Horizontal Compressive Stress Research Articles

Cyclic response of laterally loaded pile after storm loading

Offshore wind farms in China are in tropical cyclone zones, where typhoons or tropical revolving storms will pass through every year and can produce huge loads on the offshore wind turbines. Therefore, the stability of OWT foundations after storm loading is critical to ensure the safe operation and design life of turbines. While monopile is the most used foundation type for offshore wind, little studies exist on the cyclic response of monopile after storm loading. To fill the gap, this study presents a unique centrifuge and numerical study on the cyclic deflection response of monopile in medium dense sand after extreme storm loading. The centrifuge test was performed at 100g on a typical monopile with a prototype diameter (D) and embedded depth (L) of 4 m and 60 m, respectively. Finite element (FE) simulations using the advanced hypoplastic model for sand were performed to back-analyze the centrifuge test and provide further insights into the test results. From the centrifuge test, a unique “self-healing” beahviour of monopile was observed for the first time, where the foundation exhibited a deflection recovery response and tilted back to its initial position under the small amplitude cyclic load after the storm loading. This “self-healing” beahviour was also replicated by the FE model. By looking into the computed pile-soil interaction and soil element states around the pile, it was found that the “self-healing” response is attributed to the “lock-in” bending moment after the storm loading. This “lock-in” bending moment generated significant compressive horizontal stress on the soil elements behind the pile and produced more compressive strain under cyclic loading. As a result, the pile exhibited the “self-healing” response and tilted back against the loading direction.

ON THE POSSIBLE FORMATION MECHANISM OF THE OPEN FRACTURING IN SEDIMENTARY BASINS

The paper shows that the formation of orthogonal systems of open macrofractures (cleavages) of sedimentary basins can occur at the stage of diagenesis during the first hundreds of years for a small range of shelf depths (up to 100 m) during sedimentation. During this period, sediments are compacted and the excess water is slowly squeezed out, which determines the decrease in the Poisson’s ratio from values close to 0.5 to values 0.3–0.2. Because of this, in sediments, the stress state of which is almost completely determined only by the gravity of the overlying rocks, is reduced by 50 % or more of the horizontal compression stress level from the initial lithostatic pressure level. On the other hand, if the limit of pseudoplastic flow is reached in the rock, vertical compaction occurs, accompanied by an increase in horizontal compression. These two competing processes together with the factor of fluid pressure in fractures and pores determine the possibility of brittle fracture formation. It is shown that for rocks with a low level of yield strength (clay), compaction leads to an increase in the level of horizontal compression stresses, which makes brittle fracture in such rocks at the stage of diagenesis impossible. In rocks with a high level of strength (sandstone, limestone), brittle fracture due to excess water pressure in the sub­vertical microcracks starts earlier than the shear yield limit is reached.

Wrinkling instabilities of swelling hydrogels.

We investigate the formation of wrinkling instabilities at the interface between layers of hydrogel and water, which arise to relieve horizontal compressive stresses caused by either differential swelling or confinement. Modelling the gel using a linear-elastic-nonlinear-swelling approach, we determine both a criterion for marginal stability and the growth rates of normal modes. Furthermore, our formalism allows us to understand the influence of differential swelling on the stability of hydrogels brought into contact with water, and we find three distinct phases of the instability. Initially, when only a thin skin layer of gel has swollen, buckles grow rapidly and the gel deforms as an incompressible material. A balance between normal elastic stress and pore pressure selects a wavelength for these buckles that increases with the square root of time. At late times, when the gel approaches a uniformly swollen state, buckles can only grow by differential swelling on much slower timescales determined by solvent transport. At intermediate times, growth is driven by the same fluid transport process as at late times but gradients in fluid pressure in the gel as it swells destabilize the interface, driving faster growth of wrinkles. We also explain why some instabilities can be transient, "healing" as time progresses, while others must remain for all time.

