Current Stress Field Research Articles

The 16 March 1999 mN 5.1 Cote-Nord Earthquake: The Largest Earthquake Ever Recorded in the Lower St. Lawrence Seismic Zone, Canada

On 16 March 1999 at 12:50 UT, a moderate earthquake of mN 5.1 occurred in the Lower St. Lawrence Seismic Zone (LSZ), 60 km south of Sept-Iles, Quebec. Due to its location under the St. Lawrence River, more than 50 km away from inhabited areas, the earthquake was named the Cote-Nord earthquake (the French name for the Quebec North Shore region along the St. Lawrence River). In this zone, approximately 60 events are located yearly; historically, only one earthquake was nearly as large as the Cote-Nord event. Felt up to 300 km distant, the mainshock did not cause any damage. Peak ground accelerations of 0.22 g and 0.13 g were measured at epicentral distances of 130 km and 360 km by digital accelerographs. Three different determination methods each suggest a depth of approximately 20 km, within the Precambrian basement. A mixed strike-slip/reverse-faulting focal mechanism was obtained using regional data. Surface-wave inversion suggests a similar solution but with slightly different nodal planes. No foreshock was recorded by the regional network, which has a mN 2.0 detection threshold. The number of aftershocks was anomalously high for a mN 5.1 mainshock; more than 75 aftershocks with mN ranging from 0.9 to 3.5 were recorded in the 30 days following the mainshock. Characteristic parameters of aftershock decay were normal. Within these aftershocks, five doublets and one triplet were found, representing 13 out of 77 events. We interpret the northwest-southeast nodal plane to be the fault plane for the mainshock from the preferred alignment of aftershocks and from the orientation of nearby magnetic and gravimetric lineaments. This orientation is, however, different from the general assumption that northeast-southwest faults are those reactivated in the current local stress field.

ISRM Suggested Methods for rock stress estimation—Part 2: overcoring methods

Adopting 47 sets of in situ stress measurements taken at depths ranging from 417 to 1123 m in a coal mining district in central China by the overcoring method, we characterize the magnitude and orientation of the current stress field and its relationship to tectonism in this district to determine the controls on in situ stress in underground coal energy exploration and seismogeology. The measurements reveal that this district is in a relatively high horizontal stress environment. The stress regimes are generally favorable for strike-slip faulting and reverse faulting and are characterized by σH > σv > σh and σH > σh > σv, revealing that the regional stress field is absolutely dominated by horizontal tectonic stress and reflects a typical tectonic stress field type. The maximum horizontal principal stress is predominantly oriented between the ENE–WSW and WNW–ESE directions and averages nearly E–W,​ which is closely related to the tectonic plate movement and crustal deformation and agrees well with the direction derived from focal mechanism solutions and other stress indicators in and around the region. The current stress field in this district generally inherited the characteristics of the tectonic stress field in the third stage and partially retained the traces generated by the tectonic stress field in the first and second stages. In addition, the frictional parameter μm, which is calculated using the derived stress data, is used as a strong predictor to reflect the capability of the level of crustal stress accumulation. This region is in a state of critical or subcritical equilibrium, and regional faults at shallow depths have a medium to slightly high frictional strength, indicating that some well-oriented faults may theoretically approach or reach frictional equilibrium and that frictional sliding may occur.

Effects of crustal stresses on fluid transport in fractured rock: case studies from northeastern and southwestern USA

