Principal Stress Orientations Research Articles

Theoretical analysis of cross‐joint geometries and their classification

One commonly observed bedrock joint pattern consists of cross joints that terminate against an older set of longer systematic joints. Dyer [1988] showed that unique cross‐joint geometries may develop in response to a combination of remote stresses and local perturbations of the stress field in the vicinity of preexisting joints. We expand upon Dyer's analysis by providing the general solutions for cross‐joint geometry as a function of remote principal stress orientations and ratios. Specific cross‐joint geometries are determined by solving for the local cross‐joint angle as a function of distance from the preexisting joint. On the basis of theoretical derivations, cross‐joint geometries are grouped into five main categories: curving parallel, curving perpendicular, quasi‐curving parallel, quasi‐curving perpendicular and noncurving geometries. Furthermore, by discussing the stress state in the vicinity of the preexisting joint, a general expression for the dimension of compressive zone is derived, and a more detailed classification of cross‐joint geometry is provided. Results from this study provide a method to estimate the relative magnitudes of remote principal stresses simply based on the observed cross‐joint geometry, which may help constrain the tectonic history of a region. In addition, knowledge of precise cross‐joint curvature and intersection geometries may provide important insights into their connectivity and fluid flow characteristics.

A graphical technique to predict slip along a pre-existing plane of weakness

A graphical technique is proposed to determine whether a pre-existing plane of weakness will be reactivated by slip under a stress field. This technique is based on Coulomb-Navier criteria and the method of Yin and Ranalli (Yin, Z., Ranalli, G., 1992. Critical stress difference, fault orientation and slip direction in anisotropic rocks under non-Andersonian stress systems. J. Struct. Geol. 14, 237–244). It consists of calculating which mechanism, rupture or sliding, needs the smaller stress difference to liberate the deformation. Using the results of calculations over a wide range of plane orientations, we plotted, in an equal-area net, the line which separates the orientation fields where rupture needs less stress difference from the fields where slip on pre-existing plane is favored. We named these plots slip-rupture graphs. For the three Andersonian fault regimes, the graphs are presented as dendrograms. These dendrograms show the variation of the range of orientations favorable for reactivation as a function of cohesion and friction of the plane of weakness, depth, pore fluid pressure and the stress ratio. The slip-rupture graphs are compared with the Mohr diagram and slip-tendency graphs (Morris et al., 1996). Relative to Mohr diagrams, our graphs have the advantage that it is possible to work with geographic orientations of planes and principal stresses, and it is not necessary to transform the field data to a stress space. The slip-rupture graphs are similar to slip-tendency graphs; however, the former can lead to estimate physical parameters that make reactivation possible along planes with unfavorable orientations.

Finite Element Analysis Applied to Bone Remodelling in Revision Arthroplasty of the Femur using Morselized Allograft and Cement

Finite element models have extensively been developed and reported for primary cemented and uncemented total hip replacement, but not for revision arthroplasty of the hip. A two-dimensional equivalent thickness model was developed of the proximal femur with bone grafts, a thin but uniform cement layer and a standard femoral component. This model represents the reconstructed femoral revision setting described by Gie et al (1). The effects of surface texture, cement-stem friction, cement creep and stem malplacement were evaluated. Within the range of coefficients of friction which was investigated in this study (0.03, 0.136 and 0.3) it was observed that the influence of the value of the coefficient upon the stresses in the bone-implant structure are less than 5%. Also static cement creep and certain stem malalignments which were simulated showed a relatively small influence upon the stress distribution within this structure. An agreement between trabecular orientations in the remodeled bone allograft and the principal stress orientations in this structure was observed in the calcar region. It was concluded that the remodelling which occurs in the bone allografts follows Wolff's law of optimal material use in the trabecular structure.

