Extensional Cracks Research Articles

Precise source location of AE doublets by spectral matrix analysis of triaxial hodogram

We have developed a precise relative source location technique using acoustic emission doublets (AE doublets) in the triaxial hodogram method to evaluate the direction and distance of subsurface extension cracks. An AE doublet is a pair of acoustic emissions with similar waveforms and adjacent locations on the same crack but which occur at different times. The relative source location is estimated by an analysis in the frequency domain. The relative distance between two AE sources is determined from the difference of P-S arrival time delays by cross‐spectrum analysis. The relative direction is derived using a spectral matrix from the difference in P‐wave polarization directions. We also propose a method to optimize the estimated relative location by using a group of AE doublets. The accuracy of the estimated source location was confirmed by performing field experiments. The relative locations of artificial wave sources about 150 m from a triaxial detector can be estimated with distance errors of less than 1 m, and direction errors of less than 3.8 degrees in both azimuth and inclination. Results of the application of this analysis on AE doublets in a geothermal field demonstrate its ability to evaluate deeper subsurface fractures.

Egan Range Volcanic Complex, Nevada: geochronology, paleomagnetism and magnetic fabrics

In a relatively brief interval near the Eocene-Oligocene boundary, trachydacite lavas were erupted over an area of about 300 km 2 to form the Egan Range Volcanic Complex, in the southern Butte Mountains and adjacent Egan Range of east-central Nevada. The mean radiometric age for this volcanic episode is 34.1 ± 0.2 Ma, based on whole-rock and biotite KAr ages. Detailed comparisons were made between anisotropy of magnetic susceptibility (AMS) and the macroscopic petrofabric at 31 sites. Great variation in flow structures is attributed to complex flow in viscous lava over a surface of considerable relief. The magnetic fabric, as expressed by AMS axes, agrees well with the flow petrofabric. Typically, flow lineations are parallel to K 1 and flow foliation is perpendicular to K 3, as expected. Extension cracks and joints are normal to K 1. Late-stage deformation of pipe vesicles also impressed a measurable magnetic fabric. Because of significant initial dip and complexity of flow fabric, paleohorizontal is not generally demonstrable. Paleomagnetic studies indicate an exclusively normal polarity. The mean in situ characteristic direction is relatively well grouped ( D = 350°, I = 52°, α 95 = 8°) for 28 selected sites and provides a mean pole at 124°E, 81°N, which is not significantly different from the coeval cratonic reference pole. These results indicate that large Oligocene or younger rotations have not affected this sector of the Basin and Range province. The combination of age and magnetic polarity suggests a correlation with a normal polarity subchron of chron 13 or 15.

Small‐scale fracture patterns on the volcanic plains of Venus

Some of the most striking features of the Magellan radar images of Venus are the intricate and varied deformation patterns on the volcanic plains. Particularly puzzling are remarkably linear features with a regular spacing of about 1 km first observed in the gridded plains of Guinevere Planitia. The very close spacing of these features requires an unreasonably thin lithospheric layer in conventional geophysical models for regular spacing. We have found many sets of parallel, regularly spaced lineations similar to the closely spaced features of the gridded plains. They are composed of parallel, thin, straight lineations whose microwave reflectivity does not appear to depend on radar illumination direction. These sets of parallel lineations, which we interpret to be fractures, typically cover areas with dimensions of hundreds of kilometers. Several examples of these regular lineations are characterized in terms of their average fracture spacing, which is between 1 and 2.5 km; the scatter in individual spacings is about ±1/3 the average. Based on observations, we hypothesize that these features are extension fractures in the brittle upper layers of the volcanic plains material, which formed well after emplacement and solidification of the flows. For parallel extension cracks, models based on the stress distribution around a crack and observations of regular jointing in sedimentary layers predict that the spacing of fractures should be roughly equal to the depth of fracturing, or to the thickness of the jointed layer. However, we observe that fracture sets in diverse locations all have nearly the same spacing, implying either that the fractures always form with a depth extent of about 1 km or that the spacing is independent of the thickness of the layer containing the fractures. We propose a model based on a shear lag mechanism used by materials scientists to explain morphologically similar patterns of regularly spaced tensile cracks on the brittle surfaces of layered composites. In this model, lithospheric extension causes a fracture to form in a surface layer of basalt several hundred meters to a kilometer thick. A horizontal detachment, corresponding to an older plains surface below the fractured layer, allows frictionally resisted lateral displacement of the surface layer in the region near the extension fracture. The model gives a spacing that is independent of layer thickness and is able to reproduce the observed fracture spacing using reasonable values of material constants. It provides a framework for using these sets of lineations to help determine mechanical properties of the volcanic plains and to place constraints on the sequence of their tectonic evolution. Another unresolved problem of the gridded plains has been the presence of an orthogonal set of brighter structures with a markedly different appearance and a somewhat larger scale of spacing. We utilize the observed morphology and self‐similarity of feature lengths to determine the reasons for their contrasting morphology. It is concluded that they are primarily extensional structures which penetrate the entire 2–4 km thick brittle crustal layer and which are likely to have accommodated some shear deformation.