Disagreements in Geodetically Inferred Strain Rates in the Western US With Stress Orientations and Geologic Moment Rates

AbstractI employ an elasticity‐based method to invert a geodetically derived surface velocity field in the western US using for present‐day surface strain rate fields with uncertainties. The method uses distributed body forces in a thin elastic sheet and allows for discontinuities in velocity across creeping faults using the solution for dislocations in a thin elastic plate. I compare the strain rate fields with previously published stress orientations and moment rates from geological slip rate data and previous geodetic studies. Geologic and geodetic moment rates are calculated using slip rate and off‐fault strain rates from the 2023 US National Seismic Hazard Model (NSHM) deformation models. I find that computed total geodetic moment rates are higher than NSHM summed moment rates on faults for all regions of the western US except the highest deforming rate regions including the Western Transverse Ranges and the northern and southern San Andreas Fault (SAF) system in California. Computed geodetic moment rates are comparable to the moment rates derived from the geodetically based NSHM deformation models in all regions. I find systematic differences in orientations of maximum horizontal shortening rate and maximum horizontal compressive stress in the Pacific Northwest region and along much of the SAF system. In the Pacific Northwest, the maximum horizontal stress orientations are rotated counterclockwise 40–90° relative to the maximum horizontal strain rate directions. Along the SAF system, the maximum horizontal stresses are rotated systematically 25–40° clockwise (closer to fault normal) relative to the strain rates.

Intraplate stress distribution within the Indian Plate: Insights from finite element modelling

The Indian Plate is bounded by different types of plate boundaries like divergent, convergent/collisional, transform and consists of oceanic and continental lithosphere. Due to such tectonic settings, the intraplate tectonic stress distribution across the Indian Plate is very complex. Further, available in-situ stress information over the Indian Plate is relatively sparse; therefore, numerical modelling of maximum horizontal compressive stress (SHmax) is needed to comprehend regional stress patterns. In the present work, stress simulations are carried out to explore the effects of rigidity, lithospheric thickness, and boundary forces on the distribution of intraplate stress across the Indian Plate. The rigidity is derived from effective elastic plate thickness (Te), which is estimated through joint inversion of admittance and coherence using the wavelet transform approach. The stress field is simulated for four cases for different combinations of Te, lithospheric thickness and tectonic forces and compared with in-situ stress observations. The analysis of models shows that the magnitude of SHmax is primarily controlled by boundary forces, Te and scaled by lithospheric thickness; and its orientations are mostly governed by the applied tectonic forces and rigidity. Spatial distribution of earthquakes correlated well with the von-Mises stress and high von-Mises stresses are accumulated at earthquake prone regions. The stress field and von-Mises stress over major geological features are also analyzed and discussed. Apart from boundary forces, it is inferred that Te is one of the most significant factors which controls the stress field distribution.

Prediction of fractures by using the stress field in medium to deep shale gas reservoirs: A case study of Late Ordovician–early Silurian shale gas reservoirs in the Nanchuan region, South China

Containing representative shale gas reservoirs, the Wufeng–Longmaxi Formations in the Nanchuan region in South China have been modified by three compressional tectonic movements (in the Late Jurassic to Early Cretaceous, Late Cretaceous, and Cenozoic to present). These movements formed deeply buried reservoirs with complex in situ stresses and rapid directional changes, influencing the state of shale gas reservoirs fractures. Numerical modeling applied to model the stress field in the three periods showed that the maximum compressive horizontal principal stress (σHmax), strain rate, and fault slip rate regimes transformed from a thrust and strike-slip regime to a thrust and strike-slip regime to a thrust regime. The σHmax direction changed from NW–SE and WNW–ESE to WNW–ESE and nearly E–W and then to multiple directions. The magnitude of strain rate changed from -16 ∼ -15 to -17.7 ∼ -15.6 to -18.8 ∼ -15.8. The fault slip rate changed from 0 to 0.029 mm/a to 0–0.0026 mm/a to 0–0.0012 mm/a. These data strongly agree with data from drilled wells and outcrops and show that the present-day stress field is the superposed effect of multistage movements. Semiquantitative research was conducted on shale gas fracture openness and fracture development. Fracture openness is favorable when the angle between the σHmax direction and nearby faults is ≤45° and the distance from the fault is >2 km, and fracture development is favorable when the magnitude of the strain rate is <-17.6 and the fault slip rate is <0.00036 mm/a. Accordingly, we predicted fractured shale gas reservoirs.