Contrasting hydrogeologic regimes along strike-slip and thrust faults in the Oregon convergent margin: Evidence from the chemistry of syntectonic carbonate cements and veins

Research Article| January 01, 1998 Contrasting hydrogeologic regimes along strike-slip and thrust faults in the Oregon convergent margin: Evidence from the chemistry of syntectonic carbonate cements and veins James C. Sample; James C. Sample 1Department of Geological Sciences, California State University, Long Beach, California 90840-3902 Search for other works by this author on: GSW Google Scholar Mary R. Reid Mary R. Reid 2Department of Earth and Space Sciences, University of California, Los Angeles, California 90095-1567 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1998) 110 (1): 48–59. https://doi.org/10.1130/0016-7606(1998)110<0048:CHRASS>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation James C. Sample, Mary R. Reid; Contrasting hydrogeologic regimes along strike-slip and thrust faults in the Oregon convergent margin: Evidence from the chemistry of syntectonic carbonate cements and veins. GSA Bulletin 1998;; 110 (1): 48–59. doi: https://doi.org/10.1130/0016-7606(1998)110<0048:CHRASS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Samples of carbonate-cemented sedimentary rocks were collected during 11 Alvin dives in two regions along the Cascadia margin, a northern strike-slip fault zone and a southern thrust-fault zone (“second ridge”). We characterized 35 samples petrographically and chemically. Northern-area samples are dominantly sandstones; second-ridge samples are all mudstones. Vugs and high percentages of carbonate cement in the mudstones suggest that these rocks were never deeply buried. The abundance of veins and brecciation in samples from both regions attests to their proximity to faults. Northern-area cements are dominantly calcite or magnesian calcite, whereas second-ridge cements generally are more dolomitic.Oxygen and Sr isotopic values of carbonate cements indicate the involvement of two very different fluid reservoirs during cementation. One reservoir generally has low δ18O, very low δ13C, and high 87Sr/86Sr ratios, but the other has high δ18O, moderately negative δ13C, and 87Sr/86Sr ratios lower than modern seawater values. In the northern area, diagenetic carbonates are subdivided into two populations of coupled oxygen and carbon isotopic values. Population I has δ18OPDB values of +3‰ to +5‰ and δ13CPDB values of −55‰ to −45‰. Population II has δ18O values of −13‰ to −4‰ and δ13C values of −25‰ to −1‰. Such common low δ18O values in carbonate cements have not been observed at any other accretionary wedge. 87Sr/86Sr ratios of northern-area samples range from 0.7128 to 0.7088. Oxygen and Sr isotopic values show a correlation between increasing 87Sr/86Sr ratios and decreasing δ18O values. Oxygen and carbon isotopic values of most diagenetic carbonates from the second ridge range from +3‰ to +10‰ and −55‰ to −38‰, respectively. Enrichments of 18O probably result from a combination of dolomitic mineralogy, cold bottom-water temperatures, and the presence of decomposing gas hydrate in the subsurface. 87Sr/86Sr ratios of carbonates from the second ridge range from 0.7086 to 0.7091.The geochemical signatures of diagenetic cements from the northern area suggest that fluids were derived from greater than 2 km depth, perhaps from the decollement. Prominent gullies throughout the northern area are probably underlain by strike-slip faults that provide conduits for upward fluid migration. Cements at the second ridge precipitated from shallowly derived fluids. The difference in fluid source depth is related to proximity to the fluid conduits and different orientations of minimum principal stress and resultant hydrofractures in the two fault regimes. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Contemporary tectonic stress in northeastern Iberia. New results from borehole breakout analysis

In situ data on the orientation of tectonic stress in northern Iberia are scarce. Fault-slip and focal mechanism data show a variety of stress orientations difficult to interpret in terms of a general stress pattern. Breakout analysis performed on oriented caliper logs from 17 hydrocarbon exploration wells has provided new data on the orientation of tectonic stress in northeastern Iberia: the southern Pyrenees, the Ebro Basin and the northwestern Valencia Trough. The majority of breakout data indicate a rather consistent NE-SW to ENE-WSW directional signature of the larger horizontal principal stress orientation. This orientation differs significantly from the NW western European mean stress orientation. It is consistent with depth within the first 4.7 km of both the sedimentary fill and the sedimentary basins substrate and also within allochthonous thrust sheets in the southern Pyrenees and in the underlying autochthonous and parautochthonous foreland basin sediments.

