摘要:

The currently referenced age of 730 ka for the Brunhes‐Matuyama (B‐M) geomagnetic polarity boundary [Mankinen and Dalrymple, 1979] is based largely on K‐Ar and paleomagnetic data from postcollapse rhyolites in the Valles Caldera, New Mexico [Doell and Dalrymple, 1966; Doell et al., 1968]. Dating of sanidine from Valles rhyolites by the40A/39Ar laser fusion technique indicates that samples critical to defining the B‐M boundary are up to 90 ka older than previously thought. The new data combined with earlier results yield an age for the B‐M boundary of 780 ± 10 ka.40Ar/39Ar ages from other samples suggest the Jaramillo normal polarity subchron extended from 1010

ISSN:0094-8276    

DOI:10.1029/92GL01125

年代:1992

数据来源: WILEY

摘要:

A number of previous analyses have suggested that the relative sea level record in Hudson Bay constrains the viscosity of the lower mantle to be near 1021Pa s. These conclusions have been based on a limited number of forward calculations which have assumed an isoviscous lower mantle region. We compute Frechet kernels for the RSL data in Hudson Bay to determine the detailed depth‐dependent sensitivity of the data to variations in a viscosity profile which is consistent with the past inferences. We find that the RSL data provide a robust constraint on the average viscosity in the top half of the lower mantle only (the average must be near 1021Pa s). The data are less sensitive to variations in the viscosity below approximately 1800 km depth and in the upper mantle. Forward analyses included herein show that the data admit models whose average viscosity in the deep mantle (below 1800 km depth) and in the upper mantle can differ significantly from the value (near 1021Pa s) estimated for the top half of the lower mantle. The total variation in the viscosity from the surface to the CMB can exceed an order of magnitude or more and still satisfy the constraint provided by the Hudson Bay RSL data set. The necessity of invoking an isoviscous mantle model in previous studies is thus seen to be a consequence of the limited class of viscosity model solutions employed in those studie

ISSN:0094-8276    

DOI:10.1029/92GL01285

年代:1992

数据来源: WILEY

摘要:

We developed a model which can naturally explain the entire process of nucleation, dynamic propagation, and stop of earthquake rupture. In this model we represent frictional interaction between fault surfaces by a slip‐dependent constitutive relation, and use it as the fundamental law governing the entire process of earthquake rupture. We considered a broad weak zone with a locally strong part (asperity) on a fault plane and examined the entire rupture process proceeding with the increase of external shear stress through numerical simulations. The nucleation process first proceeds quasi‐statically at the weakest portion with the increase of external stress and brings about high stress concentration at the asperity. Then dynamic rupture starts at the asperity, but this dynamic rupture is arrested soon. After the arrest of the local dynamic rupture, the quasi‐static nucleation process proceeds again and brings about high stress concentration at both ends of the weak zone. When the stress concentration reaches a critical level, dynamic rupture starts at the endzones and propagates outward. The rupture propagation is gradually accelerated toS‐wave velocity until it reaches much stronger parts (barriers). The dynamic rupture is decelerated and finally arrested, with it propagating into the barrier. Our model successfully describes the transition process from nucleation to dynamic rupture propagation observed in laboratory experiments of sti

ISSN:0094-8276    

DOI:10.1029/92GL01072

年代:1992

数据来源: WILEY

摘要:

A multiple regression analysis method is applied to detect anomalous changes of ground water level due to earthquakes. Corrected water level is calculated by eliminating responses due to barometric pressure, earth tide and precipitation from the observed water level based on a model obtained by the minimum AIC (Akaike Information Criterion) procedure. The data observed at Haibara well, Shizuoka, central Japan during the period from Feb. 1981 to Dec. 1990 is analyzed using this method. It is shown that the corrected water level decreases more than 1 cm after each of fifteen earthquakes.

ISSN:0094-8276    

DOI:10.1029/92GL01042

年代:1992

数据来源: WILEY

摘要:

Rare earth element (REE) concentrations have been determined in terrigenous, siliceous (nodule barren and nodule bearing), calcareous, and red clay from the Central Indian Basin. The bulk distribution of REE, and in particular the relative cerium fractionation, in these surface sediments has been studied in relation to bottom water conditions. REE fractionations within a single depositional environment are found to be characteristic for each sediment type, with flat shale‐normalized patterns associated with terrigenous sediments, positive Ce anomalies with siliceous sediments, negative Ce and positive Eu anomalies with calcareous sediments, and LREE‐depleted patterns with pelagic red clays. There is no relation of any REE fractionation to bottom water redox conditions. Our results indicate that the REE signatures in marine sediments are not only related to depositional setting, but also to the lithological variations, and surficial diagenetic processes. Therefore, these further constraints on understanding REE behaviour indicate that REE fractionations for paleotectonic and paleoredox reconstructions should be used with caut

ISSN:0094-8276    

DOI:10.1029/92GL01243

年代:1992

数据来源: WILEY

摘要:

Satellite altimeter data collected by the U.S. Navy's Geosat Exact Repeat Mission were used to investigate turbulent stress resulting from the variability of surface geostrophic currents in the Antarctic Circumpolar Current. The altimeter measured sea level along the subsatellite track. The variability of the along‐track slope of sea level is directly proportional to the variability of surface geostrophic currents in the cross‐track direction. We used the two components of the slope at points where the satellite ground tracks cross to calculate orthogonal components of the current's variability. Because the grid of crossover points is dense at high latitudes, the satellite data could be used