Tectonic stress around the South Caspian basin deduced from earthquake focal mechanisms

ABSTRACT In this paper, we study the tectonic stress around the South Caspian Basin (SCB), which includes the Kopeh Dagh, Alborz, Talesh, eastern Greater Caucasus mountain belts, the Apsheron sill, and the Balkhan-Ashkabad fault zone. We apply the stress inversion to focal mechanisms of 410 mainshocks that have occurred over the last 69 years. These mechanisms indicate that the surrounding fault zones of the SCB exhibit diverse types of faulting, ranging from thrust to strike-slip, normal, and their combinations. The results of the stress inversion align with the kinematics of the major fault zones bounding the SCB and emphasize the spatial heterogeneity of the stress field in this region. The region is predominantly under compression, but transpressive and strike-slip regimes are also present. This highlights the role of obliquely oriented basement faults with respect to the maximum horizontal compressive stress (S Hmax ) in accommodating deformation through convergent zones. The orientation of the S Hmax is in the NE to NNE direction in the Kopeh Dagh, Alborz, Talesh, and Ashkabad-Balkhan fault zones, being rotated to NNE-N in the Greater Caucasus. The orientation of the S Hmax relative to the convergence direction of the Arabian and Eurasian plates indicates that the Arabian-Eurasian oblique convergence-derived tectonic stress is the primary contributor to the total stress and deformation in this region.

Seismic anisotropy and stress-field variations along the Dead Sea Fault zone in northern Israel

Geological observations based mainly on stress (strain)-indicating meso-scale markers suggest that the local maximum horizontal compressive stress (SHmax) direction along the Dead Sea Fault (DSF) system in northern Israel changes considerably over short distances from a ∼ N-S, fault-parallel orientation in the south to an ∼E-W, fault-perpendicular orientation in the north. However, the association of the meso-scale markers to the present-day stress field is not well constrained as many of them could have formed at any stage during the DSF evolution. In this study, we use a large dataset of Shear-Wave Splitting (SWS) measurements, taken from local DSF-related earthquakes, to characterize the azimuthal seismic anisotropy of the upper-crust at this critical turning point of the DSF system, near the Lebanese Restraining Bend. The SWS directions commonly align parallel to the local SHmax as revealed by the meso-scale markers, supporting their association with the current stress field. In several stations, SWS fast directions indicate azimuthal anisotropy at high angles to the trace of the master fault, indicating that stress-induced anisotropy represents a more plausible mechanism for SWS than structure-controlled anisotropy. The SWS measurements suggest that a northward counterclockwise rotation of the crustal SHmax is present along this segment of the DSF, supporting previous notation on the transition from divergent to convergent strike-slip faulting in this area. Plain langauge summaryGeological observations based mainly on stress (strain)-indicating meso-scale markers suggest that the local maximum horizontal compressive stress direction along the plate-boundary Dead Sea Fault (DSF) system in northern Israel changes considerably over short distances. However, the association of the meso-scale markers to the present-day local stress field is not well constrained since many of them could have formed at any stage of the DSF evolution. In this study, we use a common method of measuring local seismic anisotropy called “shear wave splitting” (SWS) to characterize the directional-dependent seismic wave speeds in the upper crust. The SWS measurements are obtained from local seismic and micro-seismic DSF-related events, and characterize the upper-crust seismic anisotropy at this critical turning point of the DSF system. The SWS directions commonly align parallel to the maximum compression as revealed by the meso-scale markers, supporting their association with the current stress field. The SWS measurements suggest that a northward counterclockwise rotation of the crustal maximum compression is present along this segment of the DSF.

