摘要:

Order from the American Geophysical Union, Suite 435, 2100 Pennsylvania Ave., N.W., Washington, D.C. 20037. Document J69‐001; $1.00. Payment must accompany order.The spectral amplitudes and travel times of seismic body waves are used to determine mantle velocity structures appropriate to distinct structural provinces within the western continental United States. In addition to basic amplitude and time data, travel‐time delays and Pnvelocity data from other studies are used as constraints in the systematic inversion of the data for mantle structure. The regional structures for the upper mantle determined in this manner show collectively rather sharp zones of transition (high velocity gradients) near 150, 400, 650 km and possibly near 1000 km. Comparatively, the regional structures indicate strong lateral variations in the upper mantle structure down to 150 km and possibly as deep as 200 km. The structures appropriate to the Rocky Mountain and Colorado plateau physiographic provinces show low‐velocity zones capped by high‐velocity lid zones, with variability in both the lid and the low‐velocity zone properties from province to province and within these provinces to a much lesser degree. The mantle properties obtained for the Basin and Range contrast sharply with the plateau and mountain structures, with the lid zone being very thin or absent and abnormally low velocities extending from, or very near, the base of a thin crust to 150 km. The velocity determinations are coupled with estimates of the variation of the intrinsic dissipation function (Q) as a function of depth and frequency. These results show a pronounced low‐Qzone corresponding to the average low‐velocity zone depth range for the velocity models. The data suggest a frequency‐dependentQ, withQincreasing with frequency. In total the results of the study strongly suggest phase transitions in the mantle, including a partially melted region corresponding to the low‐velocity zone, the latter being highly variable in its properties over the region studied and strongly correlated with

ISSN:0148-0227    

DOI:10.1029/JB074i025p05825

年代:1969

数据来源: WILEY

摘要:

A wide variety of terrestrial rocks has been investigated to determine the range of variation of the high‐frequency electrical properties. Both solid rocks and powders of various types have been measured at frequencies of 450 MHz and 35 GHz; some measurements have been made at elevated temperatures. Solid materials show wide variations in permittivity and absorption length, but apart from a small trend with silica content it is not clear that there is any pattern to the variations, and it is unlikely that measurements limited to these radar frequencies will be useful for identification. With powdered rocks, on the other hand, the variation from rock type to rock type is much smaller, but depends on the powder density in a quite predictable way. Consequently, radar information can be employed to make precise estimates of density and of density profiles even in the absence of information about composition. The long absorption lengths in powdered rocks indicate that radar reflections may occur at considerable depths in a regolith of the type found on the moo

ISSN:0148-0227    

DOI:10.1029/JB074i025p05867

年代:1969

数据来源: WILEY

摘要:

The second version of the MIT VSA Surface‐Ship Gravimeter is described. It consists of the military surplus (Atlas program) American Bosch Arma Lot D Vibrating String Accelerometer mounted on the Sperry Mark 19 Mod 3C Meridian Gyrocompass. Data read‐out is by a uniquely simple integrated circuit phase‐lock frequency filter, presenting output data in binary coded decimal and analog format directly in milligals of apparent free‐air gravity. An accuracy of better than 1 mgal has been established over open‐ocean gravity ranges. The system described should prove to be relatively inexpensive t

ISSN:0148-0227    

DOI:10.1029/JB074i025p05882

年代:1969

数据来源: WILEY

摘要:

The acceleration of the earth's rotation averaged over the past 3000 years has been re‐studied using a new analysis of the ancient eclipses. The relaxation time for a second‐order harmonic distortion of the earth has been calculated to fall in the range 870–1600 years. This short relaxation time seems to require a viscosity for the deep mantle of ∼1022poises, permitting the convective transport of heat from the deep interior of the earth. If the viscosity is this low, deep convective currents are required to support the excess equatoria

ISSN:0148-0227    

DOI:10.1029/JB074i025p05895

年代:1969

数据来源: WILEY

摘要:

An exact solution is obtained for the displacement of a surface caused by an explosive point source within a layered sphere that consists of a homogeneous solid mantle and a fluid core. For all modes of periods 4 seconds and longer, the variation of amplitude, phase changes, and range of existence of several reflected, refracted, and diffracted pulses are found and compared with observed and model experimental results.

