Principal Stress Orientations Research Articles

Metamorphic record of catastrophic pressure drops in subduction zones

When deeply buried in subduction zones, rocks undergo mineral transformations that record the increase of pressure and temperature. The fact that high-pressure metamorphic parageneses are found at the Earth’s surface proves that rock burial is followed by exhumation. Here we use analysis of available data sets from high-pressure metamorphic rocks worldwide to show that the peak pressure is proportional to the subsequent decompression occurring during the initial stage of retrogression. We propose, using a simple mechanical analysis, that this linear relationship can be explained by the transition from burial-related compression to extension at the onset of exhumation. This major switch in orientation and magnitude of principal tectonic stresses leads to a catastrophic pressure drop prior to actual rock ascent. Therefore, peak pressures are not necessarily, as commonly believed, directly dependent on the maximum burial depth, but can also reflect a change of tectonic regime. Our results, which are in agreement with natural data, have significant implications for rock rheology, subduction zone seismicity, and the magnitudes of tectonic pressures sustained by rocks. Current views of subduction dynamics could be reconsidered in that perspective. Rocks are altered by high pressure during subduction. Analysis of exhumed metamorphic rocks suggests that the peak pressures recorded within minerals mark a change in tectonic regime within a subduction zone, rather than burial depth.

Numerical Modeling and Investigation of Fluid-Driven Fracture Propagation in Reservoirs Based on a Modified Fluid-Mechanically Coupled Model in Two-Dimensional Particle Flow Code

Hydraulic fracturing is a useful tool for enhancing rock mass permeability for shale gas development, enhanced geothermal systems, and geological carbon sequestration by the high-pressure injection of a fracturing fluid into tight reservoir rocks. Although significant advances have been made in hydraulic fracturing theory, experiments, and numerical modeling, when it comes to the complexity of geological conditions knowledge is still limited. Mechanisms of fluid injection-induced fracture initiation and propagation should be better understood to take full advantage of hydraulic fracturing. This paper presents the development and application of discrete particle modeling based on two-dimensional particle flow code (PFC2D). Firstly, it is shown that the modeled value of the breakdown pressure for the hydraulic fracturing process is approximately equal to analytically calculated values under varied in situ stress conditions. Furthermore, a series of simulations for hydraulic fracturing in competent rock was performed to examine the influence of the in situ stress ratio, fluid injection rate, and fluid viscosity on the borehole pressure history, the geometry of hydraulic fractures, and the pore-pressure field, respectively. It was found that the hydraulic fractures in an isotropic medium always propagate parallel to the orientation of the maximum principal stress. When a high fluid injection rate is used, higher breakdown pressure is needed for fracture propagation and complex geometries of fractures can develop. When a low viscosity fluid is used, fluid can more easily penetrate from the borehole into the surrounding rock, which causes a reduction of the effective stress and leads to a lower breakdown pressure. Moreover, the geometry of the fractures is not particularly sensitive to the fluid viscosity in the approximate isotropic model.

Deformation style in the damage zone of the Mondy fault: GPR evidence (Tunka basin, southern East Siberia)

The Mondy strike-slip fault connects the W-E Tunka and N-S Hovsgol basins on the southern flank of the Baikal rift system. Ground penetrating radar (GPR) surveys in its damage zone provide constraints on thicknesses, dips, and plunges of fault planes, as well as on the amount and sense of vertical slip. Strike-slip faulting in the southern segment of the Mondy fault within the territory of Russia bears a normal slip component of motion along the W-E and NW planes. These motions have produced negative flower structures in shallow crust appearing as grabens upon Pleistocene fluvioglacial terraces. The amount of normal slip estimated from the displacement of reflection events varies over the area and reaches its maximum of 3.4 m near Mondy Village. In the Kharadaban basin link, left-lateral strike slip displaces valleys of ephemeral streams to 22 m, while normal slip detected by GPR reaches 2.2 m; this normal-to-strike slip ratio corresponds to a direction of ~6° to the horizon. The angles of dips of faults are in the range 75°-79°; the thicknesses of fault planes marked by low- or high-frequency anomalies in GPR records vary from 2.5 to 17.0 m along strike and decrease with depth within a few meters below the surface, which is common to near-surface coseismic motions. Many ruptures fail to reach the surface but appear rather as sinkholes localized mainly in fault hanging walls. The deformation style in the damage zone of the Mondy fault bears impact of the NW Yaminshin fault lying between its two segments. According to photoelasticity, the stress field changes locally at the intersection of the two faults, under NE compression at 38°, till the inverse orientations of principal compression and extension stresses. This stress pattern leads to a combination of normal and left-lateral strike slip components.

