Direction Of Horizontal Principal Stress Research Articles

열-역학적 연계해석 모델을 이용한 다중 열저장공동 안정성 분석

암반공동을 이용한 열에너지 저장은 대용량 저장이 가능하며 열저장매체를 선택할 수 있는 장점이 있다. 본 연구에서는 사일로 형태의 열저장공동이 지반 내 두 개 이상 배치될 때 공동 사이에 형성되는 암반 필라의 안정성에 대해 3차원 유한차분해석 프로그램인 <TEX>$FLAC^{3D}$</TEX>를 이용하여 분석하였으며, 저장된 열에너지로 인해 암반에 발생하는 열응력을 반영할 수 있도록 열-역학적 연계모델을 사용하였다. 해석 결과, 열에너지 장기 저장으로 인해 암반 필라에 작용하는 최대주응력이 상당량 증가하였으며, 필라 폭이 좁아질수록 근접한 열원 때문에 열응력 증가량도 커짐을 확인하였다. 필라 안정성에 영향을 미치는 주요인자로서 저장공동 간격, 측압계수, 심도를 선정하고 민감도 분석을 실시한 결과, 측압계수, 저장공동 간격, 심도 순서로 영향력이 크게 평가되었다. 저장공동 간격의 경우 동일한 크기의 공동 건설 시 필라 폭을 최소 저장공동 직경 이상 확보해야 할 것으로 판단되었다. 큰 규모의 저장공동 주변에 소규모 수직갱이 설치될 때는 최소한 저장공동 직경의 0.5배 이상 이격함으로써 크기 차이로 인해 수직갱에 응력이 집중되는 현상을 해소할 수 있었다. 또한 최대수평주응력 작용방향과 공동 중심을 잇는 축이 평행하도록 배치하여 저장공동에 의한 방패효과가 발휘될 수 있게 함으로써 현지응력이 공동 사이 암반 필라에 미치는 영향을 최소화할 수 있었다. Cavern Thermal Energy Storage system stores thermal energy in caverns to recover industrial waste heat or avoid the sporadic characteristics of renewable-energy resources, and its advantages include high injection-and-extraction powers and the flexibility in selecting a storage medium. In the present study, the structural stability of rock mass pillar between these silo-type storage caverns was assessed using a coupled thermal-mechanical model in <TEX>$FLAC^{3D}$</TEX>. The results of numerical simulations showed that thermal stresses due to long-term storage depended on pillar width and had significant effect on the pillar stability. A sensitivity analysis of main factors indicated that the influence on the pillar stability increased in the order cavern depth < pillar width < in situ condition. It was suggested that two identical caverns should be separated by at least one diameter of the cavern and small-diameter shaft neighboring the cavern should be separated by more than half of the cavern diameter. Meanwhile, when the line of centers of two caverns was parallel to the direction of maximum horizontal principal stress, the shielding effect of the caverns could minimize an adverse effect caused by a large horizontal stress.

Full-waveform based microseismic source mechanism studies in the Barnett Shale: Linking microseismicity to reservoir geomechanics

Seismic moment tensors (MTs) of microearthquakes contain important information on the reservoir and fracturing mechanisms. Difficulties arise when attempting to retrieve complete MT with conventional amplitude inversion methods if only one well is available. With the full-waveform approach, near-field information and nondirect waves (i.e., refracted/reflected waves) help stabilize the inversion and retrieve complete MT from the single-well data set. However, for events which are at far field from the monitoring well, a multiple-well data set is required. In this study, we perform the inversion with a dual-array data set from a hydrofracture stimulation in the Barnett Shale. Determining source mechanisms from the inverted MTs requires the use of a source model, which in this case is the tensile earthquake model. The source information derived includes the fault plane solution, slip direction, VP/VS ratio in the focal area and seismic moment. The primary challenge of extracting source parameters from MT is to distinguish the fracture plane from auxiliary plane. We analyze the microseismicity using geomechanical analysis to determine the fracture plane. Furthermore, we investigate the significance of non-DC components by F-test. We also study the influence of velocity model errors, event mislocations, and data noise using synthetic data. The results of source mechanism analysis are presented for the events with good signal-to-noise ratios and low condition numbers. Some events have fracture planes with similar orientations to natural fractures delineated by core analysis, suggesting reactivation of natural fractures. Other events occur as predominantly tensile events along the unperturbed maximum horizontal principal stress direction, indicating an opening mode failure on hydraulic fractures. Microseismic source mechanisms not only reveal important information about fracturing mechanisms, but also allow fracture characterization away from the wellbore, providing critical constraints for understanding fractured reservoirs.

