Pre-existing Planes Of Weakness Research Articles

Use of Reverse Extrusion Method to Determine Undrained Shear Strength

The unconfined compression test (UCT) is one of the most common tools to measure the undrained shear strength of fine-grained soils. Soft to very soft soils often pose problems such that the determination of failure load is unclear using this test. Similarly, fissured clays would endeavor failure along pre-existing planes of weakness. The reverse extrusion test (RET), a mechanical metallurgy tool, has been used recently in some soil mechanics applications. The scope of this investigation is to evaluate the potential of the RET as a substitute for the UCT. Sixty remolded soils and 75 natural soils were subjected to the RET and UCT. Remolded soils were tested on mixtures prepared at the plastic limit. It was found that the ratio of extrusion pressure at failure to the undrained shear strength is relatively consistent and concentrates around 14. The same ratio was found to be 15 for natural soils. The conclusion reached, upon a series of tests on both the remolded and natural soil samples, is that the RET better represents the undrained shear strength of soil than does the UCT, that the practical difficulties with UCT is eliminated by the RET, and the results may be improved by taking into account soil plasticity.

Interpreting Microseismic Measurements: Practical Guide

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 144067, ’A Practical Guide To Interpreting Microseismic Measurements,’ by C. Cipolla, SPE, S. Maxwell, SPE, M. Mack, SPE, and R. Downie, Schlumberger, prepared for the 2011 SPE North American Unconventional Gas Conference and Exhibition, The Woodlands, Texas, 14-16 June. The paper has not been peer reviewed. Thousands of hydraulic-fracture treatments have been monitored in the past 10 years with microseismic mapping, providing measurements that show a surprising degree of diversity in event patterns. Interpretation of the microseismic data to determine the geometry and complexity of hydraulic fractures continues to focus on visualization of event patterns and on qualitative estimates of the stimulated volume, which has led to wide variations and inconsistencies. Comparing the energy input during a hydraulic-fracture treatment with the resultant energy released by microseismic events demonstrates that the seismic deformation is a very small portion of the total deformation. Introduction Hydraulic-fracture microseismic monitoring (MSM) has increased our understanding of fracture propagation, especially in unconventional reservoirs. MSM, which is a straightforward application of earthquake-seismology principles, consists of the detection, location, and further analysis of extremely small seismic events induced by the fracturing process. Typically, multiple-receiver arrays are positioned over a 300- to 1,000-ft vertical interval in a single nearby offset well. The microseismic events usually are not detectable as discrete events at the surface, but only from other wellbores within approximately 3,000 ft of the fracture-treatment well. Typically, microseisms are represented adequately by a double-couple source, which implies shear slippage that releases stress over some area. In competent rocks, shear slippages occur on pre-existing planes of weakness as a result of the changes in stress and pore pressure induced by the fracturing process. When fracture growth is relatively planar, these changes in stress and pore pressure usually result in a narrow band of microseisms that clearly defines the azimuth and geometry of the hydraulic fracture. However, complex fracture behavior is often interpreted from microseismic data, as evidenced by very intricate event patterns with width/length aspect ratios greater than 0.5. MSM data have revealed a large variation in hydraulic-fracture growth, from relatively simple planar fractures to very complex fracture networks. Understanding fracture growth and the degree and nature of hydraulic-fracture complexity is essential to selecting the appropriate stimulation design and completion strategy.

Australian paleo-stress fields and tectonic reactivation over the past 100 Ma

Even though a multitude of observations suggest time-dependent regional tectonic reactivation of the Australian Plate, its large-scale intraplate stress field evolution remains largely unexplored. This arises because intraplate paleo-stress models are difficult to construct, and that observations of tectonic reactivation are often hard to date. However, because the Australian plate has undergone significant changes in plate boundary types and geometries since the Cretaceous, we argue that even simple models can provide some insights into the nature and timing of crustal reactivation through time. We present Australian intraplate stress models for key times from the Early Cretaceous to the present, and link them to geological observations for evaluating time-dependent fault reactivation. We focus on the effect time-dependent geometries of mid-ocean ridges, subduction zones and collisional plate boundaries around Australia have on basin evolution and fault reactivation through time by reconstructing tectonic plates, restoring plate boundary configurations, and modelling the effect of selected time-dependent plate driving forces on the intraplate stress field of a rheologically heterogeneous plate. We compare mapped fault reactivation histories with paleo-stress models via time-dependent fault slip tendency analysis employing Coulomb-Navier criteria to determine the likelihood of strain in a body of rock being accommodated by sliding along pre-existing planes of weakness. This allows us to reconstruct the dominant regional deformation regime (reverse, normal or strike-slip) through time. Our models illustrate how the complex interplay between juxtaposed weak and strong geological plate elements and changes in far-field plate boundary forces have caused intraplate orogenesis and/or tectonic reactivation in basins and fold belts throughout Australia.

