The groups represent magmatic processes operating at depth and the interplay between local crustal stress and regional tectonics. • Dike emplacement at Llaima is controlled by the load of the edifice and the crustal deformation generated Liquiñe-Ofqui Fault Zone. [ABSTRACT FROM AUTHOR]

A moderate earthquake with a moment magnitude of Mw 5.5 struck the Sub-Bibanique region of eastern Algeria on 14 May 2010, killing three people, injuring hundreds of others, and causing moderate damages in the epicentral area, mainly in the villages of Beni-Ilmane and Samma. The focal mechanism of the seismic source for the first shock, obtained by near-field waveform modelling, exhibits left-lateral strike-slip faulting with the first nodal plane oriented at N345°, and right-lateral strike-slip faulting with the second nodal plane oriented at N254°. A second earthquake that struck the region on 16 May 2010, with a moment magnitude of Mw 5.1, was located 9 km SW of the first earthquake. The focal mechanism obtained by waveform modelling showed reverse faulting with nodal planes oriented NE–SW (N25° and N250°). A third earthquake that struck the region on 23 May 2010, with a moment magnitude of Mw 5.2, was located 7 km S of the first shock. The obtained focal mechanism showed a left-lateral strike-slip plane oriented at N12° and a right-lateral strike-slip plane oriented at N257°. Field investigations combined with geological and seismotectonic analyses indicate that the three earthquake shocks were generated by activity on three distinct faults. The second and third shocks were generated on faults oriented WSW–ENE and NNE–SSW, respectively. The regional stress tensor calculated in the region gives an orientation of N340° for the maximum compressive stress direction (σ 1 ) which is close to the horizontal, with a stress shape factor indicating either a compressional or a strike-slip regime. [ABSTRACT FROM AUTHOR]

Abstract: A method to evaluate regional stress state and Young’s modulus by analyzing the mining-induced deformation of ground is proposed. The effect of changes in regional strain state due to tectonic plate motion can be accounted for using the proposed method. Mining-induced deformation can manifest itself as changes in ground surface geometry resulting from mining activity. There are three underlying factors that can contribute to the deformation, these being: displacement induced by the effect of gravity; displacement induced by the effect of horizontal regional strain; and, incremental change in the horizontal regional strain. Both regional strain and Young’s modulus can be simultaneously estimated, because the displacement induced by the regional strain is independent of the Young’s modulus and the displacement induced by the gravity is inversely proportional to the Young’s modulus. The relative displacement arising from the mining excavation at the Torigatayama limestone mine in Japan has been measured by GPS. Both the regional stress state and Young’s modulus were estimated by back analysis of the relative displacement using a 3-D finite element method. It was shown that back analysis based on changes in distance between the measurement points provides a more reliable estimation than that could be achieved based on the direct relative displacement. [Copyright &y& Elsevier]

Abstract: Defining regional stress for a heterogeneous rock mass composed of different rock bodies and a large fault as a displacement boundary condition that is consistent with the mean trend of the in situ stress, we propose a method for determining regional stress from limited stress data by using a 3D finite element method (FEM). First, this method was applied to a small-scale heterogeneous model with a large planar fault that penetrates the region, to clarify the estimation accuracy and the effects of poor modeling and incomplete stress data. The results showed that (1) the method proposed in this study can accurately estimate the stress distribution in the region under consideration except at the bottom when the sliding area of the fault is small, (2) the reproducibility of the given stress can be used to estimate appropriate values of the normal and shear stiffnesses of the fault when the fault is homogeneous, and (3) both incomplete stress data and the assumption that the fault does not slide decrease the accuracy of stress estimation while the given stresses are approximately reproduced. Next, the method was applied to the Tono area, Japan, where a large fault with a complex geometry, called the Tsukiyoshi fault, exists. The results obtained under the assumption that the Tsukiyoshi fault does not slide showed that (1) the stress distribution was estimated reasonably well only for a narrow region that contained the boreholes at which the stress used in the inverse analysis was measured, (2) the stress in the area shows a large scatter and a non-linear trend with depth, which is mainly caused by the Tsukiyoshi fault, and (3) when a fault is heterogeneous, comprehensive examination for the states of the stress and displacements on the fault is needed to obtain a reasonable estimation for the regional stress. [Copyright &y& Elsevier]

Major sources of stress perturbation are expected in the brittle crust, and documented by local studies in Iceland. Whether or not a significant average stress state may emerge from regional-scale inversion of very large sets of focal mechanisms is thus subject to doubt. We carried out stress inversion of double couple focal mechanisms recorded by the Icelandic seismological network within a set of 126 588 shallow earthquakes from July 1991 to July 1999. We performed mass inversion of two main data sets of 12 191 and 71 889 focal mechanisms in the transform zone areas of North and South Iceland respectively. The inversion reveals surprisingly high levels of consistency within these data sets, with regard to the expected dispersion. Adopting a threshold value of +40% in an individual fit scale from −100% (total misfit) to +100% (perfect fit), the proportion of acceptable data is as high as 78% for 65 571 focal mechanisms of the major regime in these two areas (53% for 18 519 mechanisms of the minor regime). The major stress regimes thus calculated show nearly vertical intermediate principal axes and the azimuths of extension (minimum stress σ3) are 065° in the Tjo¨rnes Fracture Zone and 140° in the South Iceland Seismic Zone. With respect to the 105° azimuth of plate separation, these directions show nearly symmetrical angular deviations: 40° anticlockwise in the right-lateral transform zone area of North Iceland and 35° clockwise in the left-lateral one of South Iceland. Such large deviations reveal first-order stress perturbation in the areas where major rift offset resulted from Late Cainozoic rift jumps related to plate boundary migration with respect to the Iceland Mantle Plume. [Copyright &y& Elsevier]