ISSN:0148-0227    

DOI:10.1029/JB074i025p05903

年代:1969

数据来源: WILEY

摘要:

The hypocenters of about 1000 earthquakes in the Izu‐Bonin, Mariana, Palau, Caroline, and Ryukyu‐Taiwan regions were relocated by computer, and earthquake mechanism solutions based on the first motions ofP,pP, andSwere determined for 26 earthquakes. The spatial distribution and the mechanisms were compared with major tectonic features such as volcanic zones, island arcs, and trenches. Although a nearly continuous zone of shallow earthquakes can be traced from Honshu to Palau, deep shocks were not detected south of 16°N; intermediate‐depth events were not found south of the Marianas. A region of low seismicity for shallow earthquakes and a discontinuity for deeper events occurs in the Volcano Islands adjacent to a prominent gap in the trench where the Marcus‐Necker ridge intersects the arc. In some profiles of this series of arcs, the deep seismic zone is nearly planar, but in other cases it is more contorted and the thickness of the zone appears to vary from place to place. Maxima and minima in seismic activity occur for deep and intermediate depths, and these features migrate in depth similar to that found in the Tonga arc. Activity as deep as about 680 km occurs in the northern Marianas where the deep focal zone is nearly vertical for depths greater than about 200 km. This nearly vertical distribution suggests that gravitational tectonics may play an important role as a motive force for global tectonics. Seismicity in the Ryukyu arc is concentrated in a thin planar zone that dips 35° to 45° northwest to a depth of about 280 km. Near the northwest coast of Taiwan all tectonic elements change strike abruptly, and the mechanisms change from thrusting of the Philippine Sea plate beneath the Ryukyu arc to strike slip near the east coast of Taiwan. South of this point, most of the events are shallower than 100 km, and the focal zone is not well defined in vertical sections. No events were detected from the Kyushu‐Palau ridge, from the Central Basin fault, or from the rest of the interior of the Phillipine Sea. Thus, this region appears to be a single lithospheric plate. Normal faulting is found for some earthquakes either in the deepest parts of the trench or along the seaward wall. A zone of shallow earthquakes also can be identified on many of the profiles near the volcanic axis. Mechanism solutions for two events near this zone are also of the normal‐fault type. Mechanism solutions for shallow earthquakes beneath the islandward margins of the Izu‐Bonin and Mariana trenches indicate that the Pacific plate is underthrusting the Philippine Sea plate in a westerly direction.The results of the relocations and other pertinent data are given in a separate appendix, which is available on microfiche along with the entire article. Order from the American Geophysical Union, Suite 435, 2100 Pennsylvania Ave., N.W. Washington, D.C. 20037. Document J69‐002; $1.00. Payment mu

ISSN:0148-0227    

DOI:10.1029/JB074i025p05923

年代:1969

数据来源: WILEY

摘要:

The nine adiabatic elastic stiffness constants of single‐crystal forsterite (Mg2SiO4) have been measured as a function of hydrostatic pressure and temperature using a pulse super‐position technique. The pressure dependence was linear, within experimental error, for all nine coefficients up to 10 kb. Temperature measurements were made in the range from 300° to 700°K. At lower temperatures some curvature was present, but above 500°K the data were linear. The nine adiabatic stiffnesses and their isothermal pressure derivatives at 25°C are presented, and the isobaric temperature derivatives are given at zero pressure in the high temperature limit above 500°K. The elastic constants and their pressure derivatives agree approximately with theoretical data for a hexagonal close‐packed structure of oxygen anions assuming only nearest neighbor interactions and central forces. This agreement indicates that the elastic properties of forsterite are predominantly determined by the oxygen anion framework. In addition, the results were used to calculate the isothermal pressure dependence of the volume and of the lattice parameters of forsterite at high pressure from the Murnaghan and Birch equation

ISSN:0148-0227    

DOI:10.1029/JB074i025p05949

年代:1969

数据来源: WILEY

摘要:

Ultrasonic wave velocities in single‐crystal forsterite (F) and single‐crystal olivine (O) have been measured as a function of pressure and of temperature near ambient conditions. Shear and longitudinal velocities were measured in eighteen independent modes, so that each of the nine elastic constants could be calculated by at least two independent equations. The adiabatic stiffness constantscij(in Mb), their temperature derivativesdcij/dT(10−4Mb/deg), and their pressure derivativesdCij/dP, areij11 22 33 44 55 66 23 31 12Cij(F) 3.284 1.998 2.353 0.6515 0.8120 0.8088 0.738 0.688 0.639 (O) 3.237 1.976 2.351 0.6462 0.7805 0.7904 0.756 0.716 0.664dcij/dT(F) 3.31 2.81 2.83 1.30 1.32 1.51 0.46 0.82 1.04 (O) 3.40 2.85 2.86 1.28 1.30 1.57 0.51 0.94 1.05dcij/dP(F) 8.47 6.56 6.57 2.12 1.66 2.37 4.11 4.84 4.67 (O) 7.98 6.37 6.38 2.17 1.64 2.31 3.76 4.48 4.74The density ρoand other isotropic properties including the adiabatic bulk modulusKs, the shear modulus μ, the Poisson ratio σs, and their pressure and temperature derivatives (using the same dimensions as above) computed by the VRH (Voigt‐Reuss‐Hill) method are ρKsμ σsdKs/dTdμ/dTdσs/dT, 10−5°C−1dKs/dPdμ/dPdσs/dP(F) 3.224 1.286 0.811 0.240 −1.50 −1.30 0.93 5.37 1.80 0.42(O) 3.311 1.294 0.791 0.246

ISSN:0148-0227    

DOI:10.1029/JB074i025p05961

年代:1969

数据来源: WILEY

摘要:

The elastic constants of a very small orthopyroxene gem (1.9 grams) of the bronzite variety (density 3.335 g/cc) was measured by the pulse superposition method. The adiabatic stiffness constants in Mb at room temperature and 1 atm pressure are:c11= 2.299,c22= 1.654,c33= 2.057,c44= 0.8306,c55= 0.7637,c66= 0.7853,c23= 0.496,c31= 0.573, andc12= 0.701. Using the Voigt‐Reuss‐Hill average, the dilational wave velocity is 7.85 km/sec, shear wave velocity is 4.76 km/sec, bulk modulus is 1.050 Mb, rigidity is 0.755 Mb, and Poisson's ratio is 0.21. In contrast with the properties of clinopyroxenes, the a axis in this crystal is elastically the most stiff direction, showing the maximum dilational velocity and minimum linear compressibility. The maximum velocity anisotropy of dilational waves is 126 km/sec, or 17%, which is comparable to 18% for oliv

ISSN:0148-0227    

DOI:10.1029/JB074i025p05973

年代:1969

数据来源: WILEY

摘要:

In conjunction with the Apollo lunar passive seismic experiment, a study was undertaken to determine the efficiency of meteoroid impacts on the lunar surface as potential sources of seismic energy. This study is based on experiments performed at Ames Research Center to determine the coupling between an impacting projectile and seismic waves. High‐velocity guns at Ames were used to fire projectiles into targets in a vacuum chamber. Projectile velocities ranged from less than 1 to more than 7 km/sec; projectile masses ranged from about 0.25 to 5 grams. Targets were of two types having considerably different elastic properties. Experimental results indicate that the seismic source function of an impact can be expressed as a function of the kinetic energy of the projectile. Extrapolating our results up to the kinetic energies of meteoroids, it is possible to predict the number of impacts that will be detected by the Apollo passive seismic experiment during its nominal lifetime of one year if assumptions are made about the physical properties of the moon. For the most optimistic set of assumptions, the instrument will detect about 370 meteoroid impacts; for the most pessimistic, about 3 impacts. Most of these impacts will be within 10 to 20 km from the seismometers. The experimental data of this study can be used to estimate the effectiveness of impacting spent spacecraft stages as artificial seismic sources. The results suggest that surface waves generated by the impact of the Saturn S‐IVB stage of the Apollo booster would be detected at distances of from 41 to 681 km from the seismometer, depending on the properties assumed for the lunar surface mater

ISSN:0148-0227    

DOI:10.1029/JB074i025p05981

年代:1969

数据来源: WILEY