Hydraulic fracturing experiments of highly deviated well with oriented perforation technique

Abstract. In order to investigate the effect of different perforation angles (the angle between the perforation direction and the maximum horizontal principal stress) on the fracture initiation and propagation during hydraulic fracturing of highly deviated well in oil & gas saturated formation, laboratory experiments of the hydraulic fracturing had been carried out on the basis of non-dimensional similar criteria by using 400^3 mm 3 cement cubes. A plane fracture can be produced when the perforations are placed in the direction of the maximum horizontal principal stress. When the perforation angle is 45°, the fractures firstly initiate from the perforations at the upper side of the wellbore, and then turn to the maximum horizontal principal stress direction. When the well deviation angle and perforation angle are both between 45° and 90°, the fractures hardly initiate from the perforations at the lower side of the wellbore. Well azimuth (the angle between the wellbore axis and the maximum horizontal principal stress) has a little influence on the fracture geometries; however it mainly increases the fracture roughness, fracture continuity and the number of secondary fractures, and also increases the fracture initiation and propagation pressure. Oriented perforating technology should be applied in highly deviated well to obtain a single plane fracture. If the well deviation angle is smaller, the fractures may link up.

Prediction of the horizontal stress of the tight sandstone formation in eastern Sulige of China

In recent years, with the advancement of exploitation for domestic tight gas reservoir, more and more attention has been paid to the borehole stability based on the quantitative prediction of ground stress. In this paper, related research has been made in a tight sandstone gas reservoir in eighth segment of Shihezi group (P1x) and first segment of Shanxi group (P1s) in Eastern Sulige of Inner Mongolia; limitation of application of Huang model in the area has been discussed on the basis of an in-depth analysis of the geological character according to the logging information, and a new optimized Newberry model has been obtained. By using this model, the horizontal ground stress of this region was quantitatively forecasted with minor error and the results prove that the Newberry model can perfectly fit the quantitative calculation of horizontal ground stress. In the meantime, according to borehole collapsed technique and the analysis of the trend of induced slot, the horizontal ground stress direction has been determined. The study shows that the maximum horizontal principal stress direction in eastern Sulige is close to east–west and the minimum horizontal principal stress direction is close to north–south. The elaborate research about the horizontal stress for the tight sandstone formation in eastern Sulige in this paper can meet the need of judgment of ground stress better in this area and it is of great significance to the safety of drilling of profitable reservoir in this area, reservoir fracturing and stimulating, and the safety of production in the subsequent process of development.

In situ stress measurements by hydraulic fracturing in the Western Route of South to North Water Transfer Project in China

This technical note summarizes and discusses the results of the in situ stress measurements by hydraulic fracturing conducted in 7 holes at depths ranging from 21 to 111m in planned dam sites, and in 13 holes at depths varying from 28 to 532m in planned tunnel sites in the Western Route of South to North Water Transfer Project in China. The measured maximum and minimum horizontal principal stresses increase linearly with depth in tunnel sites, and piecewise-linearly in dam areas. The horizontal stresses are greater than the vertical stress, indicating a thrust faulting regime at all tested locations. Such a stress regime is confirmed by local geological structures. The direction of the maximum horizontal principal stress is generally NEE–SWW, which is highly correlated with the tectonic plate movement and crustal deformation in the study area, and consistent with that revealed by focal mechanism solutions from earthquakes. The results in the present study may provide in situ stress information for the design of planned dams and tunnels, and make a good complement to the World Stress Map.

