摘要:

A new global transfer function (admittance) between oceanic geoid and seafloor depth has been derived for the wavelength band 1000‐4000 km, after removal of lithospheric cooling effects using the most recently published crustal ages of the oceans. In this wavelength band, the admittance is positive, reaches a maximum of ∼ 5m/km around 2750 km wavelength, then decreases to ∼ 2.5 m/km, on the average, at shorter wavelengths (between 2500 km and 1000 km). These values are systematically higher by 30‐40% than those we derived in a previous study using crustal ages of poorer quality.In view of these new results, we cannot exclude that part of the signal at the 2750 km wavelength is of convective origin, unlike the conclusions of the previous study. On the other hand, at shorter wavelengths, the observed admittance can still be interpreted in terms of crustal thickening and sediment

ISSN:0094-8276    

DOI:10.1029/GL014i001p00001

年代:1987

数据来源: WILEY

摘要:

A trilateration network was set up in early 1982 by a team of French and Mexican institutions across the central part of the Gulf of California in order to study the plate boundary related movements in this transition area between the San Andreas fault system and the East Pacific Rise. The reobservation of this network in March 1986 provides a first set of data on the present day deformations in this area. Both surveys used AGA8 Laser geodimeter measurements between 11 stations located on elevated points of Baja California and Sonora and on the islands between the peninsula and mainland coasts. Deformation patterns during the 1982‐1986 interval, obtained through three different methods indicate mainly a right lateral shear movement in the Gulf axis direction N46°W. Between Baja California Peninsula and Angel de la Guarda Island 17 ± 4 cm of dextral slip occurred. Between the coast of Sonora and the central islands of the Gulf the mean displacement amount to about 23 ± 12 cm. In the southwestern part of the network, weaker movements seem to have occurred, and are smaller than estimated errors. This may indicate either that the boundary is locked in this part or is deflected towards the ESE. These results, which give an estimation of the relative plate velocity of 8 ± 3 cm/a, are consistent with the generally accepted relative movement between North American and Pacific plates (about 6

ISSN:0094-8276    

DOI:10.1029/GL014i001p00005

年代:1987

数据来源: WILEY

摘要:

P‐waves propagating from shallow earthquakes in the Tonga‐Kermadec seismic zone to regional stations in Raratonga and Niue are associated with large negative travel‐time residuals (observed minus Jeffreys‐Bullen time) of the order of −10 s. For focal depths below 100 km the magnitude of residuals decreases sharply, to about −2.5 s for depths near 200 km. The pattern of residuals is explained by a model of oceanic lithosphere possessing an upper mantle P‐wave velocity of 8.3 km/s and having its base at a dept

ISSN:0094-8276    

DOI:10.1029/GL014i001p00009

年代:1987

数据来源: WILEY

摘要:

In this paper we present a method for analyzing seismic signals recorded at an array of seismometers. The method is based on the multiple signal characterization (MUSIC) method. Four important features of this approach are: (1) it has the ability to resolve multiple closely spaced sources, (2) it works with both stationary, and nonstationary signals, (3) it is most sensitive to the strongest sources, and (4) it provides a geometric interpretation of the solution to the direction finding problem. We have developed a numerical algorithm that extends the MUSIC method to nonstationary and correlated signals. We present results using this algorithm to measure directions of arrival and amplitudes of multiple plane waves in a homogeneous medium.

ISSN:0094-8276    

DOI:10.1029/GL014i001p00013

年代:1987

数据来源: WILEY

摘要:

Cellular automata are arrays of discrete variables that follow local interaction rules and are capable of modeling many physical systems. A successful recent application has been in fluid dynamics, in which the Navier‐Stokes equations are solved by creating a model in which space, time, and the velocity of particles are all discrete. Acoustic waves can be obtained from these fluid models when perturbations of the idealized fluid are small. Because seismicP‐waves can be approximated by the acoustic wave equation, cellular automata can be adapted for seismic wave computations. This study shows how to modelP‐waves in two dimensions by using a modified form of the cellular‐automaton rules for fluids. Propagation, reflection, and the computation of synthetic seismograms are demonstrated. Because no arithmetic calculations are needed and each lattice site can be updated simultaneously, this method is well‐suited for implementation on massively parallel computers. Among the many potential advantages are unconditional stability, no round‐off errors, and the possibility for devising novel approaches for modeling waves in inhomoge

ISSN:0094-8276    

DOI:10.1029/GL014i001p00017

年代:1987

数据来源: WILEY

摘要:

The Rikitake two‐disk dynamo is known to display chaotic oscillations of the current direction which resemble occasional polarity reversals of the geomagnetic field. In order to see how well the actual behavior of the geodynamo is simulated by this simple model, statistical properties of the two‐disk dynamo system are examined in two aspects; the field intensity and the length of polarity intervals. Paleomagnetic data indicate that the occurrence frequency of the dipole intensity is well approximated by superposition of two normal distributions, and that of the polarity intervals by an exponential distribution. For various parameters tried for the Rikitake two‐disk dynamo, the distributions of the dipole intensity and of the polarity intervals are not similar to the paleomagnetically observed ones. Thus the similarity between the two‐disk dynamo and the geomagnetism appears to be