The link between stress and hydrologic properties was examined at two sites that are distinguished by different rock types and different stress states. This investigation is based upon the analysis and interpretation of geophysical logs obtained in water wells at the two locations. At the northeast site (Newark Basin), the hydrologic characteristics of sedimentary rocks are dependent upon the relationship to the current regional stress field of two primary types of orthogonal features that serve as preferential pathways for fluid flow. Subhorizontal bedding-plane partings are highly transmissive near the surface and delineate transversely isotropic fluid flow at shallow depths. With increasing depth, the subhorizontal planes become less dominant and steeply dipping fractures become more influential hydrologically. These high-angle features define anisotropic flow pathways that are preferentially oriented along strike. At the southwest site (west Texas), extrusive rocks are subjected to topographically modified tectonic and gravitational stresses that vary spatially within a valley setting. The attendant changes in stress invariants cause fracture connectivity within the rock mass to systematically increase with depth along the valley flanks, but to remain relatively low in the central valley. The degree of fracture connectivity predicted within this valley configuration is consistent with variations in transmissivity determined at several well locations. In each of these cases, the idealized understanding of the hydrologic system is enhanced by considering the effects of regional and local stresses that act upon the fractured-rock aquifer.

The June 2000 Mw 7.9 earthquakes south of Sumatra: Deformation in the India–Australia Plate

Two large (Mw 7.9) earthquakes occurred on 4 and 18 June 2000, south of Sumatra, beneath the Indian Ocean. Both earthquakes were predominantly left‐lateral strike‐slip on vertical N‐S trending faults that we interpret to be reactivated fracture zones. The 4 June Enggano earthquake occurred at the edge of the rupture area of the 1833 subduction earthquake. The first strike‐slip subevent within the subducting plate triggered a thrust subevent on the plate interface, which comprised at least 35% of the total moment and ruptured SE away from the 1833 earthquake. The 18 June earthquake in the Wharton Basin is one of the largest shallow strike‐slip faulting earthquakes ever recorded. A small second subevent with reverse slip is required to fit the body waves. The orientation of both subevents in our preferred model is consistent with the current stress field in the region. Both the June 2000 earthquakes are consistent with recent models of distributed deformation in the India–Australia composite plate. The occurrence of the Enggano earthquake implies that the stress field within the Indian plate continues to a depth of 50 km in the subducting slab. The purely strike‐slip source model of the Wharton Basin earthquake obtained by Robinson et al. [2001] matches the P waves very poorly and fits the S waves no better than our preferred model. The strike‐slip subevents of both earthquakes had few aftershocks and higher stress drops than the subduction thrust subevent of the Enggano earthquake. This difference is consistent with previous observations of oceanic and subduction earthquakes.

A case study of hydrocarbon transport along active faults and production-related stress changes in the Monterey Formation, California

Abstract Recent field studies show that critically stressed faults, that is, faults that are close to frictional failure in the current stress field, serve as conduits for fluid flow. Similarly, geological field studies of the low permeability siliceous shales in the Monterey Formation, California, clearly indicate that faults influence hydrocarbon transport. We report here a study of the Antelope Shale, a low permeability siliceous shale hydrocarbon reservoir in the Buena Vista Hills field in the southern San Joaquin Valley to determine the influence of the stress state on the relative hydraulic conductivity of the fractures and faults present in the subsurface. Because production has both lowered reservoir pressure and the horizontal stresses, it was necessary to “restore” the reservoir stress state to initial conditions in order to identify correctly the most highly productive intervals. This analysis demonstrates that prior to production, faults in the reservoir were active in a transitional reverse/strike-slip faulting stress state, consistent with regional tectonics. Initial production rates in the field were 2000 barrels of oil per day, principally from intervals where critically stressed faults were encountered.

Paleoseismicity of Two Historically Quiescent Faults in Australia: Implications for Fault Behavior in Stable Continental Regions

Paleoseismic studies of two historically aseismic Quaternary faults in Australia confirm that cratonic faults in stable continental regions (SCR) typically have a long-term behavior characterized by episodes of activity separated by quiescent intervals of at least 10,000 and commonly 100,000 years or more. Studies of the approximately 30-km-long Roopena fault in South Australia and the approximately 30-km-long Hyden fault in Western Australia document multiple Quaternary surface-faulting events that are unevenly spaced in time. The episodic clustering of events on cratonic SCR faults may be related to temporal fluctuations of fault-zone fluid pore pressures in a volume of strained crust. The long-term slip rate on cratonic SCR faults is extremely low, so the geomorphic expression of many cratonic SCR faults is subtle, and scarps may be difficult to detect because they are poorly preserved. Both the Roopena and Hyden faults are in areas of limited or no significant seismicity; these and other faults that we have studied indicate that many potentially hazardous SCR faults cannot be recognized solely on the basis of instrumental data or historical earthquakes. Although cratonic SCR faults may appear to be nonhazardous because they have been historically aseismic, those that are favorably oriented for movement in the current stress field can and have produced unexpected damaging earthquakes. Paleoseismic studies of modern and prehistoric SCR faulting events provide the basis for understanding of the long-term behavior of these faults and ultimately contribute to better seismic-hazard assessments.