The Experimental Study on the &lt;i&gt;In Situ&lt;/i&gt; Stress of SongNan Block

In view of the low pressure tight gas reservoir in Songnan block, the comprehensive experiment of in-situ stress is carried out. Firstly, the tuffaceous breccia of Longshen 301 and 307 has been cored and the flag line is depicted. Through the viscous remanence experiment, the secondary viscous remanence component at 0°C~200°C is gradually separated, and the average direction of the two groups core flag line are obtained, which are 92.0° and 114.7°. Then to mark the flag line as the baseline, using the wave velocity anisotropy experiment to measure the acoustic wave velocity under different phase angle, the minimum wave velocity phase angle of the two groups core are achieved, which are 23° and 44° . And combined with the direction of the flag line, the direction of maximum horizontal principal stress are determined for N69o E and N70.7o E. Finally, using DSA (differential strain) experiment, the strain recovery of 9 direction under hydrostatic pressure are measured, and the three principal strain, the magnitude and direction of the principal stress are obtained through the inversion, the maximum principal stress direction of which are N70.8o E and N71.7o E. Compared the wave velocity anisotropy experiments and DSA experimental results, both close, the direction of the regional maximum horizontal in-situ stress is determined for N70.5° E ± 1.5°. According to the above research results, the basis for the engineering design of Songnan block such as oil and gas exploration, development, drilling and production is provided.

Local stress sources in Western Europe lithosphere from geoid anomalies

We propose a method to evaluate the stress generated at the local scale by the spatial variations of the gravitational potential energy (GPE), which is related to inhomogeneous topography and mass distribution in the lithosphere. We show that it is possible to infer these local stress sources from the second spatial derivatives of a geoid height grid, used as a proxy of the GPE. The coherence of the method is validated on a passive margin, the Bay of Biscay. The result is that expected in such a geological configuration, with extensional local stress sources with the maximum horizontal principal stress parallel to the margin and compressive sources with the maximum horizontal principal stress perpendicular to the margin in the continental and oceanic lithosphere, respectively. We apply the method to Western Europe in order to provide a better understanding of the complex spatial variation of the present-day tectonic activity. Our results indicate a stress pattern from the local sources dominated by short-space-wavelength (of the order of a few tens of kilometers) variations in the tectonic style and in the direction of the maximal horizontal principal stress σ H . A comparison of the σ H orientations and tectonic style from the local sources with the ones of the World Stress Map (WSM) data set indicates that the local stress sources can be representative of the deviatoric stress state in some regions. Our results explain 71% of the faulting styles for the earthquake fault-plane solutions in the WSM, which is better than the classical compressive NW-SE stress field model. In the central part of the Pyrenees, the agreement between earthquake fault-slip directions and the direction of shear stress from the local sources acting on the associated fault planes is compatible with the extensional stress field evidenced by recent investigations.

Verification for Fault Stability by In-Situ Stress Test

In order to verify the fault stability of a tunnel in Fujian, hydro-fracturing in-situ stress measurement was conducted. The test revealed the stress magnitude and the maximum horizontal principal stress direction in testing area, which have obvious similarity with other results in intact rock mass. According to the horizontal stress distribution relationship with depth, two points’ stress state represent the fault was calculated. At last, based on the selected sliding criterion, the stability of faults was verified in this paper.

SATI algorithm — the calculation of stress aligned HTI stiffness tensor for sandstone reservoir from wireline data