Stress pattern in the double seismic zone beneath Cook Inlet, south-central Alaska

Relocation of south-central Alaskan earthquakes recorded by the regional seismograph network, using the joint hypocenter determination method, revealed a double seismic zone in the underthrusting Pacific plate beneath the Cook Inlet area. The lower zone is separated from the upper one by a decrease in seismic activity within a 5–10 km wide band, and extends from a depth of 50 km to about 90 km. Stress tensor inversion for earthquakes in the upper and lower zones shows different stress regimes. In the upper zone minimum principal stress is oriented in the down-dip direction and maximum principal stress is normal to the Wadati-Benioff zone (WBZ). In the lower zone both maximum and minimum principal stresses are horizontal and rotated by 40–60° from the down-dip direction of the WBZ. Below 80–90 km depth, stress tensor inversion results show down-dip orientation of the minimum principal stress and the maximum stress aligned in the direction of convergence of Pacific plate.

RAKE: A windows program to plot earthquake focal mechanisms and the orientation of principal stresses

RAKE is a program that is used to examine the stress regime in a region, using information from an earthquake database. The program is used to: (a) plot the fault plane solutions of earthquakes, in the conventional beach-ball presentation, and move them to a desired position, (b) calculate the orientation of the principal P and T stress axes, and plot them on a map or a Schmidt stereonet, (c) plot earthquake epicenters on a map, and (d) project fault plane solutions, earthquake epicenters, and stress axes onto vertical planes of any orientation. RAKE is a program for Windows, written in Visual Basic 3, and has all the advantages of programs running under Windows.

Estimation of the complete stress tensor to 8 km depth in the KTB scientific drill holes: Implications for crustal strength

For many years, in situ stress in the brittle crust has been measured at relatively shallow depth and related to the mechanical behavior of the crust as inferred from laboratory studies and faulting theory. A continuous profile of the magnitudes and orientations of the three principal stresses has been estimated to depths of 7.7 km and 8.6 km in the German Continental Deep Drilling Program (KTB). This was achieved by hydraulic fracturing tests at relatively shallow depth (1–3 km), estimates of the magnitude of the least horizontal principal stress provided by modified hydraulic fracturing experiments at 6 km and 9 km depths, and analysis of compressional (breakouts) and tensile (drilling‐induced tensile wall fractures) failures of the borehole wall over nearly the entire depth of the KTB borehole. The orientation of the maximum horizontal principal stress was found to be uniform with depth with an orientation of N160°±10°E, which is consistent with the average orientation found throughout western Europe. The only significant change in stress orientation was observed directly below a major fault zone crosscutting the borehole. The profile of stress magnitudes we have obtained demonstrates that to a depth of 8 km, the state of stress in the brittle crust in southern Germany is in frictional equilibrium. That is, the ratio of shear to normal stress as resolved on preexisting faults which are well‐oriented to the in situ stress field is comparable to their frictional strength based on predictions of Coulomb faulting theory for a coefficient of friction of about 0.7 and near‐hydrostatic pore pressure.

Fractographic features on joints in KTB drill cores as indicators of the contemporary stress orientation

Fractographic features on joints in cores of the superdeep KTB drilling were used to estimate the orientation of the contemporary maximum horizontal stress (SH). The unique opportunity to investigate cores from a depth down to approximately 8085 m reveals information on in situ stress orientation with increasing depth. We recognized on the cores the main normal stress axes which are reflected by common fractographic pattern on coring-induced fractures, present in all sections. The analysis is based on the symmetry of joint characteristics: 3D (out-of-plane) shape of the joint and surface features of joint planes. The symmetry axes of these joint features are related to the orientation of principal stress, which permits the determination of the orientation of the contemporary maximum and minimum horizontal stress axes (SH≠Sh), if the core orientation is determined by other methods. For that purpose, the reorientated cores of the pilot borehole (VB) could be used down to 4000 m. The main borehole (HB) cores are generally not reoriented by several reasons. Some investigations were done by members of the KTB laboratory to determine the supposedly northward orientations of HB core sections by comparison with the foliation and of drilling-induced fractures at the borehole wall (using FMI/Formation MicroImager and FMS/Formation MicroScanner log data) and within the core. Independently, the maximum horizontal in situ stress orientation down to 7800 m was determined from drilling-induced vertical joints, and the strike and dip of the foliation from borehole measurements down to ca. 8600 m, carried out by the staff members at the KTB. Additionally, rock fragments from below 9050 m represent a palaeo-tectonic joint system developed at a depth of ca. 9 km.