Magma storage in a strike-slip caldera.

Silicic calderas form during explosive volcanic eruptions when magma withdrawal triggers collapse along bounding faults. The nature of specific interactions between magmatism and tectonism in caldera-forming systems is, however, unclear. Regional stress patterns may control the location and geometry of magma reservoirs, which in turn may control the spatial and temporal development of faults. Here we provide new insight into strike-slip volcano-tectonic relations by analysing Bouguer gravity data from Ilopango caldera, El Salvador, which has a long history of catastrophic explosive eruptions. The observed low gravity beneath the caldera is aligned along the principal horizontal stress orientations of the El Salvador Fault Zone. Data inversion shows that the causative low-density structure extends to ca. 6 km depth, which we interpret as a shallow plumbing system comprising a fractured hydrothermal reservoir overlying a magmatic reservoir with vol% exsolved vapour. Fault-controlled localization of magma constrains potential vent locations for future eruptions.

Influence of Large Syncline on In Situ Stress Field: A Case Study of the Kaiping Coalfield, China

Kaiping coalfield is located in Tangshan, northern China, and the area has been mined for 130 years. The main structure is the Kaiping syncline with an axis that generally strikes north–east to east–west. Since 1998, we have measured the in situ stress using overcoring and obtained 30 sets of data in eight coal mines with depths from −260 to −1238 m. Based on the measured data, we established the trends in maximum horizontal stress, vertical stress, minimum horizontal stress, and their ratios, and the ratio of mean horizontal stress to vertical stress and depth. The relationship between magnitude and orientation of maximum horizontal stress and their relationships to the Kaiping syncline are discussed. The results show that (1) the stress field of the Kaiping coalfield is complex and non-uniform. The orientation and magnitude of principal stress from stress measurement diverge widely from the regional stress field and the stress regime, with 77 % strike-slip faulting and the remainder thrust faulting. (2) Both maximum (σ H) and minimum (σ h) horizontal stresses have significant differences at a similar depth; however, vertical stress σ v increases linearly with depth with a gradient of 0.025 MPa/m. The ratio (k) of the mean horizontal stress to vertical stress is 160/h + 1.04 < k < 300/h + 1.14. This is higher at shallow depth and gradually decreases as depth increases. Up to 90 % of σ H/σ h falls between 1.17 and 2.39 with an average of 1.96, and it is not closely related to depth. The σ H /σ v ranges from 1.06 to 2.43 with a bandwidth of 1 and decreases gradually with increasing depth. (3) The in situ stress field in the Kaiping coalfield is the result of the synthetic action of different levels of geological structures where both the orientation and magnitude of in situ stress are influenced by the Kaiping syncline. The σ H near the axis of the Kaiping syncline is remarkably high and decreases 8 km from the axis. The orientation of σ H near the axis of the Kaiping syncline is almost perpendicular to the axis of the Kaiping syncline, while far from the axis the orientation gradually changes to east–west.

Focal mechanisms and size distribution of earthquakes beneath the Krafla central volcano, NE Iceland