Effect of block rotation on the pitch of slickenlines

Abstract Rotation of faults or pre-existing weakness planes produce two effects on the slickenlines of fault planes. First, the rotation leads to changes in the pitch of slickenlines. As a result, the aspect of the pre-existing fault may change. For example, after rotation, a normal fault may show features of an oblique fault, a strike-slip fault, or a thrust fault. Second, due to rotation, stress states on the fault planes are different from those before the rotation. As a consequence some previous planes may be reactivated. For an isolated plane, the reactivation due to rotation can produce new sets of slickenlines. With block rotation, superimposed slickenlines can be generated in the same tectonic phase. Thus, it is not appropriate to use fault-slip data from slickenlines to analyze the stress tensor in a region where there is evidence of block rotation. As an example, we present the data of slickenlines from core samples in the Tunich area of the Gulf of Mexico. The results wrongly indicate that the calculated stress tensor deviates from the far-field stress tensor.

Effect of block rotation on the pitch of slickenlines

Rotation of faults or pre-existing weakness planes produce two effects on the slickenlines of fault planes. First, the rotation leads to changes in the pitch of slickenlines. As a result, the aspect of the pre-existing fault may change. For example, after rotation, a normal fault may show features of an oblique fault, a strike-slip fault, or a thrust fault. Second, due to rotation, stress states on the fault planes are different from those before the rotation. As a consequence some previous planes may be reactivated. For an isolated plane, the reactivation due to rotation can produce new sets of slickenlines. With block rotation, superimposed slickenlines can be generated in the same tectonic phase. Thus, it is not appropriate to use fault-slip data from slickenlines to analyze the stress tensor in a region where there is evidence of block rotation. As an example, we present the data of slickenlines from core samples in the Tunich area of the Gulf of Mexico. The results wrongly indicate that the calculated stress tensor deviates from the far-field stress tensor.

Geology of the Selk crater region on Titan from Cassini VIMS observations

Observations of Titan obtained by the Cassini Visual and Infrared Mapping Spectrometer (VIMS) have revealed Selk crater, a geologically young, bright-rimmed, impact crater located ∼800 km north-northwest of the Huygens landing site. The crater rim-crest diameter is ∼90 km; its floor diameter is ∼60 km. A central pit/peak, 20–30 km in diameter, is seen; the ratio of the size of this feature to the crater diameter is consistent with similarly sized craters on Ganymede and Callisto, all of which are dome craters. The VIMS data, unfortunately, are not of sufficient resolution to detect such a dome. The inner rim of Selk crater is fluted, probably by eolian erosion, while the outer flank and presumed ejecta blanket appear dissected by drainages (particularly to the east), likely the result of fluvial erosion. Terracing is observed on the northern and western walls of Selk crater within a 10–15 km wide terrace zone identified in VIMS data; the terrace zone is bright in SAR data, consistent with it being a rough surface. The terrace zone is slightly wider than those observed on Ganymede and Callisto and may reflect differences in thermal structure and/or composition of the lithosphere. The polygonal appearance of the crater likely results from two preexisting planes of weakness (oriented at azimuths of 21° and 122° east of north). A unit of generally bright terrain that exhibits similar infrared-color variation and contrast to Selk crater extends east-southeast from the crater several hundred kilometers. We informally refer to this terrain as the Selk “bench.” Both Selk and the bench are surrounded by the infrared-dark Belet dune field. Hypotheses for the genesis of the optically bright terrain of the bench include: wind shadowing in the lee of Selk crater preventing the encroachment of dunes, impact-induced cryovolcanism, flow of a fluidized-ejecta blanket (similar to the bright crater outflows observed on Venus), and erosion of a streamlined upland formed in the lee of Selk crater by fluid flow. Vestigial circular outlines in this feature just east of Selk’s ejecta blanket suggest that this might be a remnant of an ancient, cratered crust. Evidently the southern margin of the feature has sufficient relief to prevent the encroachment of dunes from the Belet dune field. We conclude that this feature either represents a relatively high-viscosity, fluidized-ejecta flow (a class intermediate to ejecta blankets and long venusian-style ejecta flows) or a streamlined upland remnant that formed downstream from the crater by erosive fluid flow from the west-northwest.

