摘要:

Crack lengths and widths in virgin and previously stressed Westerly granite have been measured by using the scanning electron microscope (SEM). Seismic velocities computed from the observed crack aspect ratios and porosities in virgin rock are compared with previously measured values both in the unconfined state and as a function of hydrostatic pressure. These comparisons suggest that even with careful SEM observation one may fail to see a substantial number of the cavities present; these are almost exclusively in the high aspect ratio fraction. Agreement between the results of noninteractive and self‐consistent velocity calculations is poor at low confining pressures but excellent at pressures in excess of a few hundred bars. The inability of present theoretical treatments to predict seismic velocities in rocks as a function of differential stress lies not with velocity calculations but with the lack of a quantitative model for the development of cracks in dilatant rocks. However, in its present form O'Connell and Budiansky's self‐consistent, isotropic theory can provide information about crack densities in stressed rock from seismic velocity data if measurement paths are carefully chosen. By combining computed crack densities with measurements of dilatant volumetric strain, it is found that the average aspect ratio of stress‐induced cracks increases with stress, reaching a maximum of 2–3 parts in 103at 80% of the fracture s

ISSN:0148-0227    

DOI:10.1029/JB081i020p03484

年代:1976

数据来源: WILEY

摘要:

The existence of precursory bulge along an incipient fracture zone in a uniaxially compressed Westerly granite sample has been investigated by two optical methods. The first method is the method of slit diffraction. The cross section of a cylindrical rock sample is monitored by bringing two straightedges next to the rock sample to form two slits (each slit being formed between one straightedge and one side of the rock sample) and by illuminating alternately the two slits with a collimated laser beam. The Fraunhofer diffraction pattern is recorded on film in the direction perpendicular to the straightedge and can be interpreted as the absolute value squared of the one‐dimensional spatial Fourier transform of the slit under certain conditions, thereby providing a simple method of magnification of the rock surface geometry. The conditions under which the Fraunhofer diffraction pattern can be interpreted as the absolute value squared of a one‐dimensional Fourier transform are related to the radius of the rock sample, width of the slit, position of the recording film plane, and nature of deformation of the rock surface and are presented in this paper. The films are digitized by a microdensitometer. The data are analyzed by digital filtering and interpolation techniques to give a strain resolution of 10−5. During a test with the slit diffraction method, strain inhomogeneities in terms of local bulges indicative of incipient failure zones were found to develop at ∼92% of the uniaxial compressive strength, and their propagation is traced at 2.66‐s intervals until failure. Local strains in the incipient failure zones are of the order of 10−2before failure takes place. Because of the large amplitude of the strain inhomogeneity prior to failure recorded by the slit diffraction method, we then tried the faster method of recording without magnification by a motion picture camera. In the second test a precursory bulge in the middle of the sample first appeared at ∼3.75 s prior to failure at a load of>99.7% of the uniaxial compressive strength. The bulge developed rapidly in successive frames until eventually a failure plane passed through this sharp bulge. The results from both tests demonstrate the formation of a concentrated weak zone as a result of the interaction and coalescence among the microcracks in the final stage of the test, which then develop into fracture zones. The bulging is the result of accentuated deformation in the weak zone because of its reduced deformation moduli. It is considered that the local bulge and orientation of the fracture zones in the first test were controlled by the stress concentration at the sample‐load block interface, whereas those in the second test were controlled mainly by the inhomogeneity in the material properties within the sample. The precursor times of both tests do not fit into the empirical relationship between precursor time and fault dimension as derived from earthquakes and mine rock bursts. The precursor times of these tests are too long by 3 orders of magnitude in comparison with those given by the empiric

ISSN:0148-0227    

DOI:10.1029/JB081i020p03495

年代:1976

数据来源: WILEY

摘要:

Thermal fluctuation analysis is a new method of determining ‘magnetic grain size’vand microscopic coercive forceHKin particles too small for direct domain observations. When it is used with single‐domain particles, thermal fluctuation analysis is a method of magnetic granulometry, sincevis the physical grain volume andHKis usually closely related to grain shape. For multidomain particles,vcorresponds to one Barkhausen jump of a domain wall, andHKdescribes the opposition to wall motion. The technique requires three steps: (1) measurement of coercive forceHcand saturation magnetization over a broad temperature range, (2) separation of the contributions ofHKand the ‘fluctuation field’HqtoHC, in a manner suggested by the Néel (1949) theory, and (3) calculation of average values ofvand room temperatureHKfromHq(T) andHK(T). Thermal fluctuation analysis is the only simple means of calculating averagev; andHKseparately. If thermal fluctuations are important, as is always the case for the fine particles or small wall displacements involved in low‐field thermoremanence or viscous remanence, thermal and alternating field demagnetization do not resolvevandHKunambiguously, because observed coercivities and blocking temperaturesTBdepend on bothvandHK. The method is tested by applying it to single‐domain materials of known grain size. Fluctuation analysis is then used to infer the possible domain structure of 500‐ to 2200‐Å magnetite particles and to point up a deficiency in the Stacey and Banerjee (1974) procedure for detecting ordering of lattice defects impeding domain wall motion. Potential broader applications include determining mean grain sizes of single, and under favorable conditions, double populations of magnetic carriers in rocks, defining average elongation for single‐domain particles in rocks and synthetic dispersions (e.g., magnetic recording tapes), and deducing the mechanism of anisotropy (shape, crystalline or magnetoelastic) or of opposition to wall motion (strain, inclusion or magnetostatic). The main limitations to the method are that the averages ofvandHKbecome nonstationary ifHcmeasurements are made within theTBrange and that step 2 (separation ofHK(T) andHq(T)) is difficult unlessHKis predominantly due to a single anisotropy or wall