AbstractSeismicity was monitored beneath the Krafla central volcano, NE Iceland, between 2009 and 2012 during a period of volcanic quiescence, when most earthquakes occurred within the shallow geothermal field. The highest concentration of earthquakes is located close to the rock‐melt transition zone as the Iceland Deep Drilling Project‐1 (IDDP‐1) wellbore suggests and decays quickly at greater depths. We recorded multiple swarms of microearthquakes, which coincide often with periods of changes in geothermal field operations, and found that about one third of the total number of earthquakes are repeating events. The event size distribution, evaluated within the central caldera, indicates average crustal values with b = 0.79 ± 0.04. No significant spatial b value contrasts are resolved within the geothermal field nor in the vicinity of the drilled melt. Besides the seismicity analysis, focal mechanisms are calculated for 342 events. Most of these short‐period events have source radiation patterns consistent with double‐couple (DC) mechanisms. A few events are attributed to non‐shear‐faulting mechanisms with geothermal fluids likely playing an important role in their source processes. Diverse faulting styles are inferred from DC events, but normal faulting prevails in the central caldera. The best fitting compressional and tensional axes of DC mechanisms are interpreted in terms of the principal stress or deformation rate orientations across the plate boundary rift. Maximum compressive stress directions are near‐vertically aligned in different study volumes, as expected in an extensional tectonic setting. Beneath the natural geothermal fields, the least compressive stress axis is found to align with the regional spreading direction. In the main geothermal field both horizontal stresses appear to have similar magnitudes causing a diversity of focal mechanisms.

Incompatible stress regimes from geological and geomechanical datasets: Can they be reconciled? An example from the Carnarvon Basin, Western Australia

Knowledge of the in-situ stress field is essential in petroleum basins, as it exerts a prime control over seal integrity, fracture stimulation, wellbore stability, and fluid flow. Previous geomechanical analysis of Australia's Carnarvon Basin, the continent's premier hydrocarbon region, highlights a normal to strike-slip fault stress regime. However, neotectonic evidence and contemporary seismicity suggest the possibility of a strike-slip to reverse-fault stress regime. We attempt to reconcile these conflicting datasets through new analysis of the in-situ stresses; Principal stress magnitudes and orientations were defined with data from 76 previously unanalysed petroleum wells. Wellbore image logs yield a maximum horizontal stress orientation of 113°N, consistent with modelling of the stress-field throughout the Indo-Australian Plate. Vertical stress magnitudes are estimated to range from 20MPakm−1 to 22.4MPakm−1. Leak-off tests (LOT) from 42 wells are analysed using two methodologies (one assuming tensile failure as traditionally accepted, the other assuming shear failure as outlined by Couzens-Schultz and Chan, 2010) and result in two different sets of horizontal stress magnitude estimates. Traditional interpretation of LOTs results in a minimum horizontal stress gradient of 16.8MPakm−1 and a maximum horizontal stress gradient of 21.8MPakm−1. The new method for LOT interpretation results in a minimum horizontal stress gradient of 18.1MPakm−1 and a maximum horizontal stress gradient of 25.4MPakm−1. Inclined natural fractures observed at LOT depths supports application of the new method of LOT interpretation. Traditional interpretation of stress magnitudes implies a normal to strike-slip fault stress regime, while the new method implies a strike-slip fault stress regime. This latter interpretation is favoured by the authors, as it allows for a reconciliation of the geological and geomechanical datasets. The assessment undertaken herein allows for reinterpretation of stress magnitudes in seismically-active basins hosting differing geomechanical and neotectonic regimes, potentially altering existing understanding of seal integrity, fracture stimulation, wellbore stability, and fluid flow.

Shear behaviour of hybrid composite-concrete beams: Experimental failure and strain analysis

The combination of high performance composites and concrete results in hybrid composite-concrete elements which no longer resemble the traditional reinforced concrete structures. Consequently, these hybrid elements have a different structural behaviour that requires new evaluation procedures. This paper focusses on hybrid TRC-composite-concrete elements which are used as a substitute for reinforced concrete beams within a floor system. Their shear behaviour is experimentally investigated by the examination of ten short-span hybrid beams with different geometries.Visual inspection shows that shear failure occurs by an inclined flexural shear crack in the concrete. A rosette strain gauge analysis proves that the principal strain and stress orientations are in accordance with theoretical predictions. Finally, the small influence of shear deformation on the total deflection of these beams is illustrated.