I present an algorithm that uses cross-dipole wireline data only in order to estimate the HTI stiffness tensor for sandstone formations under in-situ asymmetric lateral (azimuthal) stress conditions. The algorithm is based on the generalization of terms “excess compliance” and “fracture weakness” developed within the linear slip interface theory for fractured rocks and is applied here to describe the effect of grain contacts in loose sandstones. I introduce the term “plane of weakness” being oriented (aligned) orthogonal to the minimal horizontal principal stress direction in order to describe the overall effective weakness of sandstone caused by the different principal stresses. For the quantification of this phenomenon I use the anisotropic Gassmann model. As a result I am able to calculate a HTI stiffness tensor for the interval length of a saturated sandstone formation and the respective Thomsen’s parameters. The input data required for these calculations have to be provided by wireline logging and will consist of porosity, density, P-wave velocity, fast and slow shear wave velocities and oil-water saturation ratio. The algorithm in its current form is applicable to sandstone reservoirs only. Its limitation is based on two assumptions, which state that all the measured anisotropy is induced by the present stress in sandstone and that the unstressed sandstone would be nearly isotropic. From a technical viewpoint this algorithm can be implemented fairly easily in data acquisition and interpretation software relying on correct estimation of anisotropy parameters. It is also cheap because it does not require any additional measurements apart from the cross-dipole logging.

Simulation of ground stress field and fracture anticipation with effect of pore pressure

Triaxial compression tests are carried out on the cores with different confining pressure and pore pressure to study the rupture mode and fracture distribution of carbonate rocks in Kenkiyak pre-salt oilfield, and the cores are made into thin sections after experiment. It shows that shear plane, high angle crack, conjugate shear cracks and net fractures will gradually appear with the effective confining pressure, the rock texture is damaged more and more seriously with the increase of effective confining pressure. Tectonic stress field in Kenkiyak Field is simulated by finite element numerical simulation software ADINA considering the effect of pore pressure, this model contains five faults and assumes that two planes of faults could slip with the force to decompress. The simulation results indicate that the total displacement coincides with the practical formation, the simulated tectonic stress fits with the values measured by acoustic emission testing, and the direction of major horizontal principal stress is consistent with the imaging log interpretation data. The fracture rupture rate and density are predicted according to tension and shear rupture rate which derived by simulation results. The fracture density varies widely in the simulated region and cracks develop easily on the structural high position, near the fault because of the increasing pore pressure and extrusion in the process of the tectonic movements.

In‐Situ Stress Measurement and Tectonic Stress Field in the Lanzhou‐Maqu Region of China

AbstractThe study region is a strongly deformed frontal area of the Tibet plateau caused by the northeastward compression of Indian plate. The study on the present tectonic stress field in this region has important meaning for the research of continental dynamics. The values and directions of the ground stress at several sites of Lanzhou‐Maqu area are given in the paper using piezomagnetic stress gauge overcoring method. The measurement sites are situated in Ayishan, Dashui, Gahai, Ma'ai, and Qingshui. The previous stress data are also analyzed for systematically studying the tectonic stress field in the region of Lanzhou‐Maqu and its adjacent area. The result shows that the values of ground stress in depths of tens meters are medium as compared to that in other areas, and increase with depth, the increase rate differs in different tectonic units. The overall direction of maximum horizontal principal stress is northeast with a small variation in different tectonic units. The direction of maximum horizontal principal stress is nearly west‐east in Ordos block, NNW‐NE in Hexizoulang. The direction of maximum principal stress changes considerably in the southeast part of Qilanshan Mountains. The west Qinling Mountain is a transition zone of ground stress. The direction of maximum horizontal principal stress changes gradually from NE in the north to EW in the middle and SEE in the south part. The results agree with the strain field in this area from GPS observation.

전남 고흥 일대 백악기 화강암류의 아문미세균열을 이용한 백악기 말-신생대 3기 초 고응력장

Late Cretaceous to early Tertiary paleostress was evaluated by analyzing the healed microcracks in the Cretaceous granite of the Goheung area, south Korea. Healed microcracks in five granite samples (GH-1, GH-3, GH-4, GH-5, GH-8) were investigated and measured according to direction. The directions of maximum horizontal principal stress in GH-1, GH-3, and GH-4 are dominantly , while minor directions are N-S and . In GH-5 and GH-8, are the most dominant directions, while is the minor direction. Thus overall, the most dominant directions of healed microcracks in the study area are oriented , while minor directions are oriented , essentially NE. Combining the paleostress results of this study with other studies, the direction of the maximum horizontal principal stress in the study area during the late Cretaceous to the early Tertiary should perhaps be changed WNW to NE. The reason for this is thought to be the complex tectonic movements which occurred in northeast Asia at that time.