Palaeostress evolution of the Iberian Peninsula (Late Carboniferous to present-day)

Three main stages can be differentiated in Iberia from the Upper Carboniferous to the Present: the end of the Variscan orogeny (Late Carboniferous), the Permian–Early Cretaceous rifting and the Alpine orogeny (Late Cretaceous to Present). In this contribution, the main features of the stress fields determined in each of these stages are compiled and revised from the analysis of the brittle deformations, mainly microfault population. Stresses are related to the development of the principal structures of each stage. In the Late Variscan fracturing stage, sinistral strike–slip faults prevailed in a stress field with NE–SW subhorizontal compression. In the Permian-to-Early Cretaceous rifting stage, the extensional trends varied in each sector. During the Alpine orogeny, primarily the margins of the Iberian Massif (the Betic and Pyrenee mountain chains) were deformed. However, numerous deformations are also identified in its interior, where large mountain chains such as the Central System and the Iberian Chain developed. These deformations occurred in a complex stress field where zones of NW–SE to NNE–SSW horizontal compression prevailed, albeit predominantly extensional zones existed as well. At present, the stress field in most of Iberia has a NW–SE compressive trend, associated with active deformations in the Betic Cordillera, where a continental subduction developed. However, simultaneously, there are zones in which extensional settings predominate, such as the Internal Zones of the Betic Cordillera and the Gulf of Valencia region, making for quite a complex current stress field.

Stress control of hydraulic conductivity in fracture-saturated Swedish bedrock

The granitoid bedrock exposed in the Äspö Hard Rock Laboratory is the best known rock mass in Sweden but is probably generally similar to most of the country in many characteristics relevant to the long term isolation of spent nuclear fuel. Like most Precambrian rocks in the Fennoscandian shield, the rocks of Äspö are saturated by fractures with a large range of orientations after long and complicated deformation histories.The ca. 11,000 fractures documented throughout the Äspö HRL belong to six sets with distinctive orientations. Only about 8% of the total fracture population were wet when they were excavated. Although the wet fractures belong to the same six sets as the total population, their numbers in each set change abruptly at a particular gently dipping fracture zone 240m deep in the laboratory. We correlate the numbers of wet fractures in particular sets to different regimes in the current stress field. Most wet fractures are subhorizontal in a stress regime prone to thrusting above a depth of 240m and subvertical with NW trends in an underlying stress regime prone to wrench faulting.Faults favourably orientated for slip or dilation in the ambient stress field have recently been shown to be the most active groundwater flow pathways near a plate boundary. Here we show a similar relationship for old fractures in a Precambrian shield embedded deep in the Eurasian plate. However, the stress regimes, and thus the anisotropy of transmissivity, are complicated by post-glacial uplift in the Fennoscandian shield and can change 90° within a few metres at depths and locations on the scale of tens and hundreds of metres.

Influence of current annealing, stress, torsion and dc magnetic field on Matteucci effect in amorphous wires

Influence of axial dc magnetic field and applied stress on induced Matteucci voltage in as-cast and current annealed amorphous wires has been studied. Induced Matteucci voltage shows large variations with applied axial dc magnetic field and stress, which is further influenced by current annealing with or without torsion, via anisotropy induced by current annealing. Linear variation of induced Matteucci voltage with applied axial dc field and stress is reported that can be useful especially for field and stress sensors.