Triassic to recent tectonic evolution of a crestal collapse graben above a salt-cored anticline in the Glückstadt Graben/North German Basin

In this study we investigate faulting above a salt wall in the Glückstadt Graben/North German Basin. Two supra-salt faults are mapped from coast to coast over a distance of 6–9km based on offshore and onshore seismic data. These faults form a ca. 2km wide crestal collapse graben and pierce the seafloor. Salt wall evolution started in the early Late Triassic to Early Jurassic due to regional extension and resulting sub-salt faulting. The salt wall was eroded following exposure to costal and sub-aerial erosion by the regional Mid-Late Jurassic to Early Cretaceous uplift. Late Cretaceous to Early Paleogene compressional tectonics reactivated the vertical salt movement and shortened the salt wall, creating a salt-cored anticline with the crestal collapse graben above. The supra-salt faults were reactivated between the Late Eocene and Middle Miocene when the principal horizontal stress orientation changed from a NE–SW to a NW–SE, the present-day orientation. Stratigraphic data indicate that these faults moved mainly in the Cenozoic. Several observations strongly suggest that the faults continued developing during the Pleistocene until today: (i) the Pleistocene Unconformity is concave upwards and cut by faults; (ii) growth strata within the marine Holocene deposits above the graben imply recent tectonic movements; (iii) onshore high-resolution P-wave vibroseis data of the south-eastern Eckernförde Bay suggest about 10m of faulted Holocene strata; and (iv) marine seismic data show the faults piercing the seafloor. We suggest that the recent salt tectonics and upward propagation of supra-salt faults resulted from differential ice-sheet loading. That effect on the salt wall stopped once ice grew over the whole structure, at which time the wall subsided because of ice loading. The salt wall and faults were reactivated again once the ice front retreated so that the ice loaded only one side of the structure.

Production optimization in fractured geothermal reservoirs by coupled discrete fracture network modeling

In this work, a stimulation then heat production optimization strategy is presented for prototypical EGS geothermal reservoirs by comparing conventional stimulation-then-production scenarios against revised stimulation schedules. A generic reservoir is selected with an initial permeability in the range of 10−17–10−16m2, fracture density of ∼0.09m−1 and fractures oriented such that either none, one, or both sets of fractures are critically stressed. For a given reservoir with a pre-existing fracture network, two parallel manifolds are stimulated that are analogous to horizontal wells that allow a uniform sweep of fluids between the zones. The enhanced connectivity that develops between the injection zone and the production zone significantly enhances the heat sweep efficiency, while simultaneously increasing the fluid flux rate at the production well. For a 10m deep section of reservoir the resulting electric power production reaches a maximum of 14.5MWe and is maintained over 10 years yielding cumulative energy recoveries that are a factor of 1.9 higher than for standard stimulation. Sensitivity analyses for varied fracture orientations and stimulation directions reveal that the direction of such manifolds used in the stimulation should be aligned closely with the orientation of the major principal stress, in order to create the maximum connectivity. When the fractures are less prone to fail, the output electric power is reduced by a decrease in the fluid flux rate to the production well.

Coseismic Fault Model of Mw 8.3 2015 Illapel Earthquake (Chile) Retrieved from Multi-Orbit Sentinel1-A DInSAR Measurements

On 16 September 2015, a Mw 8.3 interplate thrust earthquake ruptured offshore the Illapel region (Chile). Here, we perform coseismic slip fault modeling based on multi-orbit Sentinel 1-A (S1A) data. To do this, we generate ascending and descending S1A interferograms, whose combination allows us to retrieve the EW and vertical components of deformation. In particular, the EW displacement map highlights a westward displacement of about 210 cm, while the vertical map shows an uplift of about 25 cm along the coast, surrounded by a subsidence of about 20 cm. Following this analysis, we jointly invert the multi-orbit S1A interferograms by using an analytical approach to search for the coseismic fault parameters and related slip values. Most of the slip occurs northwest of the epicenter, with a maximum located in the shallowest 20 km. Finally, we refine our modeling approach by exploiting the Finite Element method, which allows us to take geological and structural complexities into account to simulate the slip along the slab curvature, the von Mises stress distribution, and the principal stress axes orientation. The von Mises stress distribution shows a close similarity to the depth distribution of the aftershock hypocenters. Likewise, the maximum principal stress orientation highlights a compressive regime in correspondence of the deeper portion of the slab and an extensional regime at its shallower segment; these findings are supported by seismological data.

