摘要:

Very long base line interferometry (VLBI) observations of lunar radio transmitters have been combined with data from laser ranging to lunar retroreflectors to estimate simultaneously (1 ) parameters in models of the lunar orbit and libration and (2) the selenodetic coordinates of the radio transmitters and retroreflectors. For the ratio of the mass of the sun to that of the earth plus moon we obtain 328,900.50 ±0.03. For the lunar moment‐of‐inertia ratios we find β[≡ (C‐A)/B] = (631.27 ± 0.03)× 10−6and γ [ ≡(B‐A)/C] = (227.7 ± 0.7) × 10−6. The value implied forC/MR2is 0.392 ± 0.003, the uncertainty being dominated by that of the coefficientJ2of the second zonal gravity harmonic, obtained by Gapcynski et al. from analysis of Explorer spacecraft orbital data. The values and most of the uncertainties that we obtain for the third‐degree harmonics of the moon's gravity field are comparable to those which have been obtained by others from observations of lunar orbiting spacecraft. However, our determination of β appears to be the best available, and our results for the two third‐degree gravity coefficientsC31= (26 ± 4) × 10−6andC33= (2 ± 2) × 10−6have much less uncertainty than determinations based on laser data alone. For the relative position vectors of the lunar radio transmitters and the retroreflectors our estimates have uncertainties of about 30 m along the earth‐moon direction and about 10 m in

ISSN:0148-0227    

DOI:10.1029/JB081i035p06251

年代:1976

数据来源: WILEY

摘要:

An earlier analysis of Prairie Network fireball trajectory data by the authors failed to discriminate between the varieties of meteoric materials that are thought to be present. The Lost City meteorite event offers a new calibration of the fireball data and prompts a new analysis intended to distinguish between ordinary chondritic meteorites and the more fragile early type carbonaceous chondrites. The carbonaceous material is expected to ablate more readily and thus to have shorter trajectories or higher end heights. Observational error and the natural dispersion in the structures produce severe smoothing effects, and the end height diagnostic is, in itself, not entirely convincing. However, after making a tentative division of the data, one can discern differences in orbits and beginning heights among the three end height groups. These are consistent with our expectation of the behavior of chondritic stones (group I), weak carbonaceous chondrites (group II), and still more fragile cometary material (group III). We conclude that if the carbonaceous material occurs in reasonable abundance among the fireballs, it has been properly identified by this analysis.

ISSN:0148-0227    

DOI:10.1029/JB081i035p06257

年代:1976

数据来源: WILEY

摘要:

Two rubidium narrow line magnetometers were operated in a differential mode along a 12‐km base line near Boulder, Colorado, and the total field gradient measured to high accuracy. A number of runs of difference field were made from March through September of 1974. The results showed that daily averages were generally constant to within ±0.1 γ during quiet or disturbed periods with the exception of a few days in late August and early September. The agreement is excellent between the observed difference field and a difference field calculated from the measured east‐west fieldYand a transfer function appropriate to a local conductivity anomaly. Accounting for the effect of the anomaly typically permits a reduction of a factor of 3 in the standard deviation of the daily average; on very disturbed days a reduction of a factor of 5 in gradiometer noise was achieved. For periods of more than several days the corrections for anomaly edge effects are comparatively

ISSN:0148-0227    

DOI:10.1029/JB081i035p06276

年代:1976

数据来源: WILEY

摘要:

Seismic zones beneath consuming plate margins have a flattened upper section, which has been attributed to the growth of the accreted material beneath the inner trench slope. Comparison of well‐studied seismic zones shows that the width of this flattened section is proportional to the trench‐volcanic chain separation in nearly all arcs. The bend, or zone of maximum curvature between upper and lower sections, lies in front of the volcanic chain and behind the trench slope break. Combined geological and seismological data indicate that the bend location remains nearly fixed during subduction and that accretion at the trench causes progressive loading and depression of the descending lithosphere. Concurrent migration of the positive‐negative free air gravity anomaly couple and the approach to isostatic conditions over the earlier trench positions are further observational support for this conclusion. Mechanical modeling of the load applied by accretion to an elastic lithosphere not only shows the plausibility of this concept but also indicates that in wider accretionary prisms the geometry of the outer trench slope and outer rise is controlled by loading rather than by deeper level interplate stresses. This latter point is also supported by the lower dips of outer trench slopes and lower amplitudes of outer rises observed seaward of wide accretionary prisms. The growth of the flattened seismic zone can persist through several subduction episodes, the indication being that the slab remains unbroken and stressed after subduction ceases and that much of the mass imbalance is retained for long periods of

