摘要:

The geomagnetic field at a given epoch can be used to partition the surface of the liquid outer core into a finite number of contiguous regions in which the radial component of the magnetic flux density, Br, is of one sign. Theseflux patchesare instrumental in providing detail to surface fluid flows inferred from the changing geomagnetic field and in evaluating the validity of the frozen‐flux approximation on which such inferences rely. Most of the flux patches in models of the modern field are small and enclose little flux compared to the total unsigned flux emanating from the core. To demonstrate that such patches are not required to explain the most spatially complete and accurate data presently available, those from the Magsat mission, I have constructed a smooth core field model that fits the Magsat data but does not possess small flux patches. I conclude that our present knowledge of the geomagnetic field does not allow us to resolve these features reliably at the core‐mantle boundary; thus we possess less information about core flow than previously belie

ISSN:0094-8276    

DOI:10.1029/96GL03028

年代:1996

数据来源: WILEY

摘要:

Two pairs of core phases, PKPcd‐PKPdf and PKPbc‐PKPdf, are analysed to map the P‐wave velocity (Vp) of the uppermost inner core. Waveform analysis of PKPcd‐PKPdf shows that Vp relative to PREM (δVp) within 100 km from the inner core boundary (ICB) is laterally heterogeneous by −0.5% to +0.1% with a wavelength of nearly 1000 km. Differential travel times of PKPbc‐PKPdf relative to PREM (δTbc‐df) are positive (negative) for the regions of positive (negative) δVp near the ICB. This correlation indicates that the lateral Vp variation exists predominantly in the inner core side of the ICB. Absolute values of δTbc‐df for both positive and negative δVp regions are too large to be explained with models in which lateral δVp variation is restricted within the upper 200 km of the inner core. The lateral variation appears to extend at least to

ISSN:0094-8276    

DOI:10.1029/96GL03026

年代:1996

数据来源: WILEY

摘要:

The space orientation of the LAser GEO‐dynamics Satellite (LAGEOS) spin axis evolves slowly in time due to a permanent influence of perturbing torques. We extend a theoretical model of this evolution by including two, previously neglected, effects: (i) a radiation torque due to an asymmetric reflectivity of the satellite surface, and (ii) a perturbing torque related to a possible miscentering between the mass and geometric centers of the satellite. We demonstrate that both effects will contribute importantly to the scenario of the LAGEOS spin axis evolution at the end of this decade, and could be detected by both direct measurements of the LAGEOS spin axis and orbit analysi

ISSN:0094-8276    

DOI:10.1029/96GL03025

年代:1996

数据来源: WILEY

摘要:

High‐pressure Raman spectra and X‐ray diffraction patterns of gibbsite have been investigated up to 23.3 GPa and 8.7 GPa, respectively in diamond anvil cells at room temperature. More than 20 Raman‐active modes were observed at the ambient conditions and these modes were predominately distributed in two regions: the low‐wavenumber (LW, 200 to 1200 cm−1) and high‐wavenumber regions (HW, 3000 to 3700 cm−1). The wavenumber of Raman modes in LW region increases linearly with pressure while the wavenumber of those in the HW region decreases with pressure. The later observation indicates the enhancement of hydrogen bonds between octahedral layers in the lattice upon compression.A significant change in the diffraction patterns and Raman spectra was observed for Al(OH)3above 3 GPa where a phase transition takes place. The high‐pressure phase of Al(OH)3is quenchable and is tentatively determined as nordstrandite, a triclinic polymorph of gibbsite, on the basis of its diffraction pattern and

ISSN:0094-8276    

DOI:10.1029/96GL03023

年代:1996

数据来源: WILEY

摘要:

The single isotropic scattering model proposed for the origin of seismic coda predicts a regularly decaying coda wave envelope. In real data however, this regular decay is often interrupted by non‐random arrivals (e.g. reflections) coming later in the coda. This additional energy may be due to clustered scatterers and represents a departure from the inherent assumption in the theory that the scatterers are uniformly randomly distributed. We have simulated the effect of clustered scatterers in synthetic seismograms in order to investigate their influence upon the measured coda Q (Qc). We show that the position and amplitude of such arrivals within the otherwise regularly decaying coda give rise to elevatedQcestimates. Often this additional energy in the coda is difficult to detect on a single isolated seismogram as evidenced by an example from the 1994 Northridge California earthquake. We therefore recommended that where possible, the use of seismic arrays (commonly reserved for P‐wave coda studies) be extended to S‐wave coda studies to aid in the detection of clustered scatterers, prior to the calculation

ISSN:0094-8276    

DOI:10.1029/96GL03022

年代:1996

数据来源: WILEY

摘要:

At lower mantle conditions, the thermal conductivity of non‐metallic crystals is an order of magnitude lower than that of dense metallic minerals such as FeO, FeSi and Fe, which may be present in a chemical boundary layer at the base of the mantle. Because the core‐mantle boundary is nearly isothermal, variations in the thickness of a metal‐bearing layer induce lateral temperature variations of several hundred kelvin, which in turn affect the pattern of mantle convection. Upwellings should occur where the layer is thickest; the resulting stability of the metal + silicate layer with respect to the flow may also stabilize the pattern of mantle convection, and reduce its time‐dep

ISSN:0094-8276    

DOI:10.1029/96GL03021

年代:1996

数据来源: WILEY

摘要:

We investigate the crustal thickness under the Peninsular Ranges using P‐to‐S converted phases of teleseismic body waves recorded on a temporary broadband seismometer array and isolated by the receiver function method. Ps minus P times at sites west of a compositional boundary that separates the Peninsular Ranges batholith into east and west zones indicate a relatively flat, deep Moho. Ps minus P times at sites east of the compositional boundary decrease eastward, Moho depth estimates (made from the Ps delays and crustal velocities from seismic tomography) indicate a relatively constant 36 to 41 km thick crust in the western zone. In the eastern zone the crust thins rapidly from 35 km thick at the compositional boundary to 25 km at the edge of the Salton trough, a lateral distance of 30 km. The lack of correlation between topography and Moho depths suggests compensation via lateral density variations in the lower crust or upper mantle. We propose that the compositional boundary decouples the eastern and western portions of the batholith, and that the eastern portion has thinned in response to regional Miocene extension, or Salton trough rifting, or b

ISSN:0094-8276    

DOI:10.1029/96GL03020

年代:1996

数据来源: WILEY

摘要:

There are fundamental differences between low‐ and high‐porosity rocks in the evolution of permeability in the brittle faulting regime. In a rock with porosity less than ∼5%, dilatancy and permeability enhancement were observed prior to the peak stress. In porous rocks, the behavior is more complex. Before the onset of dilatancyC', both permeability and porosity decrease with increasing effective mean stress. BeyondC', permeability may decrease while the pore space dilates. After the peak stress has been attained, the development of a relatively impermeable shear band caused an accelerated decrease of permeability. A network model (with pore space statistics constrained by microstructural data) was developed to simulate the interplay of pre‐existing tubular pores and stress‐induced cracks in influencing the permeability evolution. The simulations show that while the accumulation of damage in form of microcracks may dilate the pore space, it causes the flow path to be more tortuous and as a result, the permeability may actually

ISSN:0094-8276    

DOI:10.1029/96GL03078

年代:1996

数据来源: WILEY

摘要:

We investigate the origin of the geodetically observed interseismic horizontal deformation in northern Honshu by comparing shear strain rates, principal strain rates, and velocity fields determined from geodetic data with those calculated from the elastic dislocation models involving interplate motion at the Japan trench. The agreement between the observed and predicted directions of the principal strain axes indicates that the geodetic strain field in northern Honshu is primarily elastic strain transmitted from the Japan trench. In order to match the strain rate tensors and velocity magnitudes obtained from the geodetic data, the dislocation model requires that 35% to 60% of the NUVEL1‐A Pacific‐North American plate motion is locked at the plate interface along the Japan trench. The down‐dip depth limit of the locked zone is inferred to be 55 km, which is consistent with the seismic data in the Japan t

ISSN:0094-8276    

DOI:10.1029/96GL02520

年代:1996

数据来源: WILEY

摘要:

We use Global Positioning System (GPS) measurements acquired from 1991 to 1995 to constrain the motion of sites in Bangalore, in southern India, and Kathmandu, Nepal, relative to a global GPS network. These measurements permit estimates of the northward motion of the Indian plate and convergence between the southern Himalaya and the Indian subcontinent. The velocities of Bangalore and Kathmandu in the ITRF92 reference frame agrees with that predicted by the NNR‐NUVEL1A plate motion model for Indian plate motion, and differ from that predicted for the Australian plate, confirming the independent motion of the Indian and Australian plate fragments. No significant motion was detected between Bangalore and Kathmandu during the three years from 1991–1994, even though Kathmandu is located in the hanging wall of the active Himalayan thrust system. The Himalayan thrust system is thought to accommodate 18±7 mm/yr of convergence and has been the source of several historic M ∼ 8 earthquakes. The absence of motion of Kathmandu relative to the Indian plate can be explained if the thrust system is presently locked south of the Greater Himalaya. Our preferred model has no steady slip on the detachment south of the Greater Himalaya, and steady slip at a rate greater than 6 mm/yr (1/3 of the long‐term convergence rate) can be ruled out at 95% confiden

ISSN:0094-8276    

DOI:10.1029/96GL02518

年代:1996

数据来源: WILEY