Mean and dispersion of stress tensors using Euclidean and Riemannian approaches

Stress is central to many aspects of rock mechanics, and in the analysis of in situ stress measurement data the calculation of the mean value and an assessment of dispersion are important for statistical characterisation. Currently, stress magnitude and orientation are processed separately in such analyses. This effectively decomposes the second-order stress tensor into scalar (principal stress magnitudes) and vector (principal stress orientations) components, and calculation of mean and dispersion of stress data on the basis of these decomposed components, which violates the tensorial nature of stress, may either yield biased results or be difficult to conduct. Here, by introducing tensorial techniques, we examine two calculation approaches for the mean and dispersion for stress tensors – based on Euclidean and Riemannian geometries – and discuss their similarities, differences and potential applicability in engineering practice. We compare the two approaches using stress tensor superposition and interpolation, and the analysis of actual in situ stress data. The results indicate that Euclidean and Riemannian mean tensors are in general not equal, with the disparity increasing as stress tensor dispersion increases. Both Euclidean and Riemannian approaches are shown to be capable of characterising stress dispersion, although Euclidean dispersion is scale dependent and has units of stress whereas Riemannian dispersion is a scale independent unitless number. Finally, a paradox is revealed in that despite stress tensors being Riemannian entities, it is Euclidean mean stress that is the more meaningful for engineering applications.

Full in-plane strain tensor analysis using the microscale ring-core FIB milling and DIC approach

Microscale Full In-plane Strain Tensor (FIST) analysis is crucial for improving understanding of residual stress and mechanical failure in many applications. This study outlines the first Focused Ion Beam (FIB) milling and Digital Image Correlation (DIC) based technique capable of performing precise, reliable and rapid quantification of this behaviour. The nature of semi-destructive FIB milling overcomes the main limitations of X-Ray Diffraction (XRD) strain tensor quantification: unstrained lattice parameter estimates are not required, analysis is performed in within a precisely defined 3D microscale volume, both amorphous and crystalline materials can be studied and access to X-ray/neutron facilities is not required.The FIST FIB milling and DIC experimental technique is based on extending the ring-core milling geometry to quantify the strain variation with angle and therefore benefits from the excellent precision and simple analytical approach associated with this method. In this study in-plane strain analysis was performed on sample of commercial interest: a porcelain veneered Yttria Partially Stabilised Zirconia (YPSZ) dental prosthesis, and was compared with the results of XRD.The two methods sample different gauge volumes and mechanical states: approximately plane stress for ring-core milling, and a through-thickness average for XRD. We demonstrate using complex analysis methods and Finite Element (FE) modelling that valid comparisons can be drawn between these two stress states. Excellent agreement was obtained between principal stress orientation and magnitudes, leading to realistic residual stress estimates that agree well with the literature (σAv≈460MPa). As a measure of validity of the matching approach we report the upper and lower bounds on the (101) interplanar spacing of YPSZ that are found to correspond to the range 2.9586−2.9596Å, closely matching published values.

Stress state of the Baoxing segment of the southwestern Longmenshan Fault Zone before and after the Ms 7.0 Lushan earthquake

In situ stress measurements were conducted with hydraulic fracturing and piezomagnetic overcoring method in a borehole at Qiaoqi of Baoxing region in the southwestern Longmenshan Fault Zone, to understand the current stress state and stress change after the Ms 7.0 Lushan earthquake. The stress regime of the Qiaoqi borehole is characterized by SH>Sh>Sv, indicating that the regional stress field is dominated by the maximum horizontal stress and this stress regime is prone to reverse faulting. Impression tests show that the orientations of the maximum horizontal principal stress are NW–NWW oriented. The magnitudes of the maximum horizontal principal stress at Qiaoqi are obviously higher than those before the Lushan earthquake, signifying that stress is still accumulating in this region. The real-time stress monitoring data shows that the stress in the NWW direction is increasing continuously before and after the earthquake. Authors have computed the frictional parameter, μm, using the derived stress data. The result demonstrates a high stress build-up level in the shallow crust before and after the Lushan earthquake. Adopting the Coulomb frictional-failure criteria, we conclude that Baoxing area, the southwestern section of the Longmenshan Fault Zone has already reached or exceeded a frictional limit equilibrium state. Evidence shows that the Lushan earthquake did not release the highly accumulated stress of the southwestern Longmenshan Fault Zone and the potential risk of earthquakes in this region still exists.