Collapse at the eastern Eiger flank in the Swiss Alps

On 13 June 2006, parts of the eastern flank of the Eiger peak in the Swiss Alps collapsed, leading to a major rock fall. The unstable spur on this flank is made up of several blocks that move relative to one another. Instability was initiated by a block at the rear that acted as a wedge and triggered motion of the other blocks. Landslides are a significant natural hazard in mountainous regions1 and are often triggered by external factors, such as earthquakes, rainfall, permafrost melting or retreat of glaciers2. A large landslide occurred in the Swiss Alps on 13 July 2006, when portions of an immense rock spur on the eastern flank of the Eiger peak3 collapsed. Here we use field observations and terrestrial laser scanning data to record and quantify the relative motion along the various blocks of rock that form this spur. The data show that during the year of observation the blocks moved relative to one another by up to tens of metres along fractures that can be related to pre-existing planes of weakness. Rates of motion and deformation were high throughout July 2006, particularly in the northern part of the spur that partially collapsed on 13 July. The rates decreased considerably during the subsequent months, although a slight increase was noted in June and July 2007. These observations are consistent with instability of the spur initiated by subsidence of a single block at the rear, which acted as a wedge and disintegrated over time owing to loss of lateral confinement.

Fault plane orientations of intermediate‐depth earthquakes in the Middle America Trench

Intermediate‐depth earthquakes are often attributed to dehydration embrittlement reactivating preexisting weak zones. The orientation of presubduction faults is particularly well known offshore of Middle America, where seismic reflection profiles show outer rise faults dipping toward the trench and extending >20 km into the lithosphere. If water is transported along these faults and incorporated into hydrous minerals, the faults may be reactivated later when the minerals dehydrate. In this case, the fault plane orientations should be the same in the outer rise and at depth, after accounting for the angle of subduction. To test this hypothesis, we analyze the directivity of 54 large (MW ≥ 5.7) earthquakes between 35 and 220 km depth in the Middle America Trench. For 12 of these earthquakes, the directivity vector allows us to identify the fault plane of the focal mechanism. Between 35 and 85 km depth, we observe both subhorizontal and subvertical fault planes. The subvertical fault planes are consistent with the reactivation of outer rise faults, whereas the subhorizontal fault planes suggest the formation of new faults. Deeper than 85 km, we only observe subhorizontal faults, indicating that the outer rise faults are no longer being reactivated. The similarity with previous results from the colder Tonga‐Kermadec subduction zone suggests that the mechanism generating these earthquakes, and controlling fault plane orientations, depends on pressure rather than temperature or other tectonic parameters and that the observed rupture characteristics constitute a basic feature of intermediate‐depth seismicity. Exclusively subhorizontal faults may result from isobaric rupture propagation or the hindrance of seismic slip on preexisting weak subvertical planes.

Imaging Unstable Wellbores While Drilling

Summary This paper presents a case study of borehole instability from four wellbores on the Gulf of Mexico (GOM) shelf, offshore Louisiana. Logging-while-drilling (LWD) borehole images are combined with observations of cavings and modeling of borehole shear failure to diagnose the mechanisms of instability and, thus, select the appropriate remedial action. It is observed that instability caused by shear failure of intact rock (borehole breakout) can be suppressed by increasing the MW. However, where pre-existing planes of weakness (such as bedding planes and fractures) dominate the mechanism of instability, mudweight increases do not necessarily lead to a more stable hole and can, in fact, further destabilize the wellbore.