Fracture permeability and in situ stress to 7 km depth in the KTB scientific drillhole

To better understand the mechanisms that control fluid flow and hydraulic conductivity at significant depth in the brittle crust, we have examined the relationship between fracture permeability and in situ stress in the German continental deep drillhole (the KTB main hole) through analysis of high resolution temperature profiles. Our analysis shows that over the entire 3–7 km depth range studied, permeable fractures and faults (i.e., those associated with distinct thermal anomalies) lie close to the Coulomb failure line for a coefficient of friction of about 0.6. This indicates that critically‐stressed faults in the crust are also the most permeable faults. This includes a major Mesozoic thrust fault at 7.1 km that is being reactivated as a strike‐slip fault in the current stress field. Conversely, non‐critically stressed fractures and faults do not appear to be permeable as they are not associated with identifiable thermal anomalies.

Widespread extension in the core of the western Alps revealed by earthquake analysis

The western Alps are an active collision belt whose current stress field is inhomogeneous [Müller et al., 1992]. We report new seismological data which significantly improve our knowledge of this stress field. About 1600 earthquakes which occurred in the western Alps during the last 10 years were precisely located, and 79 new focal solutions were computed. The analysis of this database shows that widespread extension affects all the internal zones of the belt. To better constrain the associated stress regime, six stress tensors were computed using the Gephart and Forsyth [1984] method. They show that the current tectonics of the western Alps are contrasted with close variation in the stress regime (transpression to the front of the belt contrasting with extension in the core of the belt). The extensional direction is radial to the arcuate geometry of the belt and bounded outboard by the former thrust of the internal zones onto the external zone, suggesting extensional reactivation of this inherited crustal discontinuity. Such widespread extension within the inner part of an actually ongoing collision belt cannot be explained by simple collision‐related tectonics. We propose that intrabelt buoyancy forces, such as those produced by a slab retreat or slab break‐off, interfere with the boundary forces driven by the ongoing Europe‐Africa convergence.

Geological investigations at Killari and Ter, central India and implications for palaeoseismicity in the shield region

The 1993 Killari earthquake occurred in the central part of the Indian shield, an area generally believed to be of low seismogenic potential. This rare event provided an opportunity to understand the seismogenesis within the shield regions. Trench excavations in the rupture zone as well as other natural exposures in the vicinity of the epicentral region of the Killari earthquake indicate episodic activity separated by long periods. Deep drilling in the epicentral area recorded a maximum of ∼6 m of slip in the deeper strata ( Gupta et al., 1998a). Assuming an average of about 1 m slip for a moderate earthquake, as revealed by the 1993 event, accumulated slip indicates at least six moderate earthquakes at Killari. While the timing of the penultimate event may work out to be hundreds of thousands of years or more, evidence for at least one moderate earthquake ∼1500 yr ago was obtained from Ter, about 40 km northwest of Killari. Observations at Killari and Ter indicate the localization of deformation along a preexisting shear zone that are being reactivated in the current stress field. Because of factors like low background seismicity, long interseismic intervals and smaller ruptures compounded by low preservation of surface exposures, behaviour of seismogenic structures in the shield regions may appear to be random in space and time. Observations presented in this paper suggest that the perceived randomness probably holds only for individual patches in the current time window. Other parts of the seismogenic fault may behave independently, showing a different temporal pattern. Comprehensive investigations of the entire structure are needed to understand the long-term behaviour of such faults, and thereby improve the seismic hazard assessment in the cratonic interiors.

