摘要:

Reversals locating the equilibrium orthoenstatite = protoenstatite + diopside in CaO‐MgO‐SiO2demonstrate that orthoenstatite cannot exist stably above 1005±10°C at one atmosphere pressure. This in turn suggests that the orthorhombic low‐Ca phase found near 1400°C at atmospheric pressure in this system (formerly identified as orthoenstatite) is in fact a previously unrecognized pyroxene p

ISSN:0094-8276    

DOI:10.1029/GL012i007p00409

年代:1985

数据来源: WILEY

摘要:

The phonon spectra, distortion mechanisms, and thermoelastic properties of CaSiO3and MgSiO3in the cubic perovskite structure are investigated as a function of pressure using a lattice dynamic approach. The bonding forces are derived from a parameter‐free rigid‐ion electron‐gas formulation. At low pressures, CaSiO3is found to be dynamically stable in the cubic perovskite structure; however, the phonon spectrum exhibits soft modes at the Brillouin zone boundary which ultimately result in a dynamic instability of the lattice near 80 GPa. The computed phonon spectrum of cubic MgSiO3perovskite exhibits complex frequencies along parts of the zone boundary at all densities investigated. These vibrational instabilities include coupled octahedral rotations which produce the observed distorted structure of MgSiO3perovskite. The measured bulk modulus of MgSiO3perovskite compares well with our calculated value for the cubic stru

ISSN:0094-8276    

DOI:10.1029/GL012i007p00413

年代:1985

数据来源: WILEY

摘要:

An homogeneous upper mantle chemical composition defined by the pyrolite model is compatible with the observed seismic velocities. Such a test is now possible for depths to 670 km as a result of new mineral elasticity and phase equilibria data. Three specific criteria for comparing the mineralogical velocity model with that of the mantle include i) the velocity jump at the 400 km discontinuity, ii) the velocity increase between 400 and 670 km, and iii) the absolute value of velocity in the 300 to 400 km region (shallower than the discontinuity). The first criterion is sensitive to the olivine content of the mantle and the pressure derivatives of the elastic properties of the high pressure phase. The application of this criterion suggests a mantle with 40 ‐ 65% olivine by volume. The second criterion is sensitive to the details of the pyroxene to garnet transformation and the relative abundances of the pyroxenes and garnet. The third criterion depends on details of this transformation as well as iron content and the temperature of the mantl

ISSN:0094-8276    

DOI:10.1029/GL012i007p00417

年代:1985

数据来源: WILEY

摘要:

We present the initial results of an ongoing study of higher‐order mantle overtones of Rayleigh waves. The main problem is to separate overtone branches (3R,4R,5R), traveling with identical group velocities at similar frequencies. Global stacking is used, both for traveling waves and normal modes, with dispersion results in generally excellent agreement with both theoretical models and values published for a well‐separated part of the3Rbra

ISSN:0094-8276    

DOI:10.1029/GL012i007p00421

年代:1985

数据来源: WILEY

摘要:

During the month of August 1984 an experiment was undertaken in the Charlevoix Seismic Zone to test the hypothesis that shear‐wave splitting could be observed in a seismically active region. Seismograms, recorded by digital 3‐component seismographs, located very close to the epicentres of three earthquakes, showed shear‐wave splitting. The amount of splitting was estimated to be 1.6% to 2% of the shear wave velocity from particle motion diagrams. The average azimuth of the direction of polarization was 30°. These observations can be explained by the presence of vertical cracks striking 30° east of north, with a minimum crack density of

ISSN:0094-8276    

DOI:10.1029/GL012i007p00425

年代:1985

数据来源: WILEY

摘要:

Australia and India are conventionally thought to be contained in a single plate divided from an Arabian plate by the Owen Fracture Zone. We propose instead that motion along the nearly aseismic Owen Fracture Zone is negligible and that Arabia and India are contained within a single Indo‐Arabian plate, divided from the Australian plate by a diffuse boundary. This boundary, which trends E‐W from the Central Indian Ridge near Chagos Bank to the Ninetyeast Ridge, and north along the Ninetyeast Ridge to the Sumatra Trench, is a zone of concentrated seismicity and deformation heretofore characterized as “intraplate”. Plate motion inversions and an F‐ratio test show that relative motion data along the Carlsberg Ridge are fit significantly better by the new model. The misclosure of the Indian Ocean triple junction is reduced by 40%. The rotation vector of Australia relative to Indo‐Arabia is consistent with the seismologically observed ∼2 cm/yr of left‐lateral strike‐slip along the Ninetyeast Ridge, N‐S compression in the Central Indian Ocean, and the N‐S extension near Chagos. This boundary, possibly initiated in late Miocene time, may be related to the opening of the Gulf of Aden and the uplift of the Himalayas. The convergent segment of this boundary may represent an early stage of convergence preceding the ini

ISSN:0094-8276    

DOI:10.1029/GL012i007p00429

年代:1985

数据来源: WILEY

摘要:

With data recorded by a deep‐ocean hydrophone array centrally located in the Northwestern Pacific Basin near Wake Island, significantly large earthquakes were found to have their epicenters within the interior of the Pacific plate. The signal/noise ratio of the largest is about 40/1 at a distance of 2268 km. Although all of these earthquakes were unreported in National Earthquake Information Service (NEIS) listings, comparisons with the Po/So phases of other earthquakes reported by NEIS indicate that these earthquakes could have body‐wave magnitudes in excess of

ISSN:0094-8276    

DOI:10.1029/GL012i007p00433

年代:1985

数据来源: WILEY

摘要:

Atmospheric concentrations of aerosol methanesulfonic acid (MSA) and non‐sea‐salt (nss) sulfate were measured in samples collected during one year periods at Fanning and American Samoa Islands in the Pacific Ocean. Mean concentrations were 0.045 (s=0.011) ug/m⊃3 MSA and 0.67 (s=0.13) ug/m⊃3 nss sulfate at Fanning, and 0.026 (s=0.012) ug/m⊃3 MSA and 0.41 (0.19) ug/m⊃3 nss sulfate at American Samoa. MSA and nss sulfate were significantly correlated at both stations (r=0.76), with a mean MSA/nss sulfate ratio of 0.065. The higher mean levels of MSA and nss sulfate at Fanning compared to American Samoa apparently reflect differences in the biological productivity of the surface ocean in the

ISSN:0094-8276    

DOI:10.1029/GL012i007p00437

年代:1985

数据来源: WILEY

摘要:

Stratospheric chlorine monoxide concentrations were measured in situ using a new and improved observational method in which the detection package is lowered and then reeled back up. Two reel down and reel up sequences covering 38 to 31 km produced [CℓO] profiles which agree with each other to ± 12%. The measured [CℓO]profile falls within the envelope of nine previous "fast" parachute drop results, but exhibits a different shape, peaking near 32 km as opposed to ∼39 km for the average of the previous profiles. These results are consistent with the profiles derived from remote sensing methods, and indicate the need for simultaneous measurements of relevant s

ISSN:0094-8276    

DOI:10.1029/GL012i007p00441

年代:1985

数据来源: WILEY

摘要:

Regularly spaced vertical structures have been observed in stratospheric aerosols revealed by the observation of scattered sunlight. The simultaneous observations of this phenomenon and of regularly spaced vertical fluctuations of temperature is presented. The aerosol structure is interpreted in terms of particle size as a function of temperature on the basis of the limited theoretical studies available about the effects of temperature and humidity on the growth and optical properties of sulfuric acid‐water dropplets in the stratospher

ISSN:0094-8276    

DOI:10.1029/GL012i007p00445

年代:1985

数据来源: WILEY