Intraplate stress distribution induced by topography and crustal density heterogeneities beneath the Killari, India, region

The Deccan Traps covered region of the Indian peninsular shield experienced a devastating earthquake of magnitude 6.3(mb&amp;) on September 29, 1993, with the epicenter (18.07°N, 76.45°E) near Killari(Latur). The event had a focal depth of 7 km, and its teleseismic moment tensor analysis suggests reverse faulting characterized by 120°strike, 46°dip, and 100°rake. The fault plane solution of the rnainshock also correlates the reverse faulting with NW‐SE trending nodal plane. In order to gain more insight into understanding the causative factors, the three‐dimensional intraplate lithospheric stresses associated with the topography and subsurface mass hetrogeneities are estimated up to a depth of 15 km beneath a region surrounding the epicenter and lying between 17°–20°N and 75°–78°E. A two‐layered lithospheric model with irregular interface of small amplitude at the Moho has been used for elastostatic stress calculations. The plate boundary force induced stresses are superimposed on the computed stress regime to assess their contribution vis‐a‐vis local causes. The estimated local stresses alone show a subdued northeasterly extensional regime at hypocentral depth. However, the superimposition of an assumed 30 MPa ridge push trending N10°E leads to an enhanced concentration of total stresses beneath the region near the hypocenter. Its maximum value at Killari is 31 MPa at 2 km depth, and it reduces to 29 MPa at 15 km depth. The total deviatoric stresses are also found maximum near Killari. Analysis of computed stresses indicates that the orientation of total principal stresses favors a strike‐slip movement at the focal depth with a NE trending maximum compression. The shear component of these stresses when resolved on the fault plane of the Killari mainshock [U.S. Geological Survey, 1993], however, suggests a thrust movement at 7 km depth beneath the Killari region. This study thus suggests that concentration of total stresses along some kind of crustal weak zones beneath the epicentral region could be an important factor for the reactivation of faults and therefore the Killari earthquake.

In situ stress from breakouts in the Danish sector of the North Sea

Borehole breakouts from 26 wells situated in the Danish Central Graben and one well situated in the northwestern section of the Ringkobing-Fyn High have been studied. The data provide information on the orientation of the contemporary principal horizontal stresses down to depths of 4.7 km. Statistics were used to obtain a mean azimuth of breakouts for each well, and each individual breakout was weighted by its length. Data quality of breakouts is assigned according to the World Stress Map quality ranking scheme. The breakout data reveal that S H orientations in the Danish Central Graben are extremely scattered; no regional trend in mean S H orientation can be identified as a result of the large standard deviation (±77°). Similar scatterings of S H orientations have been reported from adjacent regions of the North Sea. The results indicate that the state of stress in the sedimentary cover of the Danish Central Graben is influenced by a local stress generating mechanism rather than platewide stress transmissions mechanisms. Three units with different states of stress have been identified and found to correspond somewhat with the lithostratigraphic divisions of the Post Chalk Group, Chalk Group and Pre Chalk Group.

Numerical modeling of fault-controlled fluid flow in the genesis of tin deposits of the Malage ore field, Gejiu mining district, China