Quaternary grabens in southernmost Illinois: deformation near an active intraplate seismic zone

Narrow grabens displace Quaternary sediments near the northern edge of the Mississippi Embayment in extreme southern Illinois, east-central United States. Grabens are part of the Fluorspar Area Fault Complex (FAFC), which has been recurrently active throughout Phanerozoic time. The FAFC strikes directly toward the New Madrid Seismic Zone (NMSZ), scene of some of the largest intra-plate earthquakes in history. The NMSZ and FAFC share origin in a failed Cambrian rift (Reelfoot Rift). Every major fault zone of the FAFC in Illinois exhibits Quaternary displacement. The structures appear to be strike-slip pull-apart grabens, but the magnitude and direction of horizontal slip and their relationship to the current stress field are unknown. Upper Tertiary strata are vertically displaced more than 100 m, Illinoian and older Pleistocene strata 10 to 30 m, and Wisconsinan deposits 1 m or less. No Holocene deformation has been observed. Average vertical slip rates are estimated at 0.01 to 0.03 mm/year, and recurrence intervals for earthquakes of magnitude 6 to 7 are on the order of 10,000s of years for any given fault. Previous authors remarked that the small amount of surface deformation in the New Madrid area implies that the NMSZ is a young feature. Our findings show that tectonic activity has shifted around throughout the Quaternary in the central Mississippi Valley. In addition to the NMSZ and southern Illinois, the Wabash Valley (Illinois–Indiana), Benton Hills (Missouri), Crowley's Ridge (Arkansas–Missouri), and possibly other sites have experienced Quaternary tectonism. The NMSZ may be only the latest manifestation of seismicity in an intensely fractured intra-plate region.

Linear zones, seismicity, and the possibility of a major earthquake in the intraplate western Lake Ontario area of eastern North America

Seismic hazard assessments conducted in eastern Canada rely on seismological data, which are essential, but alone, inadequate. That is because the earthquake record is too short to provide a representative picture of where large earthquakes have occurred in the past. Consequently, seismic hazard may be underestimated in areas, such as that encompassing western Lake Ontario, that are devoid of documented large earthquakes. To attempt to ascertain the likelihood of a major earthquake occurring in that highly populated and industrialized area, three regionally extensive geophysically expressed lineaments were investigated and the results were combined with available seismological information. The most conspicuous is the Niagara-Pickering linear zone, within which there have been at least two, if not three, periods of brittle faulting, including displacements compatible with the current stress field. It also appears to represent the same major structure as the Akron magnetic boundary in eastern Ohio, the site of the mb = 4.9 Leroy earthquake. The second is the Georgian Bay linear zone, which extends from Georgian Bay to New York State. It displays evidence of recent outcrop-scale faulting, an alignment of earthquakes along its southwestern boundary, and a possible spatial relationship to other earthquakes, including two of M >= 5. Lastly, there is the Hamilton - Lake Erie lineament, which is parallel and proximal to a possible fault and coincides with a linear array of small to moderate earthquakes. All three converge on the western Lake Ontario area, which has a higher frequency of seismicity than the adjacent areas. Thus, the western Lake Ontario area might have a greater potential to experience a major earthquake than heretofore believed.

Tertiary and Quaternary tectonic faulting in southernmost Illinois

Tertiary and/or Quaternary tectonic faulting is documented in three areas of southernmost Illinois: the Fluorspar Area Fault Complex (FAFC) in Pope and Massac Counties, the Ste. Genevieve Fault Zone (SGFZ) in Alexander and Union Counties, and the Commerce Fault Zone (CFZ) in Alexander County. In the FAFC, faults that strike NE and NNE displace Mounds Gravel (late Miocene to early Pleistocene) and, locally, the Metropolis terrace gravel (Pleistocene; pre-Woodfordian). No Woodfordian or younger deposits are deformed. Faults typically outline narrow, linear grabens that formed under tension with a component of strike slip. North-south to NW-trending vertical faults near the southeast end of the SGFZ displace Eocene sediments. Again, faults outline narrow grabens and show indications of strike slip. Deformed Quaternary sediments have not been observed. The CFZ, which trends northeast, displaces Mounds Gravel in Illinois and units as young as Peoria Silt (Woodfordian) in Missouri. Quaternary movement has been interpreted as right-lateral strike-slip. The CFZ coincides with a subtle gravity and magnetic lineament and seems to reflect a major feature in the basement. Surface expression in Illinois is subtle, but mafic and ultramafic intrusions, hydrothermal alteration and small faults align with the Commerce geophysical lineament. Earthquake foci in Missouri and Illinois lie on or close to the CFZ; some focal mechanisms fit the fault trend. Among these structures, only the CFZ exhibits slip that conforms to the current stress field (principal compressive stress axis E-W to ENE-WSW). Possibly, the stress field changed during Neogene time. Alternatively, high fluid pressures or local stress concentrations may have induced slip on less favorably oriented fractures. Tighter constraints are needed on timing, magnitude, and direction of Neogene displacement.

