摘要:

During the last decade, the presence of velocity anisotropy inside the inner core, with a fast axis parallel to the Earth's rotation axis, has been well established. The quantitative analysis of the amplitudes of waves which sample a particular region of the inner core, under Africa, but exhibit various orientations allows us to document and characterize the presence of anisotropy in attenuation. The analysis is based on a comparison of both amplitudes and travel times of the PKP(DF) wave, which samples the inner core, and the PKP(BC) wave, which has nearly the same path but bottoms inside the liquid core. The data reveal that the direction of strong attenuation correlates with that of fast velocity. When referred to the same epicentral distance and focal depth, the PKP(DF)/PKP(BC) amplitude ratio are about five times lower for paths tilted by 25° with respect to the Earth rotation axis, than for nearly equatorial paths. A clear negative correlation is observed between travel time and amplitude residuals, when the angle to Earth rotation axis varies. This first quantitative analysis, combined with experimental results on oriented crystals and mushy media, may bring important constraints on the mechanism responsible for anisotropy in the inner core

ISSN:0094-8276    

DOI:10.1029/95GL03583

年代:1996

数据来源: WILEY

摘要:

Selected P‐wave acoustic travel times through single crystal samples of olivine and garnet with different Mg/(Mg+Fe) ratios were measured at elevated temperatures (from 293 K to 363 K) using a newly developed GHz ultrasonic interferometer. These high precision data allow us to examine aspects of the elasticity‐composition systematics of olivine (forsterite‐fayalite) and garnet (pyrope‐almandine) solid solution series. For every 10% decrease in the Mg/(Mg+Fe) content, the P‐velocity of olivine along the b‐axis decreases about 3% and ∂lnC22/∂T decreases about 7%; for garnet ∂ln⌊C44+ (C11+ C12) / 2⌋/∂T decreases about 6%. If such velocity‐composition systematics are typical and hold at the P‐T regime of the mantle, only a 3–4% variation in the Mg/(Mg+Fe) content of olivine is required to explain the lateral seismic velocity variation in the upper portion of the mantle within one tectonic region (dlnVp∼l%), and 7–8% variation in the Mg/(Mg+Fe) content to explain the 2% lateral seismic velocity varia

ISSN:0094-8276    

DOI:10.1029/95GL03604

年代:1996

数据来源: WILEY

摘要:

The effect of serpentinization on seismic wave velocity and anisotropy has been analysed in 6 peridotite samples of the Xigaze ophiolite, having harzburgitic composition and a degree of serpentinization ranging from 3% to 70%. We found: i) P‐ and S‐ wave velocities are linearly correlated with serpentine content; ii) anisotropy of P‐ and S‐waves decreases with increasing serpentinization (while in fresh peridotites anisotropy is of the order of 6%, for 70% of serpentinization it drops to 1% for P‐waves, and to 2%, for S‐waves). Comparison of published laboratory velocity data in oceanic lithologies, like gabbros and dolerites, with serpentinized peridotite data shows the difficulty of distinguishing between gabbros and peridotites having a percentage of serpentine between 10 and 40% by seismic waves velocities only. This comparison suggests that seismic velocity data are consistent with the existence of partialy serpentinized peridotites (up to 40% of serpentinization) in the oceanic crust formed at slow‐spreading ridges in accordance with geologica

ISSN:0094-8276    

DOI:10.1029/95GL03594

年代:1996

数据来源: WILEY

摘要:

Fluid flow in the Earth's crust can be accurately described by a diffusion equation, with highly anisotropic, heterogeneous and time dependent diffusivities. The so‐called Bhatnagar‐Gross‐Krook (BGK) models provide a simple and efficient way to solve the scalar diffusion equation with complex boundary conditions. We show that it can be adapted to include an anisotropic, space and time dependent diffusivity, and a gravity body force, hence providing a suitable numerical model for realistic simulations of incompressible pore fluid diffusion in heterogeneous, anisotropic rocks in the presence of a gravity field. A stability condition limits in some cases the magnitude of anisotropy that can be simulated. Numerical results are in very good agreement with a known analytical sol

ISSN:0094-8276    

DOI:10.1029/95GL03582

年代:1996

数据来源: WILEY

摘要:

We present a nonlinear dynamical model for oscillatory zoning in plagioclase based on a simple isothermal constitutive undercooling mechanism. A phenomenological partitioning is introduced to relate the concentration of An in the melt at the interface with the concentration in the solid. The non‐linearities in the model result from the coupling of the growth velocity with the local An concentration and from the boundary condition at the interface. The consideration of a nonlinear boundary condition is new and generalizes previous nonlinear growth models. It is shown that parameter values exist for which oscillatory solutions are possible via a Hopf bifurcation. As the system is driven further out of equilibrium, the model shows the development of chaotic solutions via a period‐doubling seque

ISSN:0094-8276    

DOI:10.1029/95GL03327

年代:1996

数据来源: WILEY

摘要:

We present global phase velocity distributions for fundamental mode Love and Rayleigh waves in the period range 40 to 150 seconds. The models, expressed in terms of spherical harmonic expansions up to degree and order 40, have beed derived from 28,479 Love wave and 33,662 Rayleigh wave measurements. The measurements were made using an automatic procedure based on non‐linear waveform inversion. We show that the results are characterized by lateral resolving radii between 500 and 850 km, a significant reduction from previous work. The improved global resolution is largely due to the inclusion of many more major arc measurements. The power spectra of the models are much whiter than has previously been found. The results show great similarity to the phase velocity distributions derived from the recenta priorimodel 3‐SMAC, in which the primary relevant features are the crustal thickness and crustal velocity distributions, the cooling oceanic lithosphere and the thick (300km) lithosphere of the continental cratons. The results thus confirm these features. Differences, for example, in the strengths of the crust, ridge and craton signatures should lead to refinement in the thermal and constitutive parameters upon which thea priorimodel depe

ISSN:0094-8276    

DOI:10.1029/95GL03391

年代:1996

数据来源: WILEY

摘要:

We analyzed P‐wave travel‐time data of earthquakes along a north‐south vertical cross section in the central Tibetan plateau to search for high velocity zones related to downwellings in the upper mantle. Differential travel‐time residuals of an event in southern Tibet vs. north central and Himalayan events show relatively fast arrivals at stations in the northwest ‐ southeast direction. Results of numerical simulation with simple velocity anomaly models suggest that a narrow, nearly vertical, high velocity zone extending to at least 400 km beneath the southern plateau can account for the observed fast arrivals. This high velocity zone may result from cold downwellings predicted by the delamination model for continental co

ISSN:0094-8276    

DOI:10.1029/95GL03607

年代:1996

数据来源: WILEY

摘要:

Six destructive earthquakes had successively occurred from 1939 to 1967 along the North Anatolian Fault Zone. The July 22, 1967 Mudurnu Valley earthquake (Ms=7.1) was the last of this series, with its western neighbour remaining as a seismic gap. Surface ruptures associated with this earthquake appeared within a broad shear zone, extending to a distance of 80 km. The generated teleseismic body waves were so complex that they were different even at stations in the same azimuth. In this study we inverted the observed teleseismic body waves to investigate such a presumably complicated rupture process of the Mudurnu Valley earthquake. We could identify six subevents within 2 minutes with their total seismic moment amounting to 1.1 × 1020Nm. Although right‐lateral strike‐slip faulting was found to be dominant, two minor subevents had a dip‐slip mechanism; one showed a reverse faulting mechanism and the other a normal faulting mec

ISSN:0094-8276    

DOI:10.1029/95GL03667

年代:1996

数据来源: WILEY

摘要:

We investigated the rupture process of the 28 December 1994 Sanriku‐Oki earthquake (Mw= 7.7) using broadband seismograms recorded at local distances. The earthquake rupture nucleated at the eastern end of the aftershock area. As it propagated towards the west, a major subevent occurred near the center of the aftershock area about 26 s after the initial rupture, releasing most of the seismic energy of this earthquake. About 24 s later, another subevent followed at the western end of the aftershock area, emitting large high‐frequency waves. The whole rupture time is about 55 sec. The three‐stage rupture process is very similar to the earlier‐stage rupture process of the 1968 Tokachi‐Oki earthquake (Mw= 8.2). The difference is that the rupture of the Tokachi‐Oki earthquake further propagated towards the north in the following stage instead of terminating at the western end. The occurrence of the Sanriku‐Oki earthquake is critical to the validity of the characteristic earthquake so far believed to exist in this part of the p

ISSN:0094-8276    

DOI:10.1029/95GL03584

年代:1996

数据来源: WILEY

摘要:

The 2 September 1992 Nicaragua tsunami was induced by anMS≃ 7–7.3 earthquake and caused about 170 victims and significant damage to the coastal areas. The aim of this work is to infer the slip distribution along the causative seismic fault from the available tsunami run‐up field data. We have subdivided the fault plane into five slip‐independent subfaults and computed the corresponding numerical Green's functions by integrating shallow‐water equations via a finite‐element technique. The slip distribution on the fault has been determined by applying a least‐squares procedure to the differences between the observed run‐up values and the computed maximum water levels along the coast. The main result is that the best fit is found for a nonuniform coseismic slip, with the highest displacement taking place in the southernmost part of the fault. The measure of the improvement is given by the global rms residual of the heterogeneous‐slip case being circa 25% smaller than that of the uniform case. An ancillary outcome of the analysis is the evaluation of the mean amplification factor of the tsunami wave investing the Nicaraguan coast, that, on assuming a rigidity of 1×1010N/m²,

ISSN:0094-8276    

DOI:10.1029/95GL03606

年代:1996

数据来源: WILEY