摘要:

Using the strain gauge technique, the magnetostriction constants λShave been measured on sintered polycrystalline specimens of titanomagnetites with Fe2.4‐δTi0.6A��δO4compositions (0.05 ≤ δ ≤ 0.20) in the temperature range between 25°C and Tc. The cell‐edge, saturation magnetization, and Curie temperature decrease with increasing aluminium concentration. The room‐temperature λSvalues are 95.7, 70.6, 52.9 and 25.2×10−6for δ=0.05, 0.10, 0.15 and 0.20 respectively. A linear relationship is found between the magnetostriction constant and the aluminium content at room temperature. The constant λS(T) is proportional to approximately the third power of the saturation magnetization MS(T). The values of the index n in λS(T) ∼ MSn(T) are 2.72, 3.16, 3.35 and 3.60 for δ=0.05, 0.10, 0.15 and 0.2 respectively. This if the first reported measurement of λS(T) at elevated temperatures for A��‐substituted titanomagnetites. Such data are urgently needed in theoretical and experimental magnetic domain structur

ISSN:0094-8276    

DOI:10.1029/92GL02716

年代:1992

数据来源: WILEY

摘要:

Magnetic anomalies of 52–80 Ma age have been mapped in the equatorial Pacific in a region where they were previously unrecognized. These anomalies, now at 7° N are best modeled with crustal rocks of negative inclination, apparently because the crust was formed at a spreading ridge that was south of the magnetic equator in Late Cretaceous‐Early Tertiary time. Magnetic anomalies recorded along a 5900‐km trackline that follows a flowline of crustal generation show one long period, from 12 to 31 Ma, of apparent constant half‐spreading rate over the 0 to 80 Ma rep

ISSN:0094-8276    

DOI:10.1029/92GL02928

年代:1992

数据来源: WILEY

摘要:

The magnetization intensity of oceanic crust in the Northeast Pacific, Atlantic, and Southeast Indian oceans was calculated from marine magnetic anomaly data to investigate long‐term variations in magnetization intensity with crustal age. The magnetization intensity was derived by two‐dimensional inversion of the upward‐continued marine magnetic anomalies. The principal magnetization intensity variations versus age are summarized as follows: (1) In all the oceans, a high magnetization intensity exists at the ridge crest, followed by a sharp decrease within 8 My, and a minimum is reached at 8–20 Ma; (2) In the Northeast Pacific and Atlantic oceans, the magnetization intensity gradually increases up to at least 70 Ma. This variation pattern is consistent with the previous results of NRM measurements of drilled rocks and inversion of marine magnetic anomalies in the South A

ISSN:0094-8276    

DOI:10.1029/92GL02143

年代:1992

数据来源: WILEY

摘要:

As part of Lithoprobe's Southern Cordilleran transect investigations, magnetotelluric (MT) soundings were made at 160 sites providing unprecedented coverage from the Rockies to the west coast. Striking lateral variation, which spatially correlates with the morphogeological belt boundaries, is apparent at periods sensing the lower crust (≈10 s). For the Rockies, MT phases are around 35°, indicative of a moderately resistive (100's – 1000's Ω·m) North American Basement. Foreland belt phases are transitional and increase from 60° in the east to 70° in the west. Omineca and Coast belt phases are high (75°), implying a conductive (10–30 Ω·m) lower crust, whereas Intermontane belt phases are more than 10° lower (equivalent to ≈150 Ω·m). The regional variation in conductivity correlates to first order with surface heat flow changes along the profile and is also correlative with coincident seismic reflection sectio

ISSN:0094-8276    

DOI:10.1029/92GL01457

年代:1992

数据来源: WILEY

摘要:

We consider a Laplacian field (▽2V=0) in a semi‐infinite medium bounded by a frontier ∂B, with imposed boundary values or sources at ∂B. When the boundary conditions are periodically distributed, it is well‐known that the Laplacian field V decays exponentially due to screening of all multipoles. It is demonstrated in this paper that disorder is a singular perturbation in the sense that small random fluctuations around a periodic modulation, which are always present in nature, lead to long‐range powerlaw decreasing fluctuations of the Laplacian field V. Powerlaw decay is due to the appearance of Fourier components of arbitrary large wavelengths in the boundary conditions. This result calls for a reexamination of various geophysical inverse problems (such as temperature, electrical, gravity and magnetic anomalies), in which any possible small disorder on the sources is traditionally neglected, thereby filtering out the very low wavenumbers. This singular effect of disorder must apply to more general partial differential operators as long as the Green function is

ISSN:0094-8276    

DOI:10.1029/92GL01699

年代:1992

数据来源: WILEY

摘要:

We examine seismograms from microearthquakes recorded at Krafla central volcano, in the Northern Volcanic Zone of Iceland, during a short inflation period in 1988. We produce record sections for propagation both to the north and to the south of the center of the caldera by combining seismograms from many earthquakes. The northern record section contains clear evidence of a low velocity zone (LVZ), including a low amplitude diffraction of the P wave as it impinges on top of the LVZ and high amplitude reflections from beneath the LVZ. The absence of clear shear waves indicates that the LVZ is at least partially molten and should be interpretated as a magma chamber. The magma chamber is less than 1 km thick, with a top approximately 3 km beneath the surface. We find no evidence for a LVZ in the southern part of the caldera.