Episodic nature of earthquake activity in stable continental regions revealed by palaeoseismicity studies of Australian and North American quaternary faults

Palaeoseismic investigations of recent faulting in stable continental regions of Australia, North America and India show that these faults typically have a long‐term behaviour characterised by episodes of activity separated by quiescent intervals of at least 10 000 and commonly 100 000 years or more. Long recurrence intervals such as these are well documented by detailed studies of the faults that ruptured during the 1986 Marryat Creek, South Australia and 1988 Tennant Creek, Northern Territory earthquakes. Thus, neotectonic features associated with stable continental region faults such as scarps and grabens commonly have subtle geomorphic expression and may be poorly preserved. Many potentially hazardous faults in stable continental regions are aseismic, which is one reason why the inventory of these faults is incomplete. Although they may be currently aseismic, faults in stable continental regions that are favourably oriented for movement in the current stress field could produce damaging earthquakes, often in unexpected places. Comprehensive palaeoseismic investigations of modern and prehistoric faulting events in stable continental regions are needed to understand the long‐term behaviour of these faults, and thereby, improve seismic‐hazard assessments.

Relationships Among In-Situ Stress,  Fractures and Faults, and Fluid Flow:  Monterey Formation, Santa Maria Basin, California

We used borehole televiewer (BHTV) data from four wells within the onshore and offshore Santa Maria basin, California, to investigate the relationships among fracture distribution, orientation, and variation with depth and in-situ stress. Our analysis of stress-induced well-bore breakouts shows a uniform northeast maximum horizontal stress (SH max) orientation in each well. This direction is consistent with the SH max direction determined from well-bore breakouts in other wells in this region, the northwest trend of active fold axes, and kinematic inversion of nearby earthquake focal plane mechanisms. In contrast to the uniformity of the stress field, fracture orientation, dip, and frequency vary considerably from well to well and within each well. With depth, fractures can be divided into distinct subsets on the basis of fracture frequency and orientation, which correlate with changes of lithology and physical properties. Although factors such as tectonic history, diagenesis, and structural variations obviously have influenced fracture distribution, integration of the in-situ stress and fracture data sets indicates that many of the fractures, faults, and bedding planes are active, small-scale strike-slip and reverse faults in the current northeast-trending transpressive stress field. In fact, we observed local breakout rotations in the wells, providing kinematic evidence for recent shear motion along fracture ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received April 15, 1996; revised manuscript received December 27, 1996; final acceptance July 15, 1997. 2Department of Geophysics, Stanford University, Stanford, California 94305-2215. We wish to thank Tom Zalan of the Chevron U.S.A. Production Company for providing the offshore well data, Unocal Corporation for providing the data on the onshore well, and Marcia McLaren from Pacific Gas and Electric Company for providing the earthquake focal mechanisms used in the stress inversion analysis. The data used as background seismicity in Figure 1 were extracted from the World Wide Web of the Southern California Seismic Network (SCSN) catalog operated jointly by the Seismological Laboratory at Caltech and the U.S. Geological Survey, both in Pasadena, California. We appreciate the comments and helpful discussions from Daniel Moos, Steve Graham, and Lev Vernik.