Sustainable development of geological resources: the Characteristics of Red Karst Landscape and Tourism Development in Tongren, Guizhou

&lt;abstract&gt; &lt;p&gt;China is rich in karst resources and famous for its karst landscape. As a part of karst, the popularity of red karst is obviously low and has received little attention; moreover, the related scientific research and tourism development are relatively immature. Therefore, this paper takes the red karst in Tongren, Guizhou as the research object, analyzes the characteristics and causes of the red karst, comprehensively evaluates its landscape value, and puts forward development strategies and protection suggestions. Through the above research, the following three research results are obtained. (1) On the basis of a field investigation and a literature review, the geographical situation, background, and landscape characteristics of the red karst landform and its surrounding areas in Tongren City were investigated in detail. It forms isolated peaks, rock walls, protruding rocks, stone pillars, and other forms. The most prominent features of the red karst landscape in Tongren City are that the color is red and white, and the shape is concave and convex in order. (2) Geochemical test and structural data collection and analyses of the collected samples were performed. The obtain concave layer rock is dolomitic limestone, and that convex lay rock is argillaceous dolomitic limestone. Because the convex layer rock sample is high in silicon element content, the convex layer rock sample is not easy to be weathered, is rich in clay mineral, and is not easy to be corrode; the convex layer rock sample gradually protrudes with the passage of time to form an uneven shape. The red karst landform is developed in either the rock stratum or tectonic zone with moderate tectonic deformation intensity. First, the formation of a single-layer honeycomb morphology requires that the rock stratum is subjected to a sufficient horizontal compressive stress at a certain burial depth. Second, it is necessary to widely exist in areas with relatively gentle rock formation occurrence; too strong of a structural deformation will make the rock formation occurrence steep, which is not conducive to the occurrence of differential dissolution between adjacent strata. (3) Through the construction of an evaluation index system, the red karst in Tongren City is evaluated, and it is concluded that the geological heritage value is high, and the development and utilization conditions are weak. The conclusion of this article is a supplement to the geological and geomorphological system characteristics of southern karst. From the perspective of world natural heritage, the study enriches the academic evidence of red karst and enables its exemplary role to be well played. From the perspective of the tourism development value, more people should be made aware of the diverse types of karst landforms, which provide experience and suggestions for the value of landforms, tourism resources, and tourism development in similar areas.&lt;/p&gt; &lt;/abstract&gt;

Glacial-isostatic-adjustment strain rate–stress paradox in the Western Alps and impact on active faults and seismicity

Abstract. In many regions formerly glaciated during the Last Glacial Maximum (LGM), glacial isostatic adjustment (GIA) explains most of the measured uplift and deformation rates. GIA is also proposed as a key process contributing to fault activity and seismicity shortly after the LGM and potentially up to the present day. Here, we study the impact of GIA on present-day fault activity and seismicity in the Western Alps. We show that, in the upper crust, GIA induces horizontal compressive stress perturbations associated with horizontal extension rates. The latter agree with the observed geodetic strain rates and with the seismicity deformation patterns. Yet, in nearly all cases, the GIA stress perturbations tend to either inhibit fault slip or promote fault slip with the wrong mechanism compared to the seismicity deformation style. Thus, although GIA from the LGM explains a major part of the Western Alp geodetic strain rates, it does not drive or promote the observed seismicity (which must be driven by other processes). This apparent strain rate–stress paradox results from the gradual diminution over time of the finite shortening induced in the upper crust by the Würm ice cap load. A direct corollary of our results is that seismicity and seismic-hazard studies in the Western Alps cannot directly integrate geodetic velocities and strain rates but instead require detailed modeling of the GIA transient impact.

