摘要:

Two conditions are normally considered to be characteristic of the state of stress in a system in local isostatic equilibrium. The first is that the stress is hydrostatic below some depth, the level of compensation, and the second is that above that depth there are no vertical shear stresses. In the case of a planar two‐dimensional mid‐ocean ridge these two conditions suffice to determine the stress completely except for the normal component parallel to the ridge, but to determine that component an additional assumption is required. It is proposed here that an appropriate additional assumption is that in unflexed regions where the stress is due only to the elevation of mid‐ocean ridges, the deviatoric stress in the oceanic lithosphere is a minimum. The unique state of deviatoric stress consistent with this assumption is horizontally compressive in a direction perpendicular to the axes of mid‐ocean ridges with a magnitude in old ocean basins of a few hundred bars, in agreement with oceanic intraplate earthquake mechanisms.At any given distance from the ridge the stress is maximum at the seafloor, and it decreases with depth at about the same rate as the cooling of the lithosphere, so that the entire thermal boundary layer may be regarded as acting to support the global square root of age topography of the

ISSN:0148-0227    

DOI:10.1029/JB086iB09p07801

年代:1981

数据来源: WILEY

摘要:

Two benchmark arrays located on the islands of Santo and Efate in the New Hebrides Island Arc have been releveled 2 to 4 times per year during the past five years (1975–1980). Each array is thereby used as a multicomponent tiltmeter. The arrays are located within 35–50 km of the interplate thrust zone of this convergent plate boundary. Each array has an aperture of about 1 km and in addition contains small subarrays with apertures of about 70 m. This configuration leads to a resolving power (for tilt) of about one microradian. The most important result of this study is that, at the Devils Point array on Efate Island, there has been a consistent tilting down of about five microradians in a direction subparallel to the strike of the arc. This tilting is clearly resolved by both the large array and by the small aperture sub‐arrays. The observations are accounted for by a combination of planar tilting and random error consistent with the observed misclosures. Although tilting is clearly established at the Devils Point array, it is not possible at this stage to establish a unique cause for the observed tilt. Hypotheses consistent with the tectonic setting include interseismic strain accumulation associated with the earthquake cycle of loading and interplate boundary rupture, deformations near a transition between a creeping and a locked segment of the interplate boundary, and near surface deformations associated with magmatic activity and/or block movements also reflected in the pattern of faulting of the uplifted Holocene coral terraces. At the second array at Ratard (Santo Island) no tilt trend clearly emerges from the noise level. However, two marginally significant tile excursions precede in each case large earthquakes located several hundred km north of the array. At the Ratard array the ground noise (small wavelength elevation disturbances and benchmark instability) contributes to residuals almost as much as the measurement error, thereby degrading the threshold level for detection of tilt signals. The difference between the two arrays is not clearly established but seems to reflect an inherently larger level of short wavelength ground noise associated with geological properties of the Ratard

ISSN:0148-0227    

DOI:10.1029/JB086iB09p07808

年代:1981

数据来源: WILEY

摘要:

The isostatic transfer function for the southern Mid‐Atlantic Ridge was computed from 25 profiles of gravity and topography normal to the ridge trend. As follows from the comparison with previous studies, the transfer function for the Mid‐Atlantic Ridge is insensitive to the tectonic setting of profiles used. The experimental transfer function was employed to remove all coherent short‐wavelength features in the gravity and topography from 17 long profiles across the southern Mid‐Atlantic Ridge. The profiles were then recomputed as a function oft, age of the lithosphere, and averaged over equal ages. The observed regional topography, regional gravity anomaly, and altimeter geoid anomaly agree well with conductive cooling models of the spreading lithosphere. An observed slope of the topography versust1/2and of the geoid versustcan be explained with a single set of model parameters. Comparison with the estimates by Haxby for the northern Mid‐Atlantic and Reykjanes Ridges suggests a systematic decrease in the temperature of the mantle from 1500°–1600°C near Iceland to 1000°–1100°C at the southern Mid‐Atlantic Ridge. The theoretical gravity anomaly deduced by the analytical method of Haxby and Turcotte is sensitive to the thermal parameters only in the age range 0–20 m.y. The fit of the theoretical to the observed gravity anomaly is good outside the axial strip 0–3 m.y. The poor fit at the axis is attributed to inadequacy in the boundary

ISSN:0148-0227    

DOI:10.1029/JB086iB09p07825

年代:1981

数据来源: WILEY

摘要:

The accuracy of a gravity field model depends on the amount of available data and on the variation of the gravity field. When topographic height data are available, for example, in the form of a digital terrain model, it is possible to smooth the gravity field on a local scale by removing the gravitational effects calculated from models of the topographic masses. In this way, significant improvements of the prediction results are obtained in mountainous areas. In this paper we describe methods for the calculation of such gravitational terrain effects, applicable in collocation approximation of the gravity field. The terrain effects on gravity field quantities such as gravity anomalies, deflections of the vertical, and geoid undulations are calculated using a system of rectangular prisms, representing either a quasi‐traditional model of the topography and the isostatic compensation or a residual terrain model, where only the deviation of the topography from a mean elevation surface is considered. To test the terrain reduction methods, numerical prediction experiments have been conducted in the mountainous White Sands area, New Mexico. From gravity anomalies spaced approximately 6 arc min apart, other known gravity anomalies and deflections of the vertical were predicted using collocation. When using terrain effects calculated on the basis of 0.5 × 0.5 arc min point heights, the rms errors decreased by a factor of nearly 3 to 1 arc sec for the deflections and 3–4 mGal for the gravity anomalies, quite insensitive to the actual type of terrain reduction used. The feasibility of using topographic reductions in collocation is thus effectively demonstr

ISSN:0148-0227    

DOI:10.1029/JB086iB09p07843

年代:1981

数据来源: WILEY

摘要:

Several hundred microearthquakes with local magnitudes ranging between −4 and −2 were recorded during hydraulic stimulation experiments at the hot dry rock demonstration site, Fenton Hill, New Mexico. These events are probably caused by shear failure induced by high pore fluid pressures. Since the event locations seem to cluster in a narrow band near the hydraulic fracture, we were able to use micro‐seismic techniques to locate the hydraulic fracture and monitor its growth. We calculate the minimum pore pressure increase necessary to induce rock failure using a simple model of slippage on preexisting fractures. By comparing a pore pressure distribution, calculated using a one‐dimensional diffusion model with the distribution of event foci, we demonstrate that high pore pressures are a probable cause for these microearthquakes. Stress drops for these microearthquakes, calculated from spectral parameters, increased by a factor of 3 during the course of the experiment. The stress drop increase correlates with a migration of seismic activity away from the injection well bore, which suggests relatively low in situ shear stresses near the well bore caused by strain release in this region during previous pressuri

ISSN:0148-0227    

DOI:10.1029/JB086iB09p07855

年代:1981

数据来源: WILEY

摘要:

A method is proposed for obtaining reflectivity magnitudes from seismic record sections that are densely sampled in the horizontal dimension. The technique consists of finding the spectral ratio of subcritical (target) to postcritical (reference) reflection amplitudes. The method's novelty is that both target and reference amplitude spectra are estimated from slant (also known as ray parameter) intercept time, p‐tau or velocity) stacks of the record sections. The source level is directly determined because the reference amplitude is totally reflected; source level estimation does not require knowledge of the impedance structure of the reference reflector.The technique functions well for reflections having low S/N ratios, little or no lateralcontinuity, poor near‐surface velocity control and steep dips. Application of the method to the Consortium for Continental Reflection Profiling (COCORP) survey in the Rio Grande rift supports the interpretation that its large amplitude midcrustal reflection is a magma b

ISSN:0148-0227    

DOI:10.1029/JB086iB09p07865

年代:1981

数据来源: WILEY

摘要:

Several lines of evidence from new and existing data are consistent with an extended fault slip associated with the 1886 earthquake in South Carolina. Back slip over a portion of the Appalachian detachment approximately corresponding to the Coastal Plain of South Carolina fits some of the data including coseismic and post seismic strain indicators. A small seismic source for the August 31, 1886, main shock cannot be ruled out, and a large, but only partially seismic fault slip is possible. During this event intensities of MM ≥ VIII occurred over a wide area of the Coastal Plain and are not limited to the Charleston‐Summerville area, where the traditional small source for this earthquake is located. Near‐source effects such as ground fissuring, liquefaction, changes in line‐of‐sight, foreshocks, and aftershocks occurred over a wide area of the Coastal Plain and neighboring Inner Piedmont, which we tentatively refer to as the meizoseismal area. This area is sharply bounded by the Savannah River toward the southwest and approximately by the North Carolina‐South Carolina state border toward the northeast. These boundaries are not associated with any known structure transverse to the strike of the Appalachians. During the 14 years prior to 1886 seismicity was concentrated around the extended meizoseismal area while the seismicity within that area was unusually low, suggesting a premonitory doughnutlike pattern.The intensity distribution in the far and near field of the 1886 event and of the great Indian detachment earthquakes, e.g., the 1905 M = 8 and the 1934 M= 8.3 events, are qualitatively and quantitatively similar. This is consistent with comparable source dimensions, since well‐known intermediate magnitude earthquakes in India and in eastern North America exhibit similar intensity patterns and indicate similar seismic attenuation in the two cratonic environments. Several meters of coseismic (1886) northwest compressive strain near Summerville are indicated by buckling of rails. A large postseismic normal displacement is suggested by releveling data (1935–1968) across the Brevard fault zone updip (northwest) of the presumed 1886 detachment slip. These strains are consistent with a southeastward slip of the detached crystalline sheet associated with the 1886 earthquake. Available data indicate that in the southern Appalachians the updip transition from maximum horizontal to least horizontal compressive stress in the dip direction (perpendicular to structural trends)occurs somewhere in the Piedmont and may coincide with the steepest portion of the detachment. The stress field may also contain a time‐dependent component. In the sediments of the Coastal Plain, orthogonal sets of subvertical clastic dikes oriented along and across the Appalachian strike direction suggest one or more interchange of the principal stress axes. We infer from coseismic dry fissuring and liquefaction that clastic dikes filled from above and from below were also generated during the 1886 event. Prehistoric clastic dikes along with other structures in the sediments may have been generated during prehistoric earthquakes similar to the 1886 event. Stress reversals corresponding to earthquake cycles may characterize the stress field within detached sheets or wedges that move primarily by great detachment earthquake ruptures.Appendices are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J81‐008; $1.00. Payment m