Coseismic and postseismic stress changes in a subducting plate: Possible stress interactions between large interplate thrust and intraplate normal‐faulting earthquakes

A large intraplate, normal‐faulting earthquake (Mw = 7.5) occurred in 1999 in the subducting Cocos plate below the downdip edge of the ruptured thrust fault of the 1978 Oaxaca, Mexico, earthquake (Mw = 7.8). This situation is similar to the previous case of the 1997 normal‐faulting event (Mw = 7.1) that occurred beneath the rupture area of the 1985 Michoacan, Mexico, earthquake (Mw = 8.1). We investigate the possibility of any stress interactions between the preceding 1978 thrust and the following 1999 normal‐faulting earthquakes. For this purpose, we estimate the temporal change of the stress state in the subducting Cocos plate by calculating the slip distribution during the 1978 earthquake through teleseismic waveform inversion, the dynamic rupture process, and the resultant coseismic stress change, together with the postseismic stress variations due to plate convergence and the viscoelastic relaxation process. To do this, we calculate the coseismic and postseismic changes of all stress components in a three‐dimensional space, incorporating the subducting slab, the overlying crust and uppermost mantle, and the asthenosphere. For the coseismic stress change we solve elastodynamic equations, incorporating the kinematic fault slip as an observational constraint under appropriate boundary conditions. To estimate postseismic stress accumulations due to plate convergence, a virtual backward slip is imposed to lock the main thrust zone. The effects of viscoelastic stress relaxations of the coseismic change and the back slip are also included. The maximum coseismic increase in the shear stress and the Coulomb failure stress below the downdip edge of the 1978 thrust fault is estimated to be in the range between 0.5 and 1.5 MPa, and the 1999 normal‐faulting earthquake was found to take place in this zone of stress increase. The postseismic variations during the 21 years after the 1978 event modify the magnitude and patterns of the coseismic stress change to some extent but are not large enough to overcome the coseismic change. These results suggest that the coseismic stress increase due to the 1978 thrust earthquake may have enhanced the chance of occurrence of the 1999 normal‐faulting event in the subducting plate. If this is the case, one of the possible mechanisms could be static fatigue of rock materials around preexisting weak planes involved in the subducting plate, and it is speculated that that one of these planes might have been reactivated and fractured because of stress corrosion cracking under the applied stress there for 21 years.

Sillimanite-bearing shear zones in syntectonic leucogranite: fluid-assisted brittle–ductile deformation under amphibolite facies conditions

In the Imja Kola valley (eastern Nepal), Miocene peraluminous leucogranite dykes were emplaced within transtensional shear zones synchronously with regional amphibolite–facies metamorphism in the metamorphic host rocks. Within the dykes the magmatic foliations, parallel to the metamorphic schistosity, are cross-cut by thin (millimetre to centimetre size) shear zones, which are made up of prismatic and/or fibrolitic sillimanite with a minor amount of white mica. These structures have a heterogeneous distribution and form networks of strongly deformed and foliated layers wrapping around poorly deformed granitic pods. The textural and strain features are consistent with development of sillimanite shear zones as a result of heterogeneous deformation and fluid action, which allowed the replacement of strained magmatic minerals such as feldspar and biotite by means of incongruent pressure solution. The described example illustrates that nucleation and development of heterogeneous shear zones under amphibolite facies conditions, is also strongly favoured by brittle fracturing and fluid action at the grain-scale along pre-existing planes of weakness. The growth of cracks and fractures can be regarded as a sequence of short lived brittle events, due to work hardening, which punctuate a history of plastic deformation.

Reactivated strike–slip faults: examples from north Cornwall, UK

Several strike–slip faults at Crackington Haven, UK show evidence of right-lateral movement with tip cracks and dilatational jogs, which have been reactivated by left-lateral strike–slip movement. Evidence for reactivation includes two slickenside striae on a single fault surface, two groups of tip cracks with different orientations and very low displacement gradients or negative (left-lateral) displacements at fault tips. Evidence for the relative age of the two strike–slip movements is (1) the first formed tip cracks associated with right-lateral slip are deformed, whereas the tip cracks formed during left-lateral slip show no deformation; (2) some of the tip cracks associated with right-lateral movement show left-lateral reactivation; and (3) left-lateral displacement is commonly recorded at the tips of dominantly right-lateral faults. The orientation of the tip cracks to the main fault is 30–70° clockwise for right-lateral slip, and 20–40° counter-clockwise for left-lateral slip. The structure formed by this process of strike–slip reactivation is termed a “tree structure” because it is similar to a tree with branches. The angular difference between these two groups of tip cracks could be interpreted as due to different stress distribution (e.g., transtensional/transpressional, near-field or far-field stress), different fracture modes or fractures utilizing pre-existing planes of weakness. Most of the d– x profiles have similar patterns, which show low or negative displacement at the segment fault tips. Although the d– x profiles are complicated by fault segments and reactivation, they provide clear evidence for reactivation. Profiles that experienced two opposite slip movements show various shapes depending on the amount of displacement and the slip sequence. For a larger slip followed by a smaller slip with opposite sense, the profile would be expected to record very low or reverse displacement at fault tips due to late-stage tip propagation. Whereas for a smaller slip followed by larger slip with opposite sense, the d– x profile would be flatter with no reverse displacement at the tips. Reactivation also decreases the ratio of d max/ L since for an original right-lateral fault, left lateral reactivation will reduce the net displacement ( d max) along a fault and increase the fault length ( L). Finally we compare Crackington Haven faults with these in the Atacama system of northern Chile. The Salar Grande Fault (SGF) formed as a left-lateral fault with large displacement in its central region. Later right-lateral reactivation is preserved at the fault tips and at the smaller sub-parallel Cerro Chuculay Fault. These faults resemble those seen at Crackington Haven.