Mapping of stress and structure controlled upper crustal anisotropy in Kumaon-Gharwal Himalaya

Shear wave splitting analysis of local earthquakes provides valuable insights into the structure and stress controlled upper crustal anisotropy signatures. We examine these signatures at the Kumaon-Garhwal Himalaya and provides an excellent example of anisotropic crustal structure in the ongoing continent-continent collision settings between Indian and the Eurasian plates. A total of 256 local earthquakes were selected for the analysis (1.0 ≤ M ≤ 5.4) between January 2017 and February 2021, recorded at a dense network of 51 broadband stations. The result indicates the observed crustal anisotropy is parallel to stress-aligned micro-cracks far from the major Himalayan fault zones and structure parallel near the fault zones. The dominant fast polarization directions (FPD) in the Inner Lesser Himalaya (ILH) and Higher Himalaya, are consistent with the maximum compressive horizontal stress directions (SHmax), essentially influenced by the local stress field. Whereas in the Outer Lesser Himalaya (OLH) and Sub Himalaya, fast directions are sub-parallel to the structural trends, suggesting that the anisotropy is associated with the shear fabric from recent deformation episodes related to structural induced anisotropy. The computed normalized delay times show a mean value of 1 ms/km, within the OLH and Sub-Himalaya, while in ILH and Higher Himalaya, this value increases up to 5.3 ms/km. The associated crack densities are 0.0038 and 0.0207, with shear wave velocity anisotropy of 0.38% and 2.07% respectively. Significant scatter within the depth range of 10–15 km is observed in normalized delay times, suggesting the source of anisotropy within the upper crust.

S Hmax orientation in the Alpine region from observations of stress-induced anisotropy of nonlinear elasticity

SUMMARY The orientation of SHmax is commonly estimated from in situ borehole breakouts and earthquake focal mechanisms. Borehole measurements are expensive, and therefore sparse, and earthquake measurements can only be made in regions with many well-characterized earthquakes. Here, we derive the stress-field orientation using stress-induced anisotropy in nonlinear elasticity. In this method, we measure the strain derivative of velocity as a function of azimuth. We use a natural pump-probe (NPP) approach which consists of measuring elastic wave speed using empirical Green’s functions (probe) at different points of the earth tidal strain cycle (pump). The approach is validated using a larger data set in the Northern Alpine Foreland region where the orientation of maximum horizontal compressive stress is known from borehole breakouts and drilling-induced fractures. The technique resolves NNW-SSW to N-S directed SHmax which is in good agreement with conventional methods and the recent crustal stress model. We confirm that the NPP method can be applied to dense large-scale seismic arrays. The technique is then applied to the Southern Alps to understand the contemporary stress pattern associated with the ongoing deformation due to counterclockwise rotation of the Adriatic plate with respect to the European plate. Our results explain why the two major faults in Northeastern Italy, the Giudicarie Fault and the Periadriatic Line (Pustertal–Gailtal Fault) are currently inactive, while the currently acting stress field allows faults in Slovenia to deform actively. We have demonstrated that the pump-probe method has the potential to fill in the measurement gap left by conventional approaches, both in terms of regional coverage and in depth.

Early zircon saturation in adakitic magmatic differentiation series and low Zr content of porphyry copper magmas