Calculating regional stresses for northern Canterbury: the effect of the 2010 Darfield earthquake

ABSTRACTWe model regional stresses before and after the Mw 7.1 Darfield earthquake of September 2010 in Canterbury, New Zealand including crustal structure derived from seismic tomography. Models show that the Banks Peninsula volcanic assemblage acts as a strong, rigid block that pinches out ductile layers in the mid-crust but has little effect on shallower principal stress orientations. Static stress changes from the Darfield earthquake are everywhere <25 MPa except within 5 km of the fault. When added to regional stresses, these create only small rotations of <5° in the orientation of maximum horizontal stress SHmax, even near the fault. Predicted stress rotations do not correlate strongly with those inferred from aftershock focal mechanisms. The perturbations caused by earthquake stresses are not significant enough to explain either the magnitude or the sense of SHmax rotations near the fault at seismogenic depths.

Horizontal principal stress orientation in the Costa Rica Seismogenesis Project (CRISP) transect from borehole breakouts

AbstractThe Costa Rica Seismogenesis Project (CRISP) drilled the Pacific margin of the Middle America Trench just north of where the Cocos Ridge enters the subduction zone, resulting in basal erosion of the upper plate. Here we report the orientations of the maximum horizontal principal stress (SHmax) from borehole breakouts detected by logging‐while‐drilling and wireline downhole measurements. All SHmax directions were estimated in the sediment cover of the margin, above the deeper rocks of the deformed margin wedge. We observe three overall SHmax orientations: NNE‐SSW (25° azimuth) in the deepest interval drilled at the upper slope Site U1379; ENE‐WSW (82°) in the rest of Site U1379 and in Site U1413, also drilled in the upper slope; and NNW‐SSE (157°) in the mid‐slope Site U1378. Our preferred interpretation is that the deepest interval of Site U1379 records the stress conditions in the underlying margin wedge, as SHmax is parallel to the direction of the Cocos‐Caribbean plate convergence and of the compressional axes of plate boundary fault earthquakes. The variable SHmax directions observed elsewhere are likely due to the effect of a network of normal faults that subdivide the sediment cover into a number of independently deforming blocks. In addition, the observed SHmax directions may be influenced by the subducting Cocos Ridge, which acts as an indenter causing oblique deformation, and by the transition to seismogenic subduction along the plate boundary fault.

Determination of fracture mechanics parameters structural components with surface crack under thermomechanical loads

Rotational components of aero engine turbine are the most important components. It operates at high temperature and under conditions of extreme environmental attack such as oxidation, corrosion and wear. These conditions can cause cracking of rotational components. The failure damage modes of turbine are classified in terms of main components as flow path parts, rotating such as rotor, groove, disk, and blade. Aero-engine turbine components such as discs and blades are susceptible to environmentally assisted cracking. Unlike fatigue crack growth, this involves crack growth under constant load. If the crack grows long enough, sudden failure can occur with catastrophic consequences. It is therefore desirable to identify the limiting crack size within fixings so that they can be inspected at regular intervals and removed from service before failure occurs. Three dimensional axis-symmetric finite element models were created to simulate a disc and the portion of a blade. The finite element method (FEM) allowed the prediction of the point of crack initiation and the crack propagation using the orientations of the maximum principal stresses. Stress intensity factor (SIF) is the base parameter in strength analysis regarding fracture mechanics. For a correct determination of SIF in this paper, combining J-integral approach and FEM is used. J-integral is a path independent integral around the crack tip.

2014Mw 6.0 South Napa Earthquake Triggered Exotic Seismic Clusters near Several Major Faults