Ground ruptures of the 1974 and 1983 Kaoiki Earthquakes, Mauna Loa Volcano, Hawaii

The November 30, 1974, ML = 5.5 and November 16, 1983, ML = 6.6 earthquakes generated left‐stepping, en echelon ground cracks within the Kaoiki seismic zone, on the southeast flank of Mauna Loa volcano, Hawaii. The general trend of the ruptures, N48°–55°E, parallels a nodal plane of the main shocks' focal mechanisms. The ruptures themselves consist of short, predominantly extension cracks, which are up to 20 m long and strike roughly E‐W, 30°–50° clockwise from the overall trend of the zones. Some of the cracks are linked by secondary fractures and rubble breccia to form left‐stepping crack arrays, which are themselves linked to form longer en echelon systems of ground rupture. Geologic maps and field observations indicate that these features emerge from an underlying strike‐slip fault, and they form a “fracture‐process zone” above its tip. The maximum displacement measured across cracks in the 1983 rupture zone is 0.5 m. Trilateration data, however, suggests that the overall shear displacement was about 1.5 m at depth. Elastic solutions indicate that a region of significant tensile stress can exist above buried strike‐slip faults. We suggest that these stresses generated the extensional ground cracks and that shear displacements were transmitted to the Earth's surface by subsequent growth and linkage of these cracks into the observed arrays. We infer that the crack arrays accommodate increased displacement with depth and they merge downward into the “parent” strike‐slip fault at an estimated 1–2 km depth, where strike‐slip displacement was probably more or less continuous along the ∼7 km length of the rupture. In the Kaoiki region, only three major ground ruptures traverse a series of basaltic lava flows that date back 1500 years. This suggests that the recent ∼10‐year periodicity of moderate‐magnitude Kaoiki strike‐slip events may not have extended far into the past. The tectonic significance of strike‐slip faulting on Mauna Loa volcano remains enigmatic.

Near-fault measurement of postseismic slip associated with the 1989 Loma Prieta, California, Earthquake

Five small-aperture (0.5 to 7.7 m) quadrilaterals were installed across the San Andreas fault and newly formed extensional cracks shortly after the October 17, 1989 Loma Prieta M7.1 earthquake. Two quadrilaterals were installed across the San Andreas fault near the southeast and northwest ends of the 1989 rupture, and three were installed across dominantly extensional cracks in the Summit Road area near the main-shock epicenter and off the San Andreas fault. Measurements of line-length changes from as early as 4 d up to 184 d after the earthquake in the nail quadrilaterals indicate a small amount of right-lateral postseismic slip on the San Andreas fault. The site near the southeast end of the 1989 afterstock zone on the San Andreas fault showed about 5±2 mm of postseismic right-lateral component of slip in addition to 23 mm of right-lateral coseismic movement. The site near the northwest end of aftershocks likewise showed about 5±2 mm of postseismic slip, but after only 5 mm of coseismic slip. Measurements at sites across extensional cracks indicate possible minor left-lateral postseismic slips, and possible extensional/contractional motion. No measurements were made of possible vertical movements. Sites on extensional cracks showed similarly small amounts of possible postscismic lateral slip, from ∼2 to 10 mm, even though coseismic lateral slips were much greater (up to 410 mm) than on the surface trace of the San Andreas fault. The small slip values, in spite of uncertainties, clearly show that the lack of coseismic surface slip associated with the earthquake was not followed by large postseismic slip.

