摘要:

Analysis of very‐long‐baseline interferometer (VLBI) observations yielded estimates of the distances between three radio telescopes in the United States and one in Sweden, with formal standard errors of a few centimeters: Westford, Massachusetts‐Onsala, Sweden: 5,599,714.66±0.03m; Green Bank, West Virginia‐Onsala, Sweden: 6,319,317.75±0.03m; and Owens Valley, California‐Onsala, Sweden: 7,914,131.19±0.04m, where the earth‐fixed reference points are defined in each case with respect to the axes of the telescopes. The actual standard errors are difficult to estimate reliably but are probably not greater than twice the

ISSN:0148-0227    

DOI:10.1029/JB086iB03p01647

年代:1981

数据来源: WILEY

摘要:

Detections of variations in the frequency of the Chandlerian component of polar motion have been reported by several investigators, starting at least as early as Chandler's (1892) report. The concept has never gained general acceptance because no credible cause has been identified. Investigations reported in this paper suggest that the Chandlerian component may be frequency modulated as a linear function of the polar motion magnitude. The variation in frequency is estimated to be ∼0.03 to 0.06 cycles per year increase per 0.1 second of arc decrease in the polar motion magnitude. Nonequilibrium response of the oceans to polar motion, which takes the form of a pole tide that varies with time in enhancement and phase lag, is suggested as a possible cause of the frequency modulatio

ISSN:0148-0227    

DOI:10.1029/JB086iB03p01653

年代:1981

数据来源: WILEY

摘要:

We present results for theM2andO1ocean tides based on the Starlette satellite orbit analyses from which the tidal acceleration of the moonis derived. The solution forM2is consistent with previous satellite solutions; forO1we find a phase lag which is approximately zero. The uncertainty is still large, but there is an indication that the tidal dissipation at theO1frequency is small. Using the weighted mean value of the best satellite solutions gives= −21.32 ± 1.25 arc sec/(century)2for the total dissipation in the earth at theM2frequency. In principle, comparison betweenderived from satellite solution andderived from numerical models gives the amount of dissipation in the solid earth, but uncertainties are still too large to give a reliable estimate of the tidal mantl

ISSN:0148-0227    

DOI:10.1029/JB086iB03p01659

年代:1981

数据来源: WILEY

摘要:

We have reexamined the existing ground‐based measurements of Io's thermal emission at infrared wavelengths of 8.4, 10.6, and 21 μm. Present in these data is the signature of ‘hot spots,’ presumably similar to the hot spots seen by the IRIS experiment on Voyager. It is possible to extract from these data the total amount of power radiated. Since the hot spots are believed to be a result of deep‐seated activity in Io and since the remainder of Io's surface is an extraordinarily poor thermal conductor, the power radiated by the hot spots is essentially the total heat flow. Our analysis yields a heat flow of 2 ± 1 W m−2. This value is tremendously large in comparison to the average heat flow of the Earth (0.06 W m−2) and the moon (0.02 W m−2), but it is characteristic of active geothermal areas on the Earth. A heat flow this large requires that the interior of Io be at least partially molten on

ISSN:0148-0227    

DOI:10.1029/JB086iB03p01664

年代:1981

数据来源: WILEY

摘要:

Long‐period seismograms are generated by a spectral method for body waves interacting with the upper mantle discontinuities that define the low‐velocity and transition zones of the spherically symmetric but radially inhomogeneous earth models, PEM‐C and T7. A complex velocity profile incorporates the SL8 Q model. Models PEM‐C and T7 predict distinct differences in displacement waveforms forPwaves that interact with the 400‐ and 670‐km discontinuities. The waveforms demonstrate that later arrivals such as interference head waves can be important constraints upon earth models. Theoretical seismograms were computed by a spectral method that modeled the velocity profile with arbitrarily thick inhomogeneous layers (full wave theory). The seismograms agreed well with those generated by methods that approximate the velocity profile with thin homogeneous layers (the Cagniard and reflectivity methods) provided the displacements were convolved with source‐time functions and a long‐period instrument response. Minor differences among the results of the methods do not affect the interpretation of observed waveform data but do help define the limitations of the methods. For example, the thin‐layer methods suffer from numerical limitations on layer thickness and the number of layers needed to approximate the earth model. The full wave theory may occasionally also require the use of thin layers when the zero‐order asymptotic theory it uses fails to describe wave propagation properly in layers having rapid velocity v