Seismic Anisotropy in the Crust in Northwestern Capital Area of China

AbstractSeismic anisotropy is obtained in the crust in northwestern capital area by shear‐wave splitting analysis, using the SAM technique. The seismic data was recorded at the Capital Area Seismic Network from Jan. 2002 to Dec. 2003. The results at 14 stations in all, every of which has at least 3 records available or more, are statistically discussed in this paper. The statistical results show that the average polarization of fast shearwaves is NE69.9° ±44.5° and the time delay of slow shear‐waves is 4.44±2.93 (ms/km). The average polarization of fast shear‐waves of NE69.9° ±44.5° suggests the direction of maximum horizontal principal compressive stress in this area. The most predominant polarization direction of fast shear‐wave suggests the tectonic implication of horizontal principal compressive stress at the direction NWW or nearly E‐to‐W, which exposes the Zhangjiakou‐Penglai depression fault zones with strike NWW. According to the polarization of fast shear‐wave, this study verifies that the predominant polarizations of fast shear‐wave at stations on active faults are consistent with fault strike. Possibly, both the Nankou‐Sunhe fault and Xiadian fault are two active faults while the Babaoshan fault is possibly a less active fault. The polarizations of fast shear‐wave in the North China Basin show the complexity, consistent with the complicated pattern of regional principal compressive stress controlled locally, induced by many faults crossing in the depression zone within the basin. This study also suggests that the quick change of time delays of slow shear‐waves is possibly related to the temperature change in deep crust.

Compaction bands and the formation of slot-shaped breakouts in St. Peter sandstone

Abstract Drilling of vertical wellbores in sandstone often results in stress-induced failed zones called breakouts. In the laboratory, miniature drilling under simulated far-field crustal stress conditions above a certain threshold produces breakouts that can be studied to enhance our understanding of deformation and failure mechanisms in rock. We report on drilling experiments in St. Peter sandstone, an Ordovician aeolian rock consisting of well-rounded bimodal quartz grains held together by sutured contacts. Breakouts here are slot-shaped and oriented at right angles to the far-field maximum stress direction (σ H ), resembling emptied compaction bands. We distinguish between two porosity ranges in this rock. In the high-porosity variety (16–22%), grains are bonded through narrow sutures. In the vicinity of the borehole at points aligned with the far-field least horizontal principal stress (σ h ) direction, where the maximum compressive stress concentrates, grain sutures sever at relatively low stress levels, leading to intergranular cracking. Debonded intact grains repack and produce a reduced-porosity narrow compaction band. Loose grains in the band abutting the borehole wall are then flushed out, assisted by the circulating drilling fluid. This intensifies the stress concentration ahead of the breakout tip, advancing the compaction band and lengthening the slot-shaped breakout. The process continues sequentially. Grains in the lower-porosity sandstone (11–12%) are more substantially sutured over much larger surface areas. This makes the sutures nearly as strong as the grains themselves; hence, failure requires considerably higher stresses, sufficient to bring about intragranular cracking. Consequently, and in contrast to the high-porosity sandstone, breakouts here are preceded by compaction bands of shattered and crushed grains. Both types of compaction bands have been encountered in the field and in independent laboratory experiments.