Joint structures determined by clustering microearthquakes using waveform amplitude ratios

A new method of clustering seismic events has been developed to identify features within a large cloud of microearthquakes induced by the pressurization of a 3.5-km deep, jointed region of granitic rock. The relative amplitudes of shear- and compressional-waves were taken as an expression of the source mechanism for each microearthquake. Then, the events were clustered according to their similarity in S P ratio and a second criterion, their spatial proximity to each other. Thus, each cluster contains events with closely spaced hypocenters and similar S P ratios, and therefore a high probability of the same (or very similar) focal mechanism. This method was applied to a data set of 8000 microearthquakes recorded during hydraulic stimulation of the Hot Dry Rock geothermal reservoir at Fenton Hill, New Mexico, located on the western flank of the Valles Caldera. Over 50 clusters of more than 10 events each were identified as planar features and over 20 such clusters were identified as linear features, together containing 15% of the usable events. Two predominant sets of planar features strike north to N45°W, roughly tangent to the ring fault system circling the caldera and dip 60° in either direction. These planes represent joints that may be associated with stress fields accompanying caldera formation. Most observed planes contain significant resolved shear within the current stress field. However, wide ranges of equally probable orientations are not observed, indicating a relatively unbiased sampling of the actual range of joint orientations. Most linear features are oriented parallel to one or both sets of planar features and may be associated with joint intersections or terminations. These results support a reservoir flow model composed of water storage within aseismic joints oriented perpendicular to the minimum stress direction and of limited dimension, connected by high impedance paths along joints striking north to N45°W as defined by the cluster results.

Neotectonic activity in and around the southwestern Rhenish shield (West Germany): indications of a levelling comparison

Precise levelling data are re-examined in the southwestern Rhenish shield and its foreland by comparing them analytically along levelling profiles. Instead of generalizing regional vertical movement trends, this method emphasizes the activity of individual structural elements, which can be located with a high accuracy. It is shown that present-day vertical motions concentrate on fractures which cut the massif into a number of tectonic blocks. These undergo more or less independent movements. Significant measured displacements range from 1 to 3.5 cm over an average 20-year period and correspond mostly to aseismic slip. High displacements are found near pre-existing faults, sometimes also zones of joint concentration which are favourably oriented with respect to the current regional stress field. In the Mosel area, SW-NE-trending faults are predominantly reactivated as reverse faults. The motion inferred for the Hunsrück border fault is also consistent with the compressive regime presently observed in that area, with σ 1 oriented to the northwest. The direction of vertical motions along the western border fault of the upper Rhine graben changes from south to north in relation to the different azimuths of the central and northern segments of the graben, inducing a S-N-oriented transition from compressional to extensional shear. Within the graben itself, some N160°E-trending normal faults are identified, one of them having probably ruptured in a swarm of microearthquaks not long before the second survey was performed. The western Saar-Nahe trough is characterized by N-S-oriented fractures which cannot be related to mapped faults but show a close connection with photolineaments.

Recent, tectonically induced, surficial stress-relief structures in the Ottawa–Hull area, Canada

Contemporary reverse faults, marked by offset boreholes, were identified in two roadcuts, and recently formed pop-ups, which are surficial chevron folds, have been recognized in a quarry in the Ottawa-Hull area of Ontario and Quebec, Canada. Displacement directions of the hanging walls, marked by the offset boreholes, are commonly north-northeast to east-northeast, though one set shows displacement to the northwest. The pop-ups recorded during this investigation show average orientations of 120 and 063°. These are similar to the average trends of 136 and 074° documented in a previous study from a quarry about 20 km away. East-southeast- to southeast-trending pop-ups predominate in the two quarries and are kinematically compatible with most of the offset borehole directions recognized to date in the Ottawa-Hull area. Moreover, the quarry-floor pop-up trends in the Ottawa-Hull area are consistent with those of open field, lake bottom and other quarry-floor pop-ups in an area extending from the Miramichi region of New Brunswick into the east-central U.S.A. The compressional origin of the reverse faults and pop-ups, and the predominant orientations of those structures, are compatible with the current ambient stress field in eastern North America. This implies that both the displaced boreholes and quarry-floor pop-ups are products of that stress field, and, despite their presence in excavations, are tectonic in origin.