ISSN:0148-0227    

DOI:10.1029/JB081i020p03511

年代:1976

数据来源: WILEY

摘要:

Epicenters of large shallow earthquakes (M≥ 6.9) of the past 55 yr and their aftershocks were recomputed for the western boundary of the Philippine Sea plate. These large shocks were not evenly distributed along the plate margin but clustered, particularly near Taiwan and near the islands of the central and southern Philippines. The concentration of large earthquakes near Taiwan is approximately confined to the area where continental lithosphere of the Asian plate appears to be colliding with remnants of the Luzon arc. Large shocks were infrequent near the central Ryukyus and west of Luzon along a segment of the Manila Trench. In both of these locations, major bathymetric features near the trenches may be interacting with the subduction margin. Thus the variations in size and frequency of large events correspond spatially to changes in the tectonic conditions of plate convergence along the margin. The uneven distribution of large shocks therefore may represent a long‐term pattern of occurrence. Segments of the plate boundary near the southwestern Ryukyu arc and near the Philippine Trench are suggested here as areas of seismic potential in that these regions have a past record of large shocks and have tectonic similarities to other seismic zones where long intervals of quiescence culminated in large earthquakes. The techniques used previously to identify areas of seismic potential do not appear applicable, however, to much of this plate margin, particularly to those segments which consist of broad irregular zones of deformation. Recomputation of epicentral data prior to 1964 moves several large shocks closer to the trace of the Philippine Fault from both the east and the west. These relocated epicenters, combined with both recent data and historic descriptions, suggest that much of the fault may have been active during the past century.Table 1 is available with entire article on microfiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D.C. 20006. Document J76‐007; $1.00. Payment must accompany

ISSN:0148-0227    

DOI:10.1029/JB081i020p03518

年代:1976

数据来源: WILEY

摘要:

We present a coupled thermal and mechanical solid state model of the oceanic lithosphere and asthenosphere which includes vertical conduction of heat with a temperature‐dependent thermal conductivityk(T), horizontal and vertical advection of heat, viscous dissipation or shear heating, and linear or nonlinear deformation mechanisms with temperature‐ and pressure‐dependent constitutive relations between shear stress and strain rate. A priori assumptions and inputs to the model are relatively few. We require a constant horizontal velocityu0and temperatureT0at the surface and zero horizontal velocity and constant temperatureT∞at great depth. In addition to numerical values of the thermal and mechanical properties of the medium, we specify only the values ofu0,T0, andT∞. The model determines the depth‐ and age‐dependent temperature, horizontal and vertical velocity, and viscosity structures of the lithosphere and asthenosphere. In particular, we deduce therefrom ocean floor topography, oceanic heat flow, and lithosphere thickness as functions of the age of the ocean floor. The model also yields the age‐dependent shear stress in the lithosphere and asthenosphere. From the age‐dependent geotherms and assumed values of the elastic parameters we construct seismic velocity profiles which exhibit a marked low‐velocity zone. To a first approximation, simple boundary layer cooling, i.e., the growth of a cold thermal boundary layer by upward conduction of heat withk(T) and horizontal advection of heat with constant velocityu0, determines the thermal structure and quantities derivable essentially solely there from, namely, oceanic heat flow, ocean floor topography, and seismic velocities. The simple boundary layer cooling solution gives heat flow ∝(age)−½and depth of the ocean ∝(age)½, for example. The second approximation, i.e., the self‐consistent thermomechanical solution, provides modifications to the thermal structure associated with shear heating effects. This solution also gives the horizontal and vertical velocity and viscosity structures not determined by simple boundary layer cooling and such quantities as the age‐dependent lithosphere and asthenosphere thicknesses derivable from the mechanical field. Effects of viscous dissipation at old ages tend to flatten heat flow, ocean depth, and lithosphere thickness versus age curves; e.g., although ocean depth and lithosphere thickness tend to increase with age at all ages, shear heating tends to reduce the rate of increase at old ages below that given by (age)½. The importance of shear heating increases with plate velocity, decreases with subasthenospheric temperature T∞, increases with the activation volume describing the pressure dependence of viscosity, and inc