Online Material: Figures of background seismicity rate and magnitude of completeness, 95% confidence interval on the principal stress orientation and friction coefficient, influence of fault friction on the Coulomb stress imparted to the major mapped active faults, shear-stress changes resolved on the nodal planes of small earthquakes, and observed aftershock rates. The strongest San Francisco Bay area earthquake since the 1989 M w 7.0 Loma Prieta shock struck near Napa on 24 August 2014. Field mapping (Dawson et al. , 2014; Earthquake Engineering Research Institute [EERI], 2014; Brocher et al. , 2015) and seismic and geodetic source inversions (Barnhart et al. , 2015; Dreger et al. , 2015; Wei et al. , 2015) indicate that a 15-km-long northwest-trending section of the West Napa Valley fault ruptured in the earthquake. Remarkably, it was the first indisputable surface rupture in the Bay area since 1906. The Napa event, along with other smaller earthquakes such as the 1980 M w 5.8 Livermore and 1984 M w 6.2 Morgan Hill events on the Calaveras and Hayward faults over the past 3–4 decades, may indicate that the Bay area region is emerging from the stress shadow of the 1906 M w 7.8 San Francisco earthquake (Harris and Simpson, 1998; Pollitz et al. , 2004). Since 1979, there has been a 140% increase in the rate of M w≥4.1 shocks (Fig. 1) in the broader Bay area, with most concentrated in a corridor extending north from the 1989 Loma Prieta aftershock zone through the Calaveras, Greenville, Green Valley, Napa, and Rodgers Creek faults east of the San Francisco Bay (Fig. 1a). This corridor roughly coincides with the 1906 stress shadow that is being eroded away by more than a century of stress reaccumulation. The Napa event, as well as the surrounding faults on which we calculate …

PRESENT‐DAY CRUSTAL STRAIN RATE FIELD IN THE TAIWAN ACTIVE COLLISION ZONE AND ITS GEODYNAMIC MECHANISMS: INSIGHT FROM FEM SIMULATIONS

AbstractThe Taiwan Island is the product of convergence and collision between the Eurasian and the Philippine Sea plates, where the geological structure is complex, seismicity is high and deformation strong. In order to quantitatively study characteristics of crustal deformation and to understand its geodynamic mechanisms of this area, we calculated the strain rate field in and around Taiwan by using the finite element method (FEM), utilizing GPS data from 1995 to 2005 as boundary constraints in simulation. The results show that contractions are predominant in central Taiwan, while extension exists in northeast and south, respectively. The largest deformation appears on the Coast Range and the adjacent waters in eastern Taiwan. The calculated slip rates on the Longitudinal Valley Fault (LVF) are 13.81∼23.48 mm/yr, meaning part of the convergence is absorbed by LVF, thus deformation to the west of LVF decays rapidly westwards and northwestwards. Moreover, the results show that the calculated velocities and GPS vectors are in good agreement, and calculated orientations of principal stresses are consistent with in‐situ stress measurements and focal mechanism solutions, demonstrating the finite element model established in this paper is reasonable. In addition, the calculated results imply that the general framework of present‐day deformation in Taiwan results from interactions by many factors such as plate collision between the Eurasian and the Philippine Sea plates, geometry of plate boundaries, faulting and rifting, opening of the Okinawa trough and retreat of the Ryukyu trench.

Particle Flow Simulation on Collapse Characteristics of Silt Under Mutation of Principal Stress Orientation

The behaviors of granular material and influencing factors under complex dynamic loading are studied by more and more researchers with particle flow method. Only the strain-controlled loading has been generally used in the current study, although this method was not consistent with the practice of engineering in many situations. In this article, stress-controlled dynamic simulation tests were carried out with particle flow method, which were used to study the collapse characteristics of silt under mutation of principal stress orientation. The tests were performed by PFC2D. The simulation results and the laboratory real tests’ results had a high degree of similarity, particularly in the collapse strain and vibration times. It was very useful to forecast the silt's critical failure state. Based on the verification data, the effects of confining pressure and cyclic shear stress ratio on the collapse characteristics of silt were studied further. With the increase of cyclic shear stress ratio, the deviator strain amplitude increased and the required vibration times gradually reduced to achieve the same strain level. Under the same dynamic shear stress ratio and vibration times, the initial dynamic elastic modulus slightly increased with the increase of initial confining pressure, and the variation range of final collapse deviator strain was small. In the analysis of micro-structural evolution, the redistribution of internal stress of sample was revealed during cyclic loading. With the increase of vibration times, the development of distribution gradually stabilized, and then the high shear stress appeared in some connected regions. On that stage, the particle system developed to instability and failure. The PFC simulation results confirmed that the collapse state was the critical stage to trigger the liquefaction of silt.