ISSN:0094-8276    

DOI:10.1029/92GL02541

年代:1992

数据来源: WILEY

摘要:

A later phase of large‐amplitude is observed at a station in Hokkaido from several events of the December 1991 off‐Urup earthquake swarm in the Kurile Islands region. The phase is characterized by a narrow frequency‐band with a dominant period of about 15 s and retrograde particle motion in a vertical plane with a strike very different from the great‐circle direction between the station and the epicenter. The phase is observed from only the earthquakes that are located at shallow depths beneath the inner wall of the Kurile trench where sea depth is greater than 4 km. These characteristics indicate that the phase is a packet of Rayleigh waves that arrive at the station from an azimuth considerably different from the great‐circle direction. Rayleigh waves with periods around 15–20 s are well excited by the shallow earthquakes beneath the inner wall of the trench. This is supported by the dominance of a spectral peak at 15–20 s in the spectra of the direct Rayleigh‐waves and the later phase. Rayleigh waves that propagate in an oceanic region are strongly influenced by the thickness of the water layer at periods shorter than about 25 s. The group velocity decreases by 80% at 15 s with the increase of the sea depth from 4 km to 6 km. The sea‐trench topography therefore forms a strong low‐velocity zone. The observed later phase can be interpreted as a Rayleigh‐wave packet guided by thi

ISSN:0094-8276    

DOI:10.1029/92GL02438

年代:1992

数据来源: WILEY

摘要:

Mediterranean salt lenses (meddies) are a dominant factor in the salt budget of the Atlantic at middepth. In spite of their important role, their juvenile migration has not yet been directly observed. For the first time, two RAFOS float trajectories show strong evidence of a meddy along the Iberian continental slope off Lisbon. Over six weeks we obtained drift observations from two levels (629, 847 dbar). Both instruments recorded a series of loops with an azimuthal speed O (30 cm s−1) at a radius of about 25 km. Relatively high propagation speeds of several centimeters per second indicate the meddy was probably carried along with the undercurrent of Mediterranean Water. The Tejo Plateau, a prominent feature of the continental slope and a natural obstacle for the spreading Mediterranean Water tongue, appears to act as a deflector for advected meddies possibly formed by interaction of the undercurrent with the canyon‐rich topography farther so

ISSN:0094-8276    

DOI:10.1029/92GL02139

年代:1992

数据来源: WILEY

摘要:

We have examined the effects of climate changes, induced by orbital oscillations, on Martian surface heat flow. It was found that the climatological component of the surface heat flow can be larger than the expected internal geothermal heat flow. We suggest that measurements of surface heat flow be targeted for equatorial and south polar regions to avoid climatic effects and that care be taken in interpreting measurements in mid‐latitude and north polar region

ISSN:0094-8276    

DOI:10.1029/92GL02779

年代:1992

数据来源: WILEY

摘要:

The evolution of a meander into an anticyclonic eddy which separates from the Alaskan Stream in the vicinity of the central Aleutian Islands is observed in two years of sea surface height anomalies computed from GEOSAT Exact Repeat Mission altimeter data. The trajectory of the center of mass is estimated using a modified perpendicular bisector method. Eddy separation is inferred to occur where the trajectory makes a distinct turn to the southwest. The observed region of meander growth, and eddy separation agrees with that predicted by theory. The evolution of meander/eddy height and radius is estimated from least squares fits of the data to an axially symmetric Gaussian profile. The heights and radii are used to calculate the strength (vorticity) and energetics of the meander/eddy. Ekman dynamics are assumed to govern the eddy decay. Spin down times based on vorticity are much longer than those based on energetics. This is consistent with an eddy's ability to maintain its integrity as it decays. Representative height, diameter, and velocity scales of the eddy are characteristic of rings spawned by other current systems. Assuming a ring structure for this eddy the estimated ring volume represents a 21% transport anomaly referenced to the mean annual transport into the Bering Sea.

ISSN:0094-8276    

DOI:10.1029/92GL01882

年代:1992

数据来源: WILEY