ISSN:0094-8276    

DOI:10.1029/GL014i001p00021

年代:1987

数据来源: WILEY

摘要:

A numerical simulation of internal waves using an eikonal approach is analyzed for a relation between the short wavelength, relatively high frequency waves, and the velocity and shear resulting from a background of low vertical wavenumber waves through which the short waves propagate. The background consists predominantly of quasi‐inertial motions. The results indicate a strong correlation between the horizontal group velocity of the short waves and the shear of the background when the short waves have been refracted to 10 meters or less in vertical wavelength. It appears that rapid refraction of the short waves to dissipative scales (<5 m) then occurs as they become oriented with the shear. The directional anisotropy of the short high frequency waves at dissipation scales suggests an average u‐w correlation. Thus the high Reynolds stresses predicted by Müller [1976] are primarily associated with the shear of long wave inertial motions. In nature, the u‐w correlation would only be seen if the averaging is done in a reference frame that is allowed to rotate with the shear. A fixed frame averaging would give a near zero result, thus accounting for the absence of stress‐shear correlations in the fixed frame measurements of Ruddick and Joyce [1978]. Vertical profiler data of horizontal velocity are also examined for indications of a correlation in the short wave velocity field relative to the long wave shear and velocity fields. These observations are dominated by low frequency, quasi inertial oscillations. There is no consistent evidence of a directional anisotropy in the short wave field which is dependent on the long wave shear, but weak evidence of velocity polarization such that the long and short waves are polarized in the same d

ISSN:0094-8276    

DOI:10.1029/GL014i001p00025

年代:1987

数据来源: WILEY

摘要:

The roughness of fault surfaces is important in the mechanics of fault slip and could play a role in determining whether sliding occurs via earthquakes or fault creep. We have made preliminary measurements of the power spectral density of several fault surfaces over the wavelength range from 10−5to 1 m, using field and laboratory scale profilimeters. The fault surfaces are strongly anisotropic; profiles parallel to the slip direction have amplitudes about one order of magnitude lower than those perpendicular to the slip direction over most of the wavelength interval measured. Fault roughness perpendicular to the slip direction is similar to the roughness of natural joints; the greater smoothness in the slip direction presumably represents wear resulting from slip. Combining our data with previous measurements of the roughness spectrum of the San Andreas fault shows that fault surfaces are fractal over nearly eleven orders of magnitude in wavelengt

ISSN:0094-8276    

DOI:10.1029/GL014i001p00029

年代:1987

数据来源: WILEY

摘要:

Climatic warming is shown to be capable of inducing shear heating instability and basal melting in a model ice sheet that is creeping slowly downslope. Growth times of the instability are calculated from a nonlinear analysis of temperature and flow in the model ice sheet whose surface undergoes a prescribed increase of temperature. The source of instability lies in the decrease of maximum ice thickness for steady downslope creep with increasing surface temperature. A surface temperature increase of 5 to 10 K can cause instability on a 104year time scale for realistic ice rheology. The instability occurs suddenly after a prolonged period of dormancy. The instability might be relevant to the East Antarctic ice sheet. Warming associated with the Holocene interglacial epoch that heralded the end of the last ice age may have set the East Antarctic ice sheet on a course toward widespread instability some 104years later. The present CO2‐induced climate warming is also a potential trigger for instability and basal melting of the East Antarctic ice shee

ISSN:0094-8276    

DOI:10.1029/GL014i001p00033

年代:1987

数据来源: WILEY

摘要:

Direct calculation of elasticity in the Potential Induced Breathing (PIB) model is developed. This allows calculation of elastic constants and acoustic velocities of oxides and silicates without any experimental data other than the values of fundamental constants, such as Planck's constant. The PIB model allows for spherical charge relaxation in response to the long‐range electrostatic potential. This feature is not present in any other lattice dynamical model, and leads to many‐body forces that break the Cauchy conditions for centrosymmetric crystals (i.e. C44=C12) without the introduction of explicit angle bending forces. The elastic properties are calculated here for corundum and stishovite. High pressure elastic instabilities are found for both corundum (α‐Al2O3) and stishovite (SiO2). Softening of C44for corundum is calculated to begin at approximately 120 GPa. This may require reconsideration of the ruby fluorescence pressure scale and recent ultra‐high pressure diamond cell experiments. The high pressure instability in stishovite leads to a low pressure dependence for the aggregate shear wave

ISSN:0094-8276    

DOI:10.1029/GL014i001p00037

年代:1987

数据来源: WILEY