Do intermediate‐ and deep‐focus earthquakes occur on preexisting weak zones? An examination of the Tonga subduction zone

It has been proposed that intermediate‐ and deep‐focus earthquakes occur on preexisting planes of weakness that were created at shallow depth. The purpose of this study is to test this hypothesis. We examined fault plane solutions for 360 events that occurred in the Tonga subduction zone from January 1976 to February 1999. For events in the slab, each fault plane solution is rotated by the slab dip angle so that the slab is in its original horizontal position. We then compare fault plane solutions for events in the slab with those for events at the outer rise. We find that an asymmetric fault system, corresponding to that found for outer rise events, persists down to about 450 km depth. This suggests that earthquakes down to this depth are caused by the reactivation of preexisting faults created in the oceanic plate before subduction. A similar pattern, though less well constrained, is found in the Kurile subduction zone down to 200 km depth. For earthquakes deeper than 450 km the fault plane solutions are much more scattered. In some places the fault plane pattern suggests the creation of new fault planes along orientations of maximum shear with zero internal friction. The pattern of fault plane solutions in particular localized volumes cannot be easily explained by the creation of new faults in the direction of maximum shear under a coherent regional tectonic stress field. Thus, either a highly heterogeneous stress field exists or preexisting planes of weakness also account for some deep earthquakes.

Assessing Fault Reactivation with the ReActiva Program

In this paper, we offer ReActiva, a shareware program that will aid in understanding the role of reactivation of preexisting planes of weakness in the deformation of a cortical block by faulting. Re- Activa is based on the slip and rupture Coulomb- Navier criterion. It is a program that renders graphically the orientations of planes amenable to reactivation under a given set of physical conditions. These conditions can be modified interactively by the user, and the graphical display is updated continuously. Rather than rendering the orientations of the planes in Mohr space, the program plots the poles of planes amenable to slip on an equal-area projection net. This allows the resulting diagram to be oriented relative to geographic space. ReActiva is available at http://geologia.igeolcu.unam.mx/Tolson/SoftWare/ReActivaEng.htm.

Numerical Models of Translational Landslide Rupture Surface Growth

We analyze the initiation and enlargement of the rupture surface of translational landslides as a fracture phenomenon using a two-dimensional boundary-element method. Both processes are governed largely by the stress field and the pre-existing planes of weakness in a slope. Near the ground surface, the most compressive stress becomes either parallel or perpendicular to the slope, depending on the topography and regional stresses. The shear stress available to drive slope-parallel sliding in a uniform slope thus is small, and therefore pre-existing weaknesses are required in many cases for sliding. Stresses in a uniform slope favor the initiation of sliding near the slope base. Sliding can progress upslope from there in retrogressive fashion. Most slopes are not uniform and notches in a slope will concentrate stresses and generally promote sliding there. As the region of sliding at depth enlarges, the stress concentration near the edge of the area of slip will tend to rise. Stress concentrations can become sufficient to open fractures above and below a basal slide plane, in keeping with observations. If one tip of a slide plane intersects the ground surface, then stresses near the other tip can increase markedly, as can slip. Our analyses show that slope-parallel sliding along a plane at depth will cause downslope extension in the upslope half of a slide mass and shortening in the downslope half, consistent with observations. Displacement profiles that could be interpreted as rotational can result from sliding along such a plane, however careful analysis of surface deformation can be used to understand sliding at depth.