Crystallization and deformation of amphiboles, and cummingtonite-forming reactions in the garbenschists of the Western Tauern Window (Eastern Alps, northeast Italy-western Austria)

A detailed microscopic study of the amphiboles crystallized during the late Alpine metamorphism in the hornblende garbenschists of the western Tauern Window reveals sevesal stages of crystallization (Cr 1–Cr 3) and deformation (D 1–D 3). During the first stage of crystallization (Cr 1) poikiloblastic green hornblende of tschermakitic composition overgrew a matrix showing a strong s 1 foliation. A slight tendency towards increasing Tschermak's and edenite exchange, with glaucophane component decreasing towards the rim, suggests amphibole growth during heating and decompression. This prograde metamorphism leads to simultaneous formation of cummingtonite and hornblende in 3 out of 16 studied samples. The cummingtonite-forming reactions are discussed. Both hornblende and cummingtonite show microcracks forming s 2, which is oblique to s 1. s 2 is missing in quartz and feldspar and also in some cases in the rims of the hornblende, but is traceable through trails of fluid inclusions. The last deformational event (D 3) is a tensional one producing s 3 as large cracks in the amphiboles. Correlation of deformation and exchange components with published P- T paths suggests growth of amphibole cores at amphibolite-facies conditions. Hornblende and cummingtonite rims formed at decreasing pressures but still rising temperatures. s 2 microcracks are due to tectonic deformation (D 2) while the s 3 extensional cracks were formed by ductile thinning (D 3) at rising temperatures, as revealed by the mineral parageneses found in the cracks. This paper presents the petrological framework for detailed geochronological investigations giving the time calibration of the P- T path of the Alpine metamorphism of the Western Tauern Window.

Surface effects and tectonic setting of the 13 December 1982 North Yemen earthquake

Abstract The North Yemen earthquake (Mb = 6.0) of 13 December 1982 is the first earthquake in the southern Arabian Peninsula known to be accompanied by surface displacements. The extensive destruction and loss of lives resulted entirely from widespread collapse of unreinforced masonry and mud brick structures; maximum Modified Mercalli intensity was probably VII to VIII. The only surface manifestation of tectonic activity was the occurrence of earthquake-related extensional ground cracks in the epicentral region. The cracks occur mainly in four relatively continuous north- to northwest-trending linear zones that range from a few hundred meters to 15 km in length and in irregular areas of polygonal extension cracks. The area within which the cracks occur is 22.5 km long by about 10 km wide. Continued dilation across some cracks was measured almost 1 month after the main shock. Seismicity and active volcanism in this region are inferred to reflect slow extension of the southwestern margin of the Arabia plate perpendicular to the Red Sea spreading axis.

Field relations between dikes and joints: Emplacement processes and paleostress analysis

Sedimentary rocks intruded by Tertiary mafic dikes on the Colorado Plateau typically display systematic dike‐parallel joints. These closely spaced joints occur only near dikes, their spacing commonly increasing with distance from the dike contacts. Igneous breccias along some of these joints indicate that the joints are not younger than the dikes and, therefore, did not form during cooling. Field relations are best explained if the joints form in host rocks beyond the dike tip, becoming juxtaposed against the dike with continued propagation: an interpretation supported by previously reported descriptions of ground surface cracks formed near eruptive fissures and of microcracks formed near the tips of larger extension cracks during laboratory experiments. Tensile stress generated by magmatic pressure is sufficient to fracture host rocks beyond dike tips. The tensile maxima are located on either side of the dike plane, beyond the tip. The stresses increase in magnitude closer to the tip, thus explaining the greater abundance of joints near the dike plane. Noting that dikes may parallel regional joint sets, we contrast (1) emplacement along older joints oriented arbitrarily with respect to the principal stress directions acting at the time of intrusion and (2) emplacement along self‐generated fractures propagated in a plane perpendicular to the least compressive stress direction. Magma can invade along older joints if magmatic pressure exceeds the horizontal stress acting across the joint plane. This situation is most common if the horizontal principal stress difference is small compared to the magmatic driving pressure or if joints are nearly perpendicular to the direction of least compressive regional stress. Magma must advance by filling self‐generated fractures if older, suitably oriented, joints are absent.