ISSN:0148-0227    

DOI:10.1029/JB086iB03p01673

年代:1981

数据来源: WILEY

摘要:

A least squares waveform inversion technique to determine the source parameters of moderate size earthquakes is presented. The technique involves an iterative scheme which makes use of an error function determined by the cross correlation of a long‐period seismogram and a synthetic. Both the data and the synthetics are normalized to make the solution insensitive to absolute amplitudes. Making use of the three fundamental faults, the error functions can be written as a series of cross correlations multiplied by constants corresponding to source orientation. Once the cross correlations are computed, the source orientation is determined iteratively, and only the constants have to be recalculated. The technique has been applied to long‐period regional seismograms where the beginning portion of the vertical and radial components containingPnandPL(referred to asPnl) is inverted. Three western United States earthquakes which have been well studied by other authors were used as test samples. The inversion yields fault plane solutions in good agreement with those in the literature. Moments determined in the inversion are also in excellent agreement with those determined teleseismica

ISSN:0148-0227    

DOI:10.1029/JB086iB03p01679

年代:1981

数据来源: WILEY

摘要:

The effect of local structure on the characteristics of the regional phases Pgand Lgwas investigated by analyzing events at epicentral distances less than 10° from the Nevada Test Site. The stations used are on markedly different geological structures; three on granitic intrusions, two on Pahute Mesa, a feature overlaying a buried volcanic caldera, and, finally, four on Yucca Flats, a deep alluvial valley. The crustal effect on amplitudes is quite significant for both Lgand Pg; Yucca Flats gives amplitudes as much as a full order of magnitude higher than the stations on granite, while Pahute Mesa stations are 0.7–0.8 magnitude units higher in amplitude than the granite sites. Besides gross amplitude effects, the local crust also induces prolonged ringing at Yucca Flats, resulting in the lengthening of the signal wave trains. The observed site amplification effects show no clear dependence on the azimuths of arrivals, indicating that the phenomenon is due to local structures and is not caused by lateral refraction of surface wave modes. The observed amplification is similar in magnitude for the vertical component of both Pgand Lg, thus not affecting the Pg/Lgamplitude ratio, a possible discriminant between earthquakes and explosions at regional distances. Finite difference theoretical simulations using two‐dimensional models of the Yucca basin reproduce, in a qualitative sense, the ringing effect observed at the Yucca Flats. Any attempt at matching characteristics of Pgand Lgwith synthetic seismograms computed for laterally homogeneous layered structures is apt to lead to false conclusions about the source and path properties unless the recording sites used are carefully chosen for small crustal eff

ISSN:0148-0227    

DOI:10.1029/JB086iB03p01686

年代:1981

数据来源: WILEY

摘要:

Well‐constrained fault plane solutions fromPwave first motions for mid‐ocean ridge normal faulting earthquakes usually require nonorthogonal nodal planes. Local structural effects and/or departures from a double‐couple source mechanism have been invoked to explain this phenomenon. In order to obtain an independent determination of the source mechanisms for the April 24, 1970, and April 3, 1972, events on the southern Reykjanes Ridge, we invert the Rayleigh wave radiation pattern to obtain the source moment tensor. The moment tensor formulation should be particularly well suited to this problem because it is not restricted a priori to a double‐couple source mechanism. A potential drawback of the technique, however, is the requirement that phase velocities along the earthquake‐station paths be known very accurately in order to obtain the source phase from the observed phase, and an objective of this study was to determine whether a regionalized phase velocity model compiled from published dispersion curves is adequate. The results of the moment tensor inversion for both events indicate shallow normal faulting with the tension axis approximately horizontal and perpendicular to the local strike of the ridge. Apparent departures from a pure double‐couple source seem to result from errors in the data and the poor resolution of theMxzandMyzcomponents of the moment tensor for shallow sources. After performing the inversion under a series of increasingly more stringent constraints we conclude that the data for both events are compatible with pure double‐couple sources with moments of 4.8 and 7.5 × 1024dyn cm, respectively. We then show that interference betweenP,pP, andsPdue to shallowness of the source can account for the observed nonorthogonality and match the observedPwaveforms for the April 3, 1972, event with theoretical seismograms calculated for a shear fault whose orientation is consistent with the surface wave solution. The best fit to the data is obtained for a long, narrow fault (13 km by 3 km), with rupture initiating near the seafloor. The moment indicated by thePwaves is 7.5 × 1024dyn cm. These source parameters give an average displacement of about 60 cm and a stress drop of 30–60 bars, taking into account various uncertainties. Although we might expect attentuation to be high in the mid‐ocean ridge environment, the average attenuation required to fit the teleseismic data is not higher than normal (t* = 1 s). ThePwaves from the April 24, 1970, earthquake were too small to be suitable for quantitative modeling by synthetic seismograms but are qualitatively consistent with a shallow fault model similar to that for the larger event. We conclude that the faulting process described by these two earthquake mechanisms is directly related to the formation of rif