Characteristics of recent tectonic stress field in Jiashi, Xinjiang and adjacent regions

In this paper, we analyze the general directional features of regional tectonic stress field in Jiashi, Xinjiang and adjacent regions from the data of focal mechanism solutions, borehole breakouts and fault slip. The direction of maximum horizontal principal stress given by these three sorts of stress data differs slightly, which indicates there is a NS-trending horizontal compression in the tectonic stress field in the region of interest. We also invert and analyze the temporal and spatial changes of recent tectonic stress field in the research region by using 137 focal mechanism solutions. The inverted results show that the maximum principal stress σ1 in Jiashi and adjacent regions is NNW-SSE with an azimuth of 162°. In the period from 1997 to 2003 before the occurrence of Jiashi-Bachu earthquake, the directions of the maximum principal stress σ1 and the minimum principal stress σ3 in Jiashi seismic source zone changed clockwise with respect to the tectonic stress field in the regions around. The maximum principal stress σ1 adjusted to the direction of NNE-SSW with an azimuth of 25°. Under the control of this tectonic stress field, a series of earthquakes happened, including the Jiashi strong earthquake swarm in 1997. Then, the tectonic stress field in the Jiashi seismic source zone might adjust again. And the tectonic stress field controlling the Jiashi-Bachu earthquake in 2003 was in accordance with the regions around.

Geological and geotechnical investigation on in situ stress regime around the Yangsan Fault Zone in Korea

For the purpose of designing and analyzing the stability of the B-tunnel which is a part of the Korea Train Express (KTX) Kyungbu Line and is planned to pass through the existing Kyungbu Expressway as well as the Yangsan Fault zone, the in-situ stress regime on this site has been measured by hydraulic fracturing tests to compare with the geological survey results. The Yangsan Fault is one of the major faults in Korea. Apart from a fold that shows the ductile deformation behavior, a fault shows a characteristic of the brittle deformation behavior and it is very important to perform a drastic survey on the fault to get the information on shape, dimension, mechanical properties, and so forth. Considering the impossibility of any borehole tests in fault zone, the authors used the two boreholes which are excavated in a base rock at both sides of the Yangsan fault. The hydraulic fracturing test results are proven to be in accord with the geological survey results. In other words, the geological site investigation let us know that the direction of fault movement is NS to NNE, and the geotechnical results by hydraulic fracturing tests show that the direction of the maximum horizontal principal stress is 6 to about 24 degrees from the true north. In-situ stress measurements on and around the fault zone are known to be uncommon all over the world, and this study is the first case in Korea. In view of this, the authors insist that this result will be helpful to analyze the stability of the B-tunnel, and can be applied to other similar cases in tunnel construction field. (A) This paper was presented at Safety in the underground space - Proceedings of the ITA-AITES 2006 World Tunnel Congress and the 32nd ITA General Assembly, Seoul, Korea, 22-27 April 2006. For the covering abstract see ITRD E129148. “Reprinted with permission from Elsevier”.

Thin plate neotectonic models of the Australian plate

Thin plate finite element models of the neotectonic deformation of the Australian plate have been calculated in order to estimate the stress and strain rate within the plate, specifically concentrating on the Australian continent. The model includes plate‐bounding faults, an anelastic brittle‐ductile layered rheology and the option of laterally varying elevation and heat flow. The results of the models are compared to (1) the velocity of geodetic benchmarks on the Australian plate, (2) the spreading rate of the mid‐oceanic ridges along the Australian plate's margins, (3) the direction of the maximum horizontal principal stress, (4) the stress regime within the plate, and (5) the crustal thickness estimated from the depth to the base of Mohorovicic discontinuity's transition zone. A variety of models are tested with a wide range of input parameters. The model with the smallest misfit with observations predicts that the strain rate for most of the Australian continent is approximately 10−17 s−1. This model has a slightly lower strain rate in the central Australia and is higher off the northern coast of Australia than for the rest of the continent. Strain rates of this magnitude would be difficult to observe from geodetic or geologic data for most parts of Australia but would be enough to generate much of the seismicity that has been observed over the last century.