ISSN:0148-0227    

DOI:10.1029/JB081i020p03525

年代:1976

数据来源: WILEY

摘要:

An axial valley at the northern end of the Juan de Fuca ridge was the site where two new ocean bottom seismometers were tested in a seismic refraction experiment. Detailed underway geophysical data, seismic reflection profiles, and heat flow measurements were already available for area. By combining the refraction results from surface and bottom shots, the axial valley was found to be filled with at least 1.7 km of largely unconsolidated sediments that were deposited in about 0.3 m.y.VPis estimated for the sediments at 1.7 km/s; aPtoSVconverted wave allows estimation ofVSas 0.51 km/s and Poisson's ratio as 0.450. More distant shots show that the crust in the area (all less than 2.5 m.y. in age) is matured and has velocities similar to those of the typical oceanic section given by Peterson et al. (1974). The upper mantle has a normal 8.3‐km/s velocity across the ridge, but laterally localized low‐velocity mantle must exist beneath the axial valley. This localized low velocity must be even lower than the anisotropically lower velocity commonly observed parallel to the crest in ridge flank stud

ISSN:0148-0227    

DOI:10.1029/JB081i020p03541

年代:1976

数据来源: WILEY

摘要:

Simultaneous measurements of geomagnetic field with an array of seven proton precession magnetometers along the San Andreas fault show that the most significant local changes during 1974 were recorded at a site 11 km from a magnitude 5.2 earthquake that occurred on November 28, 1974. A systematic increase in magnetic field of 0.9 γ occurred at this site during the early part of 1974. A more dramatic increase of 1.5 γ occurred about 7 weeks before the earthquake, lasting about 2 weeks. Four weeks prior to the earthquake the magnetic field returned to approximately its initial value and remained at this value through April 1975. These data cannot be explained by ionospheric disturbances or telluric currents. The most probable source is a piezomagnetic effect, which implies that the magnetic field changes represent changes in stress in the rocks nearby the anomalous statio

ISSN:0148-0227    

DOI:10.1029/JB081i020p03556

年代:1976

数据来源: WILEY

摘要:

The occurrence of a magnitude 5.2 earthquake on November 28, 1974, near Hollister in central California, provided an excellent opportunity to test the potential of a prototype tiltmeter array as a predictive tool. Tilt perturbations on instruments near the epicenter were evident about 36 days before the earthquake and coincided with a magnetic field anomaly reported in a companion paper. The anomalous tilts systematically changed with time, reaching values as great as 7μrad. These data are interpreted in terms of a preseismic and postseismic slip mechanism involving interaction between the San Andreas and other faults in the region, where slip on one results in a sympathetic slip on others

ISSN:0148-0227    

DOI:10.1029/JB081i020p03561

年代:1976

数据来源: WILEY

摘要:

The epicenter of the Hollister earthquake (ML= 5.1) of November 28, 1974, is located near the center of an 81‐line trilateration network which has been surveyed annually since 1971. Five lines in the neighborhood of the epicenter were surveyed just 2 days before the earthquake, and two other lines nearby were surveyed 48 days earlier. Although some of these measurements suggest a possible preearthquake anomaly in line length, the anomaly is within the uncertainty in measurement. A comparison of the most recent preearthquake survey (June 1974) and the most immediate postearthquake survey (December 1974 to January 1975) of the entire network indicates a uniform linear contraction of about 0.8 ppm. This is most reasonably attributed to survey error even though a systematic error of that magnitude is unexpected. We conclude that no demonstrable preseismic or coseismic deformation was detected and suggest that an upper limit for these phenomena is less than 1 ppm in line length. A reasonable dislocation model for the Hollister earthquake yields coseismic changes in line length not greater than a few parts in 107, well below the limit of detection in Geodolite surveys.1 Supplement is available with entire article on microfiche. Order from American Geophysical Union, 1909 K Street N.W., Washington, D.C. 20006. Document J76‐006; $2.00. Payment must accompany or

ISSN:0148-0227    

DOI:10.1029/JB081i020p03567

年代:1976

数据来源: WILEY

摘要:

Rupture propagation in antiplane strain is investigated by using both analytic and numerical methods. Under the assumption that a solid will absorb energy irreversibly when it is strained at a sufficiently large shear stress it is found that energy must be absorbed at the rupture front in addition to the work done against the sliding friction stress. The energy absorbed increases with propagation distance, so it is not negligible at any length scale and is much larger than the ideal surface energy of molecular cohesion. The concept of a critical crack length carries over to the case of a finite stress‐slip law on a fault plane but does not carry over to a homogeneous inelastic medium. In a dynamic slip event, while a typical value of particle velocity is proportional to stress drop, the peak value near the fault is proportional to material strengt

ISSN:0148-0227    

DOI:10.1029/JB081i020p03575

年代:1976

数据来源: WILEY