Strength of lightly reinforced elements in a compressed zone with- consideration of inelastic deformations of concrete

1. Experimental investigations established that in reinforced-concrete structural elements with a certain percentage of reinforcement, in zones with structural transverse reinforcement under loads below the standard longitudinal extension cracks of the compressed zone occur, which can lead to a decrease of reliability of the element, and in the presence of block joints in this zone to a decrease of bearing capacity. 2. Investigations of the stress state of a reinforced-concrete element with cracks established that the cause of occurrence of longitudinal cracks is the presence of considerable secondary transverse tensile stresses in the zone of the vertex of the normal crack. A physical scheme explaining the occurrence of these stress is presented. 3. A method of determining the necessary transverse reinforcement usually installed for structural considerations which is convenient for design was developed. Calculations showed the possibility in many cases of reducing the consumption of structural reinforcement, which is not regulated by existing standards.

An Inexpensive Device for Modelling Strike-Slip and Oblique-Slip Fault Zones

An appreciation of how structures form in deformed rock is greatly aided by appropriate model analogues. We describe the method of construction and use of a device to simulate structures produced in fault zones of dominantly strike-slip motion. Simple modifications to the apparatus allow simulation of transtension and transpression as well as pure strike-slip fault motion. The structures which develop in the models include both synthetic and antithetic Riedel shears, en echelon folds and extension cracks. Numerous natural examples are illustrated in the literature from strike-slip zones around the world. We illustrate the formation of these structures using the apparatus and compare them with natural examples.

Growth fault on insular slope/rise of western Iceland; comparison with SE Greenland canyons and slumps

A recently active normal fault and extensional crevasse on the western insular slope/rise boundary of Iceland adjoins a rotational slump that moved as the lower block of an active growth fault. A slump off SE Greenland is contrasted to the western Iceland slope/rise feature in translational movement and slump morphology. Bergschrund-like extensional cracks and valleys at the heads of slumps serve as troughs channeling gravity flows into established canyons on the midrise. Distinctive characteristics of buried canyons and levees may be confused with slump or slide surfaces and related subbottom features that have similar seismic reflection qualities. Examples are defined forGrowth Fault/Rotational Slumps, andTranslational Displacement Slumps.

Chrysotile asbestos at Woodsreef, New South Wales

Economic chrysotile asbestos in the Great Serpentinite belt of New South Wales is restricted to a deposit in the bifurcated Woodsreef Serpentinite. Although the Woodsreef Serpentinite is dominated by schistose serpentinite, large bodies of partially serpentinized harzburgite and massive serpentinite, as well as tectonic inclusions of dolerite, gabbro, and rodingite, occur immediately south of the Slice (the minor arm of the bifurcation) in the central part of the ultramafic body.This central area has suffered a unique stress-strain history and the asbestos deposit is localized in heavily fractured massive serpentinite which is cut by narrow S 1 and S 2 zones of schistose serpentinite. Six types of asbestos veins are recognized: all are localized in stress-induced openings--either mineralized fractures, in extension cracks, in dilation sites in folds and kink bands, or along cleavage planes in bastite grains. Within these cracks, fiber formation took place by solution and recrystallization. As growth kept pace with the opening of cracks, the fibers became elongate parallel to the direction of maximum elongation in the XY plane of the strain ellipsoid. This fiber growth took place during S 1 and S 2 formation in the surrounding schistose serpentinite and is thus an integral part of the deformation in this part of the body.