A distinct element finite difference model based on geologic data from the tin polymetallic hydrothermal deposits of Malage is constructed to simulate the geologic processes of fault-controlled fluid migration during hydrothermal ore genesis. The history of hydrothermal ore genesis is strongly affected by the interaction between fault deformation and fluid flow. An injected fluid induces fault slip by lowering the effective stress across a fault. The subsequent movement on the fault enhances the fault aperture which provides a channelway that both facilitates and focuses fluid flow. The variation of fluid injection under certain stress conditions causes cyclic movement on faults. This cyclic failure gives rise to pulsating fluid flow along the faults which may be responsible in part for the precipitation zoning of mineral deposits. As an important structural condition for ore formation in any fault-related mineralized region like Malage, the linking up of fracture networks not only allows appropriate long-distance channels for fluid migration but also provides the appropriate conditions for ore deposition at particular structural sites. This study indicates that different fluid flow patterns (i.e., changes in fluid flow rate or velocity with time and space) can reflect the variation of structural environment. For instance, the flow velocity is increased gradually over time on the relatively connected faults, such as the northwest-southeast-trending fault and the western half of the east-west-trending Yuanlao fault in the center of the Malage ore field. These are the main pathways for the ore-forming fluids. In the smaller secondary faults, the flow velocity fluctuates more rapidly with time and space, in some instances causing conditions that are favorable to mineral deposition. The main ore deposition sites occur on the smaller secondary faults. The detailed variation of fluid and aperture conditions modeled in this complex fault array can be related to the formation and location of mineral deposits, both predictively and conceptually. The modeling also demonstrates that the fault deformation and fluid flow are sensitive to the principal stress orientation. If the orientation of sigma 1 is changed by more than 15 degrees , there are very different results for the calculated fault motion pattern, hydraulic aperture, and fluid flow.

Details of stress directions in the Alaska subduction zone from fault plane solutions

We used the cumulative misfit method to divide a data set with heterogeneous stress orientations into subsets with homogeneous stress directions. We propose that slope changes of the cumulative misfit as a function of earthquake number pinpoint the boundaries of homogeneous stresses. Using a synthetic data set of 50 fault plane solutions, composed of two halves corresponding to two incompatible stress tensors, we tested the validity and efficacy of the cumulative misfit method. Our acceptance criteria for results of stress tensor inversions are (1) the directions must be well constrained, that is, the 95% confidence regions of the greatest and least principal stresses do not overlap and (2) there must be evidence for homogeneity in the sample, that is, the average misfit of the inversion is less than 6°. We estimated stress directions in the Alaska Wadati‐Benioff Zone (WBZ) using 470 fault plane solutions, which we determined for earthquakes with ML∼3, and 67 published fault plane solutions for earthquakes with Ms∼5. We succeeded in subdividing the data set of the small and large earthquakes into 25 and 3 subsets, with the average misfit ranging from 3.2° to 5.5°. The average misfits of the subsets we accept as satisfying the assumption of homogeneity are smaller than the average misfit of the overall data set (F∼10°) by factors of 2 to 3. We estimated the stress fields at two scales along the Alaska WBZ. The stress directions measured by the large earthquakes (Ms∼5) were homogeneous over large volumes, with extension downdip and the direction of greatest compression along strike. This unusual orientation of the greatest principal stress is attributed to the bend of the slab under central Alaska, which generates compressive stresses along strike. The stress orientations revealed by small earthquakes, in contrast, exhibited a great deal of heterogeneity as a function of space, although their general trend confirms the overall stress directions obtained from the large events. We propose that the ratio of the dimensions of the stress field sensed by earthquakes to the rupture dimensions is about 20 to 50. The estimated stress directions of the crustal earthquakes corresponded to the following mechanisms: (1) strike‐slip faulting with the greatest principal stress oriented N‐S near Fairbanks and (2) thrusting with the greatest principal stress oriented NW‐SE near Mount McKinley.

Investigation of microearthquake activity following an intraplate teleseismic swarm on the west flank of the Southern East Pacific Rise