Porphyry copper ore-forming magmas worldwide are chemically distinguished from ordinary arc granitoids by lower Zr and by higher Sr/Zr ratios at equivalent SiO2 contents. Low ppm Zr in zircon-saturated melts and high whole-rock Sr/Zr in granitoid samples retaining igneous plagioclase are useful discriminants of Cu-fertile intrusive complexes. These and other chemical discriminants of porphyry-copper-forming melts of tonalite-granodiorite-adamellite composition cannot develop during crystallisation-differentiation at upper-crustal pressures. They indicate unusually high dissolved H2O (≥ 9 wt%) in residual melts at depths near the base of the crust. We compare Zr behaviour during mafic-to-felsic magmatic differentiation in orogenically deforming, copper-mineralised arc segments with behaviour during differentiation in non-orogenic, unmineralised arc segments laterally adjacent along the same subduction zones. In orogenically deforming segments, horizontal compressive stress provides resistance to opening of tensile dyke fractures and promotes entrapment of mantle-derived basaltic melts in magma chambers embedded in hot country rock at Moho-vicinity depths, where magmas cool slowly and residual melts tend to last long enough to experience intermittent chamber replenishment by basaltic melts from the deeper mantle. Over several cycles of replenishment and fractional crystallisation, residual melts acquire high concentrations of inherited chemical components that were largely excluded from cumulus minerals—H2O, Cl, CO2, SO3, etc. Accumulating H2O re-orders the high-pressure crystallisation sequence of igneous minerals in successive cycles and leads to early and prolific production of hornblende and early zircon saturation in mafic melts (< 60 wt% SiO2) of later cycles and leads to low Zr contents of zircon-saturated intermediate-composition residual melts.

Determining the present-day in-situ stresses of tight-oil sandstones by conventional logs: An approach in the Triassic Yanchang Formation, southern Ordos Basin

The present-day in-situ stresses affect the drilling design, well pattern deployment, well completion modification, hydraulic fracturing and water injection of tight-oil sandstones. The measurement data of these stresses are commonly unavailable because of their high costs and limited core samples, therefore employing conventional logs for these stress determination is imperative for tight-oil sandstones. Firstly, the suitable calculation models for the present-day in-situ stress calculation by conventional logs were selected according to the geological characteristics of the sixth member of the Yanchang Formation (Chang 6) in Heshui area of the southern Ordos Basin, China. Then, the dynamic rock mechanical parameters were determined by conventional logs, and corrected by the static rock mechanical parameters obtained from the triaxial rock mechanical tests. Moreover, the pore fluid pressure was determined by the empirical formula method. Finally, the maximum and minimum horizontal compressive stresses (σH and σh), and the vertical stress (σv) of six wells were calculated according to the selected models of these stresses, respectively. The present-day in-situ stresses, determined by the proposed method in the paper, were verified by those obtained from acoustic emission tests and finite-element numerical simulations with the relative errors of less than 10%. The results show that the magnitudes of σH, σh and the horizontal differential stress (σH−h) in the study area mainly range from 32 to 43 MPa, 23 to 37 MPa and 5 to 8 MPa, respectively. The magnitude of the three-dimensional present-day in-situ stress increases with the increase of depth. The average gradients of σH, σv and σh are 0.018, 0.014 and 0.015 MPa/m, respectively, that is σH&gt;σv&gt;σh. In this stress state, the hydraulic fractures, with a trend of little expansion towards multiple directions, are commonly developed at a small angle intersecting with the direction of σH in the study area.

Temporary stress regime reversal immediately after the complete release of the localized compressive stress by the 2021 Luxian Ms 6.0 earthquake, Sichuan Basin