Origin and diagenetic evolution of Ca–Mg–Fe carbonates in some coalfields of Japan

ABSTRACTCarbonate concretions, lenses and bands in the Pleistocene, Palaeogene and Upper Triassic coalfields of Japan consist of various carbonate minerals with varied chemical compositions. Authigenic carbonates in freshwater sediments are siderite > calcite > ankerite > dolomite >> ferroan magnesite; in brackish water to marine sediments in the coal measures, calcite > dolomite > ankerite > siderite >> ferroan magnesite; and in the overlying marine deposits, calcite > dolomite >> siderite. Most carbonates were formed progressively during burial within a range of depths between the sediment‐water interface and approximately 3 km. The mineral species and the chemical composition of the carbonates are controlled primarily by the initial sedimentary facies of the host sediments and secondarily by the diagenetic evolution of pore water during burial. Based on the regular sequence and burial depth of precipitation of authigenic carbonates in a specific sedimentary facies, three diagenetic stages of carbonates are proposed. Carbonates formed during Stage I (< 500 m) strongly reflect the initial sedimentary facies, e.g. low Ca‐Mg siderite in freshwater sediments which are initially rich in iron derived from lateritic soil on the nearby landmass, and Mg calcite and dolomite in brackish‐marine sediments whose pore waters abound in Ca2+ and Mg2+ originating in seawater and calcareous shells. Carbonates formed during Stage II (500–2000 m) include high Ca‐Mg siderite, ankerite, Fe dolomite and Fe–Mg calcite in freshwater sediments. The assemblage of Stage II carbonates in brackish‐marine sediments in the coal measures is similar to that in freshwater sediments. This suggests similar diagenetic environments owing to an effective migration and mixing of pore water due to the compaction of host sediments. Carbonates formed during Stage III (> 2000 m) are Fe calcite and extremely high Ca‐Mg siderite; the latter is exclusively in marine mudstones. The supply of Ca is partly from the alteration of silicates in the sediments at elevated burial temperatures. After uplift, calcite with low Mg content precipitates from percolating groundwater and fills extensional cracks.

Compaction Effects in Fusulinid Limestone: ABSTRACT

Of 14 fusulinid genera in the Upper Pennsylvanian-Lower Permian Bird Spring Formation, only Pseudoschwagerina? shows evidence of deformation caused by compaction. Most deformation occurred in the outermost whorl; the wall and septa were jumbled, and the End_Page 530------------------------------ whorl collapsed. Only slight fracturing was observed in inner whorls. In places, the outer whorl was completely closed, the shell being flattened on top and bottom. In other specimens, chomata (internal structures) acted as a brace, preventing total flattening. At several places where the shell is particularly badly crushed, another fossil is in surface contact with the fusulinid. As compaction proceeded, grain-to-grain contacts (where stress was localized) changed to those along a surface; strain was realized by brittle failure of the fusulinid. No pressure solution took place. The major compressional stress, oriented normal to bedding, led to horizontal stresses away from the center of each fusulinid; extension cracks and buckling of the wall near the polar ends resulted. Most other fossils and the micrite matrix had sufficient strength to withstand later compaction, in part because of early submarine cementation. However, individuals belonging to this one fusulinid genus were selectively crushed during limestone compaction because of inherent weaknesses in the structural architecture of the shell. Pseudoschwagerina? was a large fusulinid with a thick, heavy wall; outer whorls were inflated, making it comparatively hollow. Septa which supported the wall were thin, straight (unfluted), and widely spaced. End_of_Article - Last_Page 531------------

Anatomy of a mining-induced fault zone

A mining-induced fault zone that formed in the East Rand Proprietary Mine, Boksburg, South Africa, during the sloping of a large, highly stressed pillar is associated with two large seismic events that occurred within 17 days of each other; each event caused appreciable rock-burst damage in the neighboring stopes. The zone consists essentially of two high-angle normal faults intersecting at about 30°, across which shear displacements of as much as 10 cm were measured. In detail, the fault zone comprises two approximately planar shear zones, each of which strikes subparallel to the stope working faces at the time of the seismic events, and which encompass areas of at least 500 and 600 m 2 , respectively. The shear zones are themselves made up of smaller, en echelon shear planes, which are connected by subsidiary conjugate shears and extension fractures. These shear planes become diffuse and are replaced by extension and bedding-plane fractures at the boundaries of the fault zone. Microscopically, the shear planes are seen to contain a layer of finely comminuted gouge material within which fluxion structure may be developed. The wall rock adjacent to the gouge is a microbreccia formed by shattered quartz grains and larger areas of rock, cut by conjugate shear and extension fractures across which some comminution has also occurred. The microscopic observations suggest the following sequence of events resulting in the formation of the fault zone: (1) development of extension cracks in the region of planes of maximum shearing stress, (2) coalescence of extension cracks to form conjugate shear planes, and (3) rapid movement along the more favorably oriented of these shear planes to form the major gouge zone and feather fractures. Dynamic analyses of the attitudes of the microfractures and macrofractures define the stress field operating at the time of the seismic event. There is a striking similarity between the fault zones and natural faults, both in macroscopic and microscopic form and in the development of cataclastic rock types.