Between February 1991 and May 1992, 33 intraplate earthquakes having body wave magnitudes between 4.3 and 6.0 were located on the west flank of the Southern East Pacific Rise by the International Seismological Center. Seven months after the last teleseismic event, we deployed four ocean bottom seismometers at the site of the teleseismic swarm. One hundred and ninety‐two microearthquakes were located using P and S travel times of events recorded by three or more instruments during the 16‐day deployment. Most of the microearthquakes were in a band about 30 km long and 6 km wide between and parallel to seamount chains. In addition, several events were distributed along a line perpendicular to the main seismicity band and parallel to the ridge axis. The focal depths of the microearthquakes range from 1 to 15 km, and most are between 5 and 12 km, similar to the depth range of the teleseismic events [Hung and Forsyth, 1996]. The composite P wave polarities indicate that the microearthquakes had a variety of focal mechanisms. We developed a new grid‐search, inversion technique that utilizes the P wave polarities and the empirically corrected ratios of P and S wave amplitudes to find the focal mechanisms of individual events. Within the acceptable travel time and amplitude misfits, focal solutions are fairly stable. Normal faulting is found in the ridge‐parallel seismicity line. The thrust and strike‐slip faulting in the main seismicity band is distinctly different from the exclusively normal faulting mechanisms of the teleseismic events. There is no apparent depth dependence of fault types. None of the existing models of the sources of stress (ridge push, thermoelastic stresses, loading by local topographic features, caldera collapse, and north‐south extension of the Pacific Plate) provides a satisfactory explanation for both the teleseismic swarm and microearthquakes. We propose a new tectonic scenario. In this scenario, the lithosphere is prestressed by the cooling of the plate. Magma rising from the deeper mantle induces normal faulting ahead of the dike tips in the lower lithosphere, which is already under extensional, thermal stress, producing the larger, teleseismically detected events. Once the dikes propagate into the lithosphere, the region surrounding the dikes behind the tips is compressed by the overpressure of magma. Depending on the geometry of the dikes, the local orientations of the minimum principal stress, and the local weaknesses in the lithosphere, thrust or strike‐slip faulting (the microearthquakes) may occur.

In situ stress measurements by hydraulic fracturing for a rock mass with many planes of weakness

When hydraulic fracturing is performed on a rock mass which contains many natural cracks and planes of weakness, most of the induced cracks are inclined transverse cracks. We propose a method which can be used under such a circumstance to determine the stress state. The method consists of two stages. The first stage determines the magnitude and orientation of principal stresses by using shut-in pressures of inclined transverse cracks and the circumferential position of longitudinal cracks on the wellbore surface. The second stage estimates the ranges of errors of the stress state determined in the first stage, by taking into account that the shut-in pressures and orientations of the cracks are the factors which give rise to errors of the determined stress state. We have applied this method to data obtained by the hydraulic fracturing performed at two wellbores drilled from a drift way at the Kamaishi mine. It was found that the ranges of errors of the principal stress magnitude were small. The orientation of the maximum principal axis of stress had a relatively high reliability in all directions, although the ranges of errors of the directions of the minimum and intermediate principal stresses showed a tendency to spread more widely in the N-S direction.

Stress measurements by hydraulic fracturing for a rock mass with many planes of weakness

When hydraulic fracturing is performed on a rock mass which contains many natural cracks and planes of weakness, most of the induced cracks are inclined transverse cracks. We propose a method which can be used under such a circumstance to determine the stress state. The method consists of two stages. The first stage determines the magnitude and orientation of principal stresses by using shut-in pressures of inclined transverse cracks and the circumferential position of longitudinal cracks on the wellbore surface. The second stage estimates the ranges of errors of the stress state determined in the first stage, by taking into account that the shut-in pressures and orientations of the cracks are the factors which give rise to errors of the determined stress state. We have applied this method to data obtained by the hydraulic fracturing performed at two wellbores drilled from a drift way at the Kamaishi mine. It was found that the ranges of errors of the principal stress magnitude were small. The orientation of the maximum principal axis of stress had a relatively high reliability in all directions, although the ranges of errors of the directions of the minimum and intermediate principal stresses showed a tendency to spread more widely in the N-S direction.

Study on the State of Stress around Earthquake Swarm Areas Using P-Wave Polarity Data: Nikko-Ashio and Teishi Knoll Areas.

