摘要:

Geodetic measurements made in this century demonstrate that modern movements correlate well with deformation that has occurred during the past 2 m.y. Both show effects of subduction and of the Quaternary collision of Izu Peninsula with Central Honshu. Westward from Suruga Bay, deformation is greatest near the coast, decreases markedly towards the uplifting Akaishi Mountains, and then increases again in the conjugate block‐faulted terranes of Chubu district. Quaternary faults, historic earthquakes, and the distribution of horizontal shear straining since ∼1880 all reflect this deformation pattern. Since 1900 the west coast of Suruga Bay has undergone secular subsidence, horizontal contraction and tilting towards Suruga Trough, all typical of the strain buildup process that occurs landward of subduction zones. However, futher westward, active mountain building is occuring, and the Akaishi Ranges are rising 3 mm/a. Unlike the episodic aseismic uplift of the Transverse Ranges of southern California, movements here have been relatively uniform. It seems likely that an increase in the tilt rate of the Suruga coast recognized since ∼1973 is part of a long‐term readjustment to the great 1944 Tonankai earthquake. Data show that a monotonic increase has occurred since at least 1949, and similar effects are seen adjacent to the 1944 rupture zone over 200 km southwest along the Nankai

ISSN:0148-0227    

DOI:10.1029/JB086iB10p09237

年代:1981

数据来源: WILEY

摘要:

The cross‐spectral method of studying the isostasy was applied to profiles of gravity and topography which run across fracture zones in the Atlantic Ocean between 40°N and 40°S; 30 sections in total were used, each section 400 km long. In wavelengths 25 to 80 km the coherence of the free‐air gravity anomaly and topography attains 0.8, while the phase is close to zero, which indicates that the disturbances in gravity are controlled by the topography and its isostatic compensation. The experimental isostatic response function (admittance) deduced in this study is compared with previous results which relied on different spatial distribution of data. A single mechanism of isostasy can explain well the gravity anomalies across fracture zones and across the median valley of the Mid‐Atlantic Ridge. A basic feature of this mechanism is the elastic lithosphere 6 to 9 km in thickness. We show that the topography and gravity anomalies over the Mid‐Atlantic Ridge creast are essentially three‐dimensional. Nevertheless, the admittance is reliably computed from a set of profiles by using the method of D. McKenzie and C. Bowin. This possibility is provided by the favorable pattern of the topography spectrum in the area under co

ISSN:0148-0227    

DOI:10.1029/JB086iB10p09248

年代:1981

数据来源: WILEY

摘要:

Quantitative calculations for the effect of a fault creep event on observations of changes in water level in wells provide an approach to the tectonic interpretation of these phenomena. For the pore pressure field associated with an idealized creep event having an exponential displacement versus time curve, an analytic expression has been obtained in terms of exponential‐integral functions. The pore pressure versus time curves for observation points near the fault are pulselike; a sharp pressure increase (or decrease, depending on the direction of propagation) is followed by more gradual decay to the normal level after the creep event. The time function of the water level change may be obtained by applying the filter—derived by A. G. Johnson and others to determine the influence of atmospheric pressure on water level—to the analytic pore pressure versus time curves. The resulting water level curves show a fairly rapid increase (or decrease) and then a very gradual return to normal. The results of this analytic model do not reproduce the steplike changes in water level observed by Johnson and others. If the procedure used to obtain the water level from the pore pressure is correct, these results suggest that steplike changes in water level are not produced by smoothly propagating creep events but by creep events that propagate discontinously, by changes in the bulk properties of the region around the well, or by some other mechanism. In addition, simplistic calculations show that significant pressure field variations and water level changes near the surface may be expected to accompany a propagating creep event on a buried fault. Water level changes of as much as several meters may be expected at the surface for a creep event having a dislocation amplitude of l m on a semi‐infinite dislocation surface extending downward from a depth of 10 km and propagating horizontally at a rate of 10 km/day. The maximum near‐surface effect should be observed at a horizontal distance from the fault about equal to the depth of the top of the dislocation surface. These results are consistent with the observations made in China of large water level changes preceding large earthquakes, if some sort of aseismic creep event at depth precedes these ea

ISSN:0148-0227    

DOI:10.1029/JB086iB10p09259

年代:1981

数据来源: WILEY

摘要:

Topographic modification of gravity‐induced near‐surface stresses results in significant departures from a lithostatic state. A perturbation scheme provides approximate analytical solutions for plane strain of an elastic half‐space with an irregularly shaped free surface of small characteristic slope, ε. The leading order effect of the topography is equivalent to that of a distributed normal load on a plane boundary, and the correction at order ε is due to a distributed shear traction on a plane boundary. In the near‐surface region, these two effects contribute at the same order to departures from the lithostatic stress field. The contribution of the shearing has been neglected in most previous analyses. We present explicit solutions for several particular geometries: a symmetric ridge or valley, an adjacent ridge and valley, a shelf, and a sharp‐crested ridge. Notable features include (1) for a sufficiently steep ridge, the horizontal normal stress is compressive at the crest and decreases, possibly becoming tensile, with depth; (2) large horizontal tension is induced in a valley bottom. At greater depth, these effects vanish, and the stresses approach lithostatic. Similar analysis is applied to a half‐space under a far‐field tectonic compression or tension. The result shows that the leading order effect of topography is equivalent to a distributed shear traction on a plane surface. Regional horizontal compression can be significantly reduced, or even change to tension, in the neighborhood of a t

ISSN:0148-0227    

DOI:10.1029/JB086iB10p09268

年代:1981

数据来源: WILEY

摘要:

A subsurface model for the Goddard pendant is constructed from a residual gravity high of about 7 mGal over the pendant. The model, which is the simplest and most geologically reasonable possibility, shows a metamorphic block that tapers with depth and extends about 3.5 km below the surface. The structures in the Goddard pendant are similar in style and orientation to those in other Sierra Nevada pendants, indicating that the country rock was neither deformed nor rotated during pluton emplacement. Consequently, emplacement must have been a passive rather than a forceful process. The pendant itself represents a piece of country rock trapped between plutons which are dome shaped in cross section.

ISSN:0148-0227    

DOI:10.1029/JB086iB10p09279

年代:1981

数据来源: WILEY

摘要:

A simple physical model is developed to understand the effect of normal stress on fluid flow through a single fracture. Roughness along the fracture walls plays a definite role in controlling the flow. In the usual parallel plate representation for a fracture, the flow is proportional to the cube of the constant aperture,b. However, when the effect of fracture roughness is taken into account, the flow follows an equivalent ‘cubic’ law where the cube of the single value for the aperture must be replaced by an appropriately weighted average 〈b3〉. To obtain this average value, a physical model was developed wherein the single fracture is represented by a collection of voids and the closure of the fracture results from a deformation of these voids. The model enables one to characterize the fracture roughness from a relationship between the stress‐displacement measurements of intact rock and those of jointed rock. This calculated value of 〈b3〉 leads to flow rate as a function of normal stress. Predicted flow rates using this model are in good agreement with results from laboratory data on granite and basalt. By making several simplifying physical assumptions, we have eliminated the necessity of incorporating fitting parameters to the flow data. In this manner, a basic understanding of the factors controlling the flow of fluids through fractures has

ISSN:0148-0227    

DOI:10.1029/JB086iB10p09287

年代:1981

数据来源: WILEY

摘要:

Uniaxial compression tests have been conducted on Oshima granite under various constant axial strain rates ranging from 10−8to 10−4. The results showed that the strength and the acoustic emission rate increased exponentially with increasing strain rate. The inelastic volumetric strain rate defined by the differentiation with respect to the stress increased with decreasing strain rate. The redistribution of microcracks due to subcritical crack growth was considered theoretically, and the equations derived from the theory were compared with the experimental results. The agreement between the theoretical and experimental results shows that stress corrosion plays not only a major role in the brittle creep under constant load but also dominates the strain rate effects on strength and dilatancy observed in the constant strain rate loadi

ISSN:0148-0227    

DOI:10.1029/JB086iB10p09299

年代:1981

数据来源: WILEY

摘要:

Incomplete knowledge of the pattern of magnetic lineations and fossil transform faults represented by sea floor spreading data on two plates generated by the same spreading center leads to uncertainties in a reconstruction of the past relative configuration of the plates. A reconstruction may be represented by the finite rotation that describes one configuration relative to another. In this paper a method of reconstruction is provided that reflects the uncertainties in the data. The method minimizes a weighted least squares measure of fit as a function of the rotation parameters. For a given rotation the measure of fit represents the sum of squares of the weighted distances of fixed and rotated data points (representing corresponding former plate margin segments) from a common plate margin. The common margin is estimated from the locations of both fixed and rotated data points. The estimated margin consists of a set of great‐circle arcs. It is shown how the method of reconstruction may be utilized to obtain an uncertainty region for the pole and angle of rotation that characterize the finite rotation. These techniques are then used to study the uncertainties of reconstructions in the South Pacific for the times of anomalies 13 and 18. The best fit reconstructions and the uncertainty region for the anomaly 18 pole are in good agreement with previous work. However, the uncertainties in the anomaly 13 pole were substantially underestimated by previous investigator

ISSN:0148-0227    

DOI:10.1029/JB086iB10p09312

年代:1981

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/JB086i010p09319

年代:1981

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/JB086iB10p09320

年代:1981

数据来源: WILEY