Fracture growth around openings in large blocks of rock subjected to uniaxial and biaxial compression

The rock surrounding tabular excavations in deep mines is cut by sets of stable induced fractures but, occasionally, collapse of an excavation occurs violently during a seismic event, with the formation of a shear fracture of different attitude and morphology to the stable fractures. To study the development of these and other fractures seen in mines, blocks of quartzite and sandstone, containing holes of circular or rectangular cross-section were tested in uniaxial and biaxial compression over long time intervals. In some experiments, the long axes of the holes were enlarged while under load and strain rosettes were used to monitor the changes in strain at various points on the front faces of the blocks. The fracture pattern developed as follows: during the initial loading, spalling occurred at the ends of the holes and extension cracks formed in the centre. With increasing axial load or enlargement of the hole, larger cracks propagated out of the zone of spalling; these cracks dipped at 60–80° towards the rock in front of the opening and with time tended to form domical cracks above the hole. When the hole reached its maximum length or the axial stress approached the failure strength of the samples, new vertical or steeply inward dipping cracks formed in front of the holes; shear movement on these cracks was such as to cause material to collapse into the holes. To facilitate this movement, cracks parallel to the long axes of the holes opened up in the rock above or below the holes. Analysis of the strain rosette data showed the build up of stresses in the rock as the hole advanced towards the measurement point and provided information on the growth of fractures. Each period of mining was accompanied by sudden jumps in strain followed by creep, lasting from a few minutes to 24 hr. The fractures developed around the holes resemble those seen in deep mines and the experiments indicate that fractures associated with seismic events are likely to form in high stress environments when the dimensions of the opening are large.

Fracture growth around openings in thick-walled cylinders of rock subjected to hydrostatic compression

Cylindrical specimens of rock into which holes of circular, elliptical and rectangular cross section had been cut, were tested in hydrostatic compression, in order to study the growth of fractures around the holes. The rock types used were an isotropic, homogenous sandstone and an anisotropic, argillaceous quartzite and the relative sizes of specimens and holes were such as to eliminate boundary effects for at least one of the specimen geometries. Failure around the circular holes initiated by shearing at diametrically opposite points and spalling continued at these loci until the opening was sufficiently large for ultimate collapse of the specimen to occur. In the elliptical and rectangular specimens spalling along similar shear planes occurred at the ends of the long axes of the holes. However, if the applied hydrostatic pressures were sufficiently large and were held for a sufficiently long time, a second set of conjugate shear fractures appeared, along which movement was such as to cause material to move into the opening. In addition, extension cracks parallel to the long axes of the holes tended to form in the region above the opening and these cracks often coalesced with shear cracks radiating out from the ends of the holes to form a series of domical cracks. There is a striking similarity between the observed fracture patterns and that predicted theoretically around an elliptical hole with an 8:1 axial ratio in an isotropic elastic medium although the effect of bedding planes in the quartzites is to modify the orientation of the fractures. There is also good agreement between the observed fractures and those observed in front of tabular excavations in mines.

Landslide and Earthpressure

Earth pressure on a vertical wall in a sliding mass changes al ong the sliding surface, and it changes also time to time . An active or passive failure will take place along a slidi ngsurface inthe sliding soil mass. The auther pointed out that this idea might be helpful for designing countermeasures by remov ing a part of soil mass or drainage . The ground surface is extended, and extension cracks are observed in the active pressure region . The ground surface is compressed and swell up in the passive region. Wh ena part of soil mass is removed, the area is extended from the activepressureregion to theinter mediate region, and should not be extended to the passive pressure region.