Aftershocks can provide fundamental information of the seismogenic structure and the stress condition of the source region to reveal the seismogenic mechanism of earthquakes without clear surface rupture. On 16 September 2021, a destructive Ms 6.0 earthquake broke the seismic quiescence of the Luxian county in the inner Sichuan Basin, China. Here, we explore the seismogenic structure and the postseismic stress condition of the Luxian earthquake through characterizing sedimentary faults in the spatiotemporal distribution of aftershocks, through the focal mechanism solutions and further stress inversion, and through the coseismic rupture model based on synthetic aperture radar interferometry (InSAR) data. Our results show that aftershocks are concentrated at two clusters in the broad Yujiasi syncline. Seismicity of the northern cluster reveals that the main shock ruptured an east-southeast striking fault with a dip angle of ∼ 45° to the southwest (F0), and activated a gently dipping (∼ 18°) fault (F1) below F0 and a nearly vertical shallow fault (F2) on the northern side of F0. These three faults outline the edges of a thrust wedge in the sedimentary cover. The northern cluster occurred on a conjugate fault system within which steep faults with different dipping direction interlaced downward forming ‘V’ shapes. The main reverse rupture on the east-southeast striking F0 fault is inconsistent with the background northwest-southeast tectonic stress field. Combined with the morphology of anticlines, we suggest that the main shock was resulted from the localized NE-SW compressive stress concentration due to the inhomogeneous deformation of the sedimentary cover. Aftershocks with magnitude ≥ 2.0 are characterized by the dominant normal slip. Coulomb stress change (ΔCFS) results show that normal slip is enhanced by the main rupture. However, the amplitude of ΔCFS (a few bars) is not large enough to reverse the dominant slip direction of faults. Postseismic stress inversion results show a temporary stress regime reversal from the nearly north-south subhorizontal compressive stress regime before the main shock to the normal slip favored stress regime with a subvertical maximum principal stress (σ1). We suggest that the Luxian Ms 6.0 earthquake as an unusual event completely released the localized horizontal compressive stress and hence caused a temporary reversal of the localized stress regime.

Unreliable determination of in situ stress orientation by cross multipole array acoustic logging in fractured shale reservoirs

AbstractWith the development of shale gas exploration in China, the use of conventional logging tools has been introduced, and cross multipole array acoustic logging tools have gradually been used to determine the stress orientation in shale. The direction of fast shear waves (FSWs) is generally parallel to the horizontal maximum principal compression stress (SHmax). However, the azimuth of FSWs is found to be parallel to the main strike (but not to the SHmax) direction of structural fractures in shale reservoirs. Outcrop and image logging data indicate that the natural fractures in this area strike NE‒SW. If the shear wave anisotropy is caused by only the stress around the borehole and the FSWs are known to be NE‒SW, SHmaxshould be parallel to NE‒SW; however, according to statistics of land movement in adjacent areas, anelastic strain recovery, earthquake focal mechanism, borehole breakouts, hydraulic fracturing data, deviated well data, and drilling‐induced fracture data in local regions, SHmaxis oriented in the NW‒SE direction, and the directions of FSWs are generally parallel to the structural fracture direction. This contradiction indicates that the development of structural fractures may affect the orientation of FSWs. Therefore, it is not reliable to use XMAC (Cross‐Multipole Array Acoustilog) logging only to determine the direction of in situ stress in fractured shale reservoirs. In addition, the direction of the FSWs in the middle of thick mudstones is NW‒SE, which may represent accurate information about the in situ stress direction.

High-resolution three-dimensional azimuthal velocity anisotropy of S-waves in southern-central Japan, based on ambient noise tomography

To investigate the dominant deformational patterns and stress conditions in the upper crustal structure of the Kinki region, southern-central Japan, we constructed a high-resolution 3D azimuthal anisotropy model to a depth of ~ 11 km. We used 6-month-long ambient noise data recorded by the densely distributed permanent and temporary stations. From this dataset, cross-correlations were retrieved. We then obtained a 3D isotropic velocity model by inverting Rayleigh wave dispersion data, followed by a direct joint inversion for both 3D azimuthal anisotropy and additional isotropic velocity perturbation. The resolved 3D azimuthal anisotropy reveals significant contrasts of anisotropy across the Kinki region. The predominant fast axes observed in the northwestern Kinki region align with the direction of the maximum horizontal compressional stress and the principal strain rate axes, suggesting that the observed anisotropy is mainly stress-induced. In the southern part of the study area, furthermore, the predominant fast axes trend NE–SW and near E–W, also indicating the presence of stress-induced anisotropy. On the depth profile of the anisotropy, we found depth-dependent variation of azimuthal anisotropy. There exists a significant consistency between the anisotropy observed beneath 3 km depth and the dense distribution of earthquake hypocenters. This interrelationship between anisotropy and seismicity demonstrates that the observed anisotropy could be linked to local crustal stress or fractures relevant to earthquake ruptures. Our 3D anisotropy model therefore contributes towards understanding the locations and features of the seismicity region.Graphical