Automated moment tensor inversion to estimate source mechanisms of hydraulically induced micro-seismicity in salt rock

Two methods for an automated moment tensor inversion are investigated. The first is a single-source inversion using amplitude spectra of observed body-wave phases and theoretical Green functions, and the second a relative moment tensor inversion using spectral moments of body-wave phases. An optional general-dislocation-source constraint is introduced to stabilize the inversions for sparse data. Both methods are successfully tested on synthetic data and are applied to hydraulically induced micro-seismicity in salt rock. The estimated moment tensors of 187 events show similar radiation patterns with a major double-couple component and a small or zero isotropic component. We explain the double-couple component with failure on horizontal, preexisting planes of weakness within the process zone of circular tension-fractures

Holocene Activity of the Mt. Currie Scarp, Coast Mountains, British Columbia, and Implications for its Origin

A conspicuous, linear scarp crosses an alpine ridge at Mt. Currie, British Columbia. Previous studies have shown that the northeast end of the scarp, which is up to 20 m high, is currently creeping, but the history and origin of the entire 1.6-km-long feature are unknown. An exploratory trench across an upslope-facing portion of the bedrock scarp and the adjacent sediment catchment revealed evidence for a minimum of three to four periods of activity dating from late Pleistocene to late Holocene time. Movement has occurred along near-vertical, gouge-filled shear zones, which represent pre-existing planes of weakness. These zones explain the continuity and linearity of the scarp. Warped and tilted sediments and a scarp-derived colluvial wedge provide evidence for the style of deformation, and are similar to structures and deposits of both gravitational spreading features (sackungen) and tectonic faults. The Mt. Currie scarp is more likely caused by gravitational forces, and is not a recently active tectonic fault, as has been previously suggested. The scarp's geometry and position in the landscape are compatible with gravitational failure. Slickenside striations in clay gouge from our trench show a displacement vector consistent with documented creep. The orientation of the scarp is not favorable for tectonic displacement, based on an evaluation of the regional seismogenic deformation field from earthquake P and T axes.

Origin and tectonic interpretation of multiple fault patterns

It is shown that in two-dimensional and three-dimensional deformation accommodated by fracture, the symmetry of the fault patterns is an intrinsic attribute because it reflects the symmetry of either stress or strain tensors. The deformation accommodated by sliding along pre-existing planes, when there is kinematic interaction between that planes, forms multiple fault pattern and multiple slickenline sets during a single deformation event. These fault patterns have no restrictions with respect to symmetry, number of fault sets or fault orientation. The kinematic analysis developed here shows that an interacting system is formed by two cross cutting faults and three slickenlines. One slickenline must be parallel to the intersection line between the planes. Also, it is demonstrated that the slickenlines generally do not correspond to the shear stress solution on the planes. Thus, the interaction between planes does not satisfy the assumption of parallelism between shear stress and slip vector. We conclude that the inversion methods to calculate paleostress tensors can lead to erroneous interpretations in structurally complex zones with many pre-existing planes of weakness. We propose four possibilities to form multiple fault patterns: (1) two or more events of faulting obeying Coulomb's law with a change of orientation of the principal stresses in each event; (2) reactivation of non-interacting planes according to the Bott (1959) model; (3) one three-dimensional strain event that obeys the “Slip Model”; this mechanism will form an orthorhombic four-fault pattern and two slickenline sets in a single strain event; and (4) one or more events obeying the interacting block model proposed here, with or without rotation of the principal stresses. We propose the last origin as the most common in continental regions.

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.

Influence of the structural framework on the origin of multiple fault patterns

We demonstrate that the general equation for three-dimensional strain by slip on orthorhombic faults can be rearranged to take a form that applies to two-dimensional strain due to slip on pre-existing planes of weakness. Therefore, either two-dimensional or three-dimensional strain may result from the same stress state. We deduce that the kinematic interaction between planes of weakness in a body is a fundamental factor to determine the type of strain produced by a stress state. Whether deformation occurs by forming new fractures or by slip on existing planes depends upon which requires a lower stress difference. The stress difference necessary to initiate slip along a plane is highly sensitive to variations in orientation, cohesion and depth. We propose a model for crustal deformation composed of an anisotropic body with existing planes of weakness that interact kinematically. The critical stress difference necessary to initiate sliding is that required by the interacting plane that needs the highest stress difference to slip. Because the stress difference will rise until it reaches a value that can cause slip on all interacting planes, once slip initiates it will occur simultaneously on all planes that require stress differences lower than the critical value. The anisotropic body model proposed here provides a mechanism for forming multiple fault sets and may pertain to the formation of low-angle normal faults.