ISSN:0148-0227    

DOI:10.1029/JB086iB09p07874

年代:1981

数据来源: WILEY

摘要:

Near‐field tsunami formation is detailed by means of mode summations using point moment tensor sources with a spherically symmetric, self‐gravitating, and elastic earth. Snapshots of vertical sea surface motion calculated within 200 km and 1000 s of an event typically show an initial tsunami pulse slowly grow and then evolve into dispersed wave trains. The general point moment tensor used here produces frequency dependent tsunami radiation patterns of azimuthal degree ≤2. Pure dip and strike slip sources, however, create waves that are symmetric or antisymmetric about the epicenter. With a moment of 1020N m and a rise time of 25 s, these sources generate initial tsunami pulses 40 km wide when buried 10 km beneath an ocean 4 km deep. Maximum dip slip and strike slip tsunami amplitudes of 209 and 56 cm occur 10 km from the epicenter, 75 s after the start of nucleation. Tsunamis from deeper earthquakes are smaller, broader, and slower to develop because they are depleted of high frequencies. In general, characteristics of tsunamis developing in this dynamic model contradict static concepts which base tsunami formation on permanent deformations of an otherwise rigid seafloor supporting incompressible water. I find that the permanent strain state of the seafloor is not important in tsunami generation because it is defined by frequencies well below the range of interest (100

ISSN:0148-0227    

DOI:10.1029/JB086iB09p07895

年代:1981

数据来源: WILEY

摘要:

Velocity‐depth profiles derived from six two‐ship expanding spread experiments, in combination with other geophysical data, define the characteristics of two distinct types of Venezuela Basin crust and the boundary between them. Each two‐ship common midpoint reflection/refraction profile is automatically transformed into the τ‐pplane, ‘picked’ and interpreted to provideV(Z) functions with appropriate confidence bounds. The results, together with gravity, magnetic, and near‐vertical incidence reflection data, reveal a 50,000 km2triangle of Venezuela Basin crust which resembles normal oceanic crust in a magnetic quiet zone. North and west of this triangle lies the previously defined, thick ‘Caribbean’ crust, having two distinct layers above theMdiscontinuity. Acoustic basement there appears unusually smooth due to extensive basaltic sills and flows which were cored at Deep Sea Drilling Project sites 146/149(sills), and 150 (flows); also, depths to mantle are greater than normal. Interpretations of near‐vertical and wide‐angle reflection data show that the extra crustal thickness is due not only to the emplacement of the flows but also to the crust below being somewhat thicker than normal. The boundary between the two crustal areas has a NE‐SW trend which parallels the dominant structural and magnetic lineations.This boundary coincides in position, though not in trend, with the previously defined ‘central Venezuela Basin fault zone’. Further study is required to determine whether this boundary is of tectonic origin or if it represents a change in

ISSN:0148-0227    

DOI:10.1029/JB086iB09p07901

年代:1981

数据来源: WILEY

摘要:

The oxygen isotopic compositions of pore fluids from sediments of older oceanic crust commonly decrease with depth. Increases in calcium ion concentrations always accompany the18O/16O decreases.These changes are attributed principally to the alteration of basalts or alteration of volcanic ash in the sediments. Oxygen isotope material balance calculations suggest that 5–20% of the upper kilometer of basalt is altered at the time of subduction of the oceanic crust. This corresponds to a flux of18O into the oceanic crust of 2.3 × 1015moles per million years. Evaluation of the oxygen and hydrogen isotope data from the pore fluids also suggests that the principal process of mass transfer in the sediments of older oceanic crust is diffusion. Convection of water in the basalts of the oceanic crust appears to continue until subducti

ISSN:0148-0227    

DOI:10.1029/JB086iB09p07924

年代:1981

数据来源: WILEY