We have estimated the stress tensor at two earthquake swarm areas in Japan, Nikko-Ashio, Tochigi Prefecture and off Ito, Izu Peninsula, by applying a technique that inverts P-wave polarity data for a large number of events under the following assumptions: 1) Stress field in a small region is uniform, 2) slips occur along pre-existing weak planes randomly distributed in the region, and 3) slip direction is parallel to the direction of the maximum shear stress. We inverted data collected by the 1993 Joint Seismic Observation in the Nikko-Ashio area. The estimated maximum principal stress in this area is nearly horizontal in the NW-SE direction and the minimum principal stress is sub-vertical. Moreover, the state of stress just beneath Nikko-Shirane Volcano is different from that in the surrounding areas. The orientation of the minimum principal stress is sub-horizontal and perpendicular to the strike direction of opening cracks found at the top of Mt. Nikko-Shirane, an active quaternary volcano. This local change is probably related to recent magmatic activity. We also analyzed data from the 1989 Teishi Knoll eruption area. The estimated direction of the minimum principal stress is parallel to that of the maximum extension of the crustal deformation measured during the submarine eruption at Teishi Knoll. Spatial variations in the state of stress off Ito were found to be small.

石炭のガス透過性およびガス圧分布に対するき裂の配向性の影響 き裂網モデルによる石炭のガス透過性の研究（第３報）

The purpose of the present study is the effect of crack orientation on the gas permeability and gradient of gas pressure in coal. Practically, the model of anisotropic crack network was applied to the computer simulations under the state of an uniaxial compressive stress. Additionally, the simulations of gas permeation and gas pressure were investigated by setting the normal distribution as the frequency distribution of crack orientation. The results of these simulations were as follows.1) The anisotropy of crack orientation in coal is very effective on gas permeability, and the gas permeability in the orientation of maximum principal stress is higher than that of minimum principal stress.2) In the anisotropy crack network, the gas permeable cracks of the direction of minimum principal stress close at their neck. Consequently, gas flow is intercepted by the difference stress between an external stress and gas pressure in the cracks of the direction of minimum principal stress become high.3) The occurrence mechanism of a gas outburst is explained by applying the bending theory of circular plates to the anisotropic crack network saturated with high pressures gas.

Orientation of crustal stresses in the North Sea and Barents Sea inferred from borehole breakouts

The orientations of the greatest horizontal principal stresses (S H) derived from borehole breakouts are presented for 30 boreholes located in the Central Graben, the Viking Graben, the Møre-Vøring region, and the Barents Sea area. Additional borehole breakout data from 20 other wells for which information about breakout length was available have been used to calculate the mean azimuth of S H for the four areas of investigation. The mean azimuth of S H is calculated from the length-weighted distribution of the breakout orientations. In the Viking Graben and the Central Graben a S H orientation of about N 78° and N 97° was found, respectively. For the Møre-Vøring region a mean S H orientation of N 127° has been evaluated. Borehole breakout data from the Barents Sea area indicate a S H orientation of about N 177°. The S H orientations presented in this study are consistent with the results from previous studies which suggests that the direction of the maximum compressive horizontal stress S H differs off the coast of Norway markedly from the well documented NW-SE trend of S H in Western Europe. The observed S H orientation changes significantly across the investigated regions which may have to be attributed to their complicated geomechanical setting.

Relationship between the present‐day stress field and plate boundary forces in the Pacific Northwest

The relationship between plate boundary forces and the observed stress field in the Pacific Northwest is established using numerical models of continental deformation. Because the orientation of the greatest horizontal principal stress throughout the Pacific Northwest differs considerably from the direction of convergence between the Juan de Fuca and North American plates, the relationship between the stress field and forces acting along the subduction zone has been unclear. To address this relationship, a two‐dimensional finite element model developed by Bird [1989] is used that incorporates critical aspects of continental deformation such as a stratified rheology and interaction between thermal and mechanical components of deformation. Boundary conditions are specified in terms of either velocity or shear traction, depending on whether the computed shear stress at the plate boundary is less than or exceeds, respectively, a prescribed limit. Shear‐stress limits on the subduction and transform plate boundaries are independently varied to determine the relative effect of forces along these boundaries on intraplate deformation. Results from this study indicate that the shear stress limit of both subduction and transform boundaries is low, and that the intraplate stress field is attributed, in part, to the normal component of relative plate motion along the transform boundaries. However, the models also indicate that although the subduction zone fault is weak, a minimum shear strength (≥ 10 MPa) for the fault is necessary to explain the observed stress field. The balance among forces along the tectonic boundaries of North America results in a surprising degree of variation in the present‐day stress field.