ISSN:0148-0227    

DOI:10.1029/JB081i035p06281

年代:1976

数据来源: WILEY

摘要:

The deformation and growth of a vertical penny‐shaped crack, fractured hydraulically, is investigated when fluid is injected from an inlet at the center of the crack at a constant flow rate. The total flow rate at the inlet is divided into three parts: flow rate extracted from an outlet hole at an arbitrary distance above the center, fluid loss rate from the crack surface, and total fluid mass change in the crack. Two cases are considered: in case 1, inlet flow rate is initially greater than the sum of the outlet flow and fluid loss rates, and in case 2 the reverse holds true. Two subcasesaandbare also considered, depending on the values of outlet pressure. Ranges of the inlet flow rate and the outlet pressure are discussed for which the crack attains stationary states and the fluid can be extracted continuously. Subcaseb, where the outlet pressure is less than or equal to the difference between the tectonic stress and the fluid head at the inlet, is found to be more practical, and reasonable outlet flow rates are obtained in this case. It is also found that case 2bis preferable to case 1bto obtain the fluid with higher temperature. Results are expected to be of use in considerations of heat extraction from hot dry roc

ISSN:0148-0227    

DOI:10.1029/JB081i035p06292

年代:1976

数据来源: WILEY

摘要:

A multiwavelength distance‐measuring (MWDM) instrument that utilizes the dispersive nature of the atmosphere has been developed and tested. With this instrument there is no need for the usual meteorological observations and corrections. This MWDM instrument simultaneously measures the optical path length at three wavelengths, two in the optical region of the spectrum and one in the microwave region. The instrument calculates correction terms from the optical path length differences and then computes the corrected base line distance, the first‐order effects of temperature, pressure, and water vapor fluctuations along the line thus being eliminated. Field testing demonstrated that the instrument was capable of making consecutive distance determinations with a standard deviation from the mean as small as 4 parts in 108. The long‐term stability of the instrument was investigated from June 1974 to November 1974. The results showed a standard deviation of 1.3 parts in 107when the data were fit to a sine wave of annual periodicity. The amplitude of this sine wave was 3 parts in 107, which was in good agreement with the predicted results of a two‐dimensional thermoelasti

ISSN:0148-0227    

DOI:10.1029/JB081i035p06299

年代:1976

数据来源: WILEY

摘要:

We have extended our investigation of lateral variations in upper mantle shear velocity structure by employing multipleScSphases. An analysis of 108 multipleScSdifferential travel times indicates that to a first approximation, upper mantle heterogeneity as expressed in vertical, travel time differences can be correlated with surface tectonic features. Travel times for continental regions with Phanerozoic orogenic histories are significantly greater than those from Precambrian continental regions and correlate with heat flow. Oceanic regions have large delays relative to stable continental regions, and the travel time residuals show a consistent decay as a function of crustal age. The average one‐way vertical shear wave travel time difference between oceans and stable continental regions is estimated to be +4.0±0.5 s, not significantly different from the value obtained in our previous study. These results support the hypothesis that shield ocean heterogeneity persists to great depths, probably exceeding 400 km. For shear waves with characteristic frequencies of 0.1 Hz, the two‐way vertical travel time through the mantle of the spherically averaged earth is 937.3±1.0 s, about 3 s less than the value derived from eigenfrequency data and 1.6 s greater than the Jeffreys‐Bullen value. This result is inconsistent with the hypothesis that the base line discrepancy between free oscillation models and classical body wave travel time models is because of continental bias in the travel times, but it is consistent with the hypothesis that the base line discrepancy is an expression of anelastic dis

ISSN:0148-0227    

DOI:10.1029/JB081i035p06307

年代:1976

数据来源: WILEY

摘要:

Many new sonobuoy solutions have been combined with published data and incorporated into a revised plot of igneous layer 2A thickness as a function of plate age at the East Pacific Rise. Each solution was redated on the basis of regional magnetic anomaly maps to improve the accuracy of crustal dating. Results show that layer 2A becomes unobservable by seismic methods on crust older than about 15 m.y. Although igneous materials must make up layer 2A at the East Pacific Rise, low‐velocity acoustic basement exists elsewhere in the Pacific. Near seamounts and seamount chains, volcanic aprons with the seismic properties of layer 2A thin distally away from eruptive centers and seem to persist indefinitely without being affected by processes that normally increase the sound velocity at the ridge crests; This is apparently due to a lack of fracturing, which prevents hydrothermal solutions from percolating through the volcanic rubble. At active spreading centers, percolation through fractures provides minerals that could fill voids and increase the sound velocity of the basaltic rubble. Statistical studies show that the velocity in layers 2B and 2C is independent of plate age, but layer 3 velocities peak up significantly on the East Pacific Rise. In the older parts of the Pacific, low‐velocity acoustic basement is sedimentary or a mixture of sediment with volcanics. An unusually thick sequence (1 km) of material that is probably mostly sediment occurs in the Nauru and Mariana bas

ISSN:0148-0227    

DOI:10.1029/JB081i035p06321

年代:1976

数据来源: WILEY

摘要:

The observed power spectrum of the earth's gravity field is interpreted as resulting from randomly distributed density anomalies of relatively small spatial coherence occurring throughout at least the first 800–1000 km of the mantle. The magnitude of these density anomalies is of the order of 1.7×105/(dL dr)½g cm−3, where dL (in centimeters)is their horizontal spatial coherence and dr the vertical coherence. Most of the power in harmonics of degree ≥6 reflects heterogeneities in the first 300–400 km of the mantle. The lower‐degree harmonics reflect conditions down to about 800–1000 km. Anomalies in density of the above magnitude occurring at greater depths will only contribute slightly to the nonhydrostatic potential of degrees 2 and 3. This anomalous density estimate therefore presents an upper limit to permissible values that can occur in the lower mantle. When Birch's law is used this indicates that lateral anomalies in the velocity of P waves are of the order of 0.2% (rms value) or less, in general smaller than the seismic results for the lower mantle. The gravity signal of these lower mantle anomalies is masked by that of upper mantle anomalies, a fact which explains the absence of correlation found between gravity and deep mantle seismic anomalies. The seismic anomalies reported for the upper mantle often exceed the rms value of 0.1–0.3 % estimated from the gravity field. This indicates that internal compensation occurs whereby mass excesses are underlain or overlain by mass deficits. Other estimates deduced from the gravity field are upper limits to the lateral temperature variations of 100°C (rms value) and upper limits to permissible topography on the core‐mantle boundary of the order of 200 m or less. Geopotential harmonics of degree ≥6 correlate significantly with the surface topography although linear regression analysis shows that only about 10% of the total power of the observed geopotential is responsible for this correlation. This is of the same magnitude as the power in the potential spectrum computed from the topography and its isostatic compensation and suggests that this correlation is a consequence of near‐surface effects rather than

ISSN:0148-0227    

DOI:10.1029/JB081i035p06333

年代:1976

数据来源: WILEY

摘要:

Data from four tilt meters and several creep meters along the San Andreas fault in central California 30 km south of Hollister were selected to investigate nonseismic short‐period tilt perturbations of minutes to hours duration. The group of short‐period tilt amplitude changes with residual deflections was chosen because many nonseismic creep events have similar durations. Although each of the four tilt meters recorded many short‐period events, the distribution in space and time suggests that most of these events are due to causes other than slip on the San Andreas fault, such as local strain release. Only at one station used in this investigation (MEL) were creep‐related tilt changes observed. A quasi‐static dislocation model was constructed to predict the tilt changes associated with propagating slip. Comparing predicted to observed tilt wave forms and amplitudes and comparing the tilt event data to creep event data suggest that (1) a tilt amplitude change of 10−8–10−6rad associated with nonseismic creep events will be observed only if the tilt meter is less than 0.5–1.0 km from the fault, (2) the vertical extent of the slip zone associated with these creep events is greater than about 0.5 km but perhaps not more than a couple of kilometers, (3) nonseismic episodic slip on this part of the San Andreas fault does not appear to propagate uniformly over more than a couple of kilometers, and (4) the model presented in this paper requires 0.5–1.0 mm of dip‐slip displacement to reproduce the residual deflection in the creep‐related t

ISSN:0148-0227    

DOI:10.1029/JB081i035p06341

年代:1976

数据来源: WILEY