ISSN:0148-0227    

DOI:10.1029/JB086iB03p01701

年代:1981

数据来源: WILEY

摘要:

During the second half of 1979, anomalously high emanation of radon was recorded at two stations of the automated radon‐thoron monitoring network operated by the W. K. Kellogg Radiation Laboratory of the California Institute of Technology. The two stations exhibiting major anomalies, Kresge and Dalton Canyon, are located approximately 30 km apart on the frontal fault system of the Transverse Ranges of southern California. At Kresge the anomaly began on June 21, 1979, and continued through December 1979. At Dalton Canyon the anomaly started about 3 weeks later and also continued through December 1979. At both sites the anomalous levels of radon decreased (but did not return entirely to normal values) shortly before October 15, 1979. During the week of October 15, 1979, a 6.6‐Mearthquake occurred about 290 km to the southeast of the two stations, and later in that week, earthquakes of magnitude 4.2 and 4,1 occurred at Malibu and Lytle Creek. The latter two events were within 60 km of the monitors. A radon‐thoron monitor at Lytle Creek recorded no long‐term anomaly but did record a sharp spikelike decrease in the radon level on October 13, 1979. Coincident with our observations of anomalous radon levels, other investigators have reported anomalies or suspected anomalies in several other geodetic, geophysical, and geochemical signals from the same general region. The rapid temporal development of several of the anomalies together with the large area over which they were observed suggests that a large‐scale strain event took place which may have been responsible both for the widespread anomalies and for the seismicity that occurred in the region subsequent to the onset of the

ISSN:0148-0227    

DOI:10.1029/JB086iB03p01725

年代:1981

数据来源: WILEY

摘要:

High‐precision geomagnetic measurement depends on eliminating variations of ionospheric and magnetospheric origin. The commonly used technique of simply taking differences between total field magnetometers is only partially successful. Our method involves finding the multichannel Wiener filters which predict the field variation at a given total field magnetometer of an array from the fields of the remaining magnetometers and a three‐component magnetometer. The difference is then taken between the total field and the predicted field, leaving a cleaned total field. Filter lengths and number and choice of input channels are determined using methods of statistical parametric model fitting. The resulting filters which are defined on short noise free record sections are found to be effective over the remainder of the record. Detailed analysis of data from the University of California, Los Angeles, array in southern California, for which we have the best vector data, shows that for optimal predictive cleaning it is essential to use vector components and that only the two components orthogonal to the total field direction are necessary. This means that the field‐aligned component may be omitted from a vector station, which simplifies the instrumentation considerably. Analysis of the standard deviation of the residuals after cleaning (0.1 nT for hourly averages) shows that it is 8 times that expected from digitization noise alone. The most probable explanation for this is that measurement noise is about 3 times the digitization interval (0.25 nT) and that higher‐precision measurements are required for further improvement. This conclusion has been confirmed by tests on the instruments at close spacing. Application of the method to data taken in central California and Hawaii reveals tectonomagnetic effects which are otherwise hidden in noise. Wiener filters are especially suitable for real‐time analysis, which is an important factor in earthquake p

ISSN:0148-0227    

DOI:10.1029/JB086iB03p01731

年代:1981

数据来源: WILEY