Numerical simulation study on the spontaneous rupture process and its influencing factors of the 2001 MS 8.1 Kunlun earthquake in China

On November 14, 2001, the MS 8.1 Kunlun earthquake occurred in the west of Kunlun Mountain pass in the East Kunlun fault zone of the Qinghai-Tibet Plateau, China. The field investigation and the results of kinematic inversion indicate that it is a strike-slip earthquake with a complex rupture process. The study of the spontaneous rupture process of this earthquake and its influencing factors is of great significance for understanding the occurrence mechanism of continental large earthquakes. First, based on the results of geological survey and kinematic inversion, a three-dimensional non-planar fault model is established. Then, using the curved-grid finite difference method (CG-FDM), the dynamic rupture process of the 2001 MS 8.1 Kunlun earthquake is numerically reproduced by adjusting the parameters for numerical simulation experiments. On this basis, the influence of background stress field, fault geometry and friction coefficient on the surface slip distribution, seismic moment rate function and rupture velocity are discussed respectively. The results show that the important reasons for the super-long rupture scale and super-shear rupture velocity of this earthquake may be the clockwise rotation of the maximum horizontal principal compressive stress along the NNE ∼ NE direction and the low friction coefficient distribution on the whole fault surface. The difference between the dynamic and static friction coefficients along the fault strike may be non-uniform, and it may be larger on the main fault than that on the secondary fault. This will result in a higher level of stress drop on the main fault, leading to the occurrence of super-shear rupture and the slip distribution with the Kusai Lake as the macroseismic epicenter. The large local geometric variation along the strike of the main fault may also have some influence on the distribution of co-seismic slip and the location of the super-shear rupture. In addition, the numerical simulation results in this paper confirm the conclusion that the East Kunlun fault zone has low friction coefficient.

Theoretical analysis of two cracks in sandstone under the simultaneous action of pressure and shear stress

AbstractThe cracked rock mass could slide along the weak side under the action of compressive loads and shear loads in landslides; thus, the stability evaluation of the crack in rock mass media is of particular importance. The formulas of stress intensity factors (SIFs) for each crack tip are theoretically derived by implementing complex function theory. The results of the analytical solutions are then compared with the predicted results of the finite element analysis via ABAQUS software, revealing a slight relative error. Photo elastic experiment and static compression tests are conducted to verify the correctness of theoretical results. The obtained results reveal that the internal pressure leads to the increasing SIFs of mode II at crack tips. Further, the larger the difference between the horizontal and vertical compressive stresses, the more likely the occurrence of shear failure in the cracked rock mass. For the two cracks, the most dangerous point is the inner crack tip of the longer crack.

Geomechanical analysis of the geothermal reservoir at San Emidio, Nevada

The WHOLESCALE (Water and Hole Observations Leverage Effective Stress Calculations and Lessen Expenses) project is aiming to simulate the spatial distribution and temporal evolution of stress throughout the geothermal system at San Emidio, Nevada, United States, via a thermo-hydro-mechanical reservoir model. Focal mechanisms for microseismic events during a temporary shutdown of the geothermal power plant in 2016 were analyzed through linear stress-inversion methods to infer the in-situ reservoir stress state. This analysis was supplemented by other geophysical and geological data, including focal mechanisms from regional earthquakes, slickenlines on exposed fracture surfaces, wellbore stress indicators observed in the surrounding region, and secular strain rate measurements. From the inferences of in-situ reservoir stress, 78 different realizations of stress models were generated over reasonable ranges for the values of maximum compressive horizontal stress (SHmax) azimuth and ratios of principal stress magnitudes. Evaluation of slip tendencies on fault planes determined for the microseismic events for each realization of the initial stress model suggests the reservoir stress state as transtensional with an SHmax azimuth between N and N30∘E.