Stress change near the Kunlun fault before and after the Ms 8.1 Kunlun earthquake

On 14 November, 2001, a great earthquake of Ms 8.1 occurred on the Kunlun fault in the Kusai Lake segment of the Kunlun Mountain, Qinghai Province in northwest China. The stresses measured at the sites near the Kunlun fault before and after the earthquake show a great drop after the earthquake. The values of the horizontal maximum principal stress at two sites are 12.9 Mpa and 6.8 Mpa before the earthquake. Measurement of the horizontal maximum principal stress at the same sites after the earthquake, showed values of 3.5 Mpa and 2.2 Mpa respectively. This is only about one‐third of that before the earthquake. Apparently stress that accumulated near the fault before the earthquake was released along with the rupturing of the surface. The direction of the maximum horizontal principal stress also varies before and after the earthquake. The direction is found to deviate from that of the regional structure stress field after the earthquake. Such stress changes offer a basis for predicting large earthquakes in the region.

Neotectonic modelling of the western part of the Africa–Eurasia plate boundary: from the Mid-Atlantic ridge to Algeria

In this work we use the thin-shell approximation to model the neotectonics of the western part of the Africa–Eurasia plate boundary, extending from the Mid-Atlantic ridge to Tell Atlas (northern Algeria). Models assume a nonlinear rheology and include laterally variable heat flow, elevation, and crust and lithospheric mantle thickness. Including the Mid-Atlantic ridge permits us to evaluate the effects of ridge push and to analyse the influence of the North America motion on the area of the Africa–Eurasia plate boundary. Ridge push forces were included in a self-consistent manner and have been shown to exert negligible effects in the neotectonics of the Iberian Peninsula and northwestern Africa. Different models were computed with systematic variation of the fault friction coefficient. Model quality was scored by comparing predictions of anelastic strain rates, vertically integrated stresses and velocity fields to data on seismic strain rate computed from earthquake magnitude, most compressive horizontal principal stress direction, and seafloor spreading rates on the Mid-Atlantic ridge. The best model scores were obtained with fault friction coefficients as low as 0.06–0.1. The velocity boundary condition representing spreading on the Mid-Atlantic ridge is shown to produce concentrated deformation along the ridge and to have negligible effect in the interior of the plates. However, this condition is shown to be necessary to properly reproduce the observed directions of maximum horizontal compression on the Mid-Atlantic ridge. The maximum fault slip rates predicted by the model are obtained along the Mid-Atlantic ridge, Terceira ridge and Tell Atlas front. Relatively high slip rates are also obtained in the area between the Gloria fault and the Gulf of Cadiz. We infer from our modelling a significant long-term seismic hazard for the Gloria fault, and interpret the absence of seismicity on this fault as possibly due to transient elastic strain accumulation. The present study has also permitted better understanding of the geometry of the Africa–Eurasia plate boundary from the Azores triple junction to the Algerian Basin. The different deformational styles seem to be related to the different types of lithosphere, oceanic or continental, in contact at the plate boundary.

Determination of Stress State from Focal Mechanisms of Microseismic Events Induced During Hydraulic Injection at the Hijiori Hot Dry Rock Site

The stress state at the Hijiori hot dry rock site was estimated based on the inversion from focal mechanisms of microseismic events induced during hydraulic injection experiments. The best fit stress model obtained by inverting 58 focal mechanisms of seismic events simultaneously indicates that the maximum principal stress σ 1 is vertical, while the minimum principal stress σ 3 is horizontal and trends north-south. The average misfit between the stress model and all the data is 6.8°. The inversion results show that the average misfit is small enough to satisfy the assumption of homogeneity in the focal mechanism data and that the 95% confidence regions of σ l and σ 3 are well constrained, i.e., they do not overlap, suggesting that the inversion results are acceptable. The stress estimates obtained by the focal mechanism inversion essentially agree with other stress estimates previously obtained. It is therefore concluded that the focal mechanism inversion method provides a useful tool for estimating the stress state. The hypocentral distributions of microseismic events associated with the hydraulic fracturing experiments are distributed around the plane that spreads to almost east-west from the injection wells and declines to the north at a high angle. The vertical orientation and east-west strike of the seismic events are essentially coplanar with the caldera ring-fault structure in the southern portion of the Hijiori Caldera. This indicates that tensile fractures of intact rock were not being created, but pre-existing fractures were being re-opened and developed in the direction of the maximum horizontal principal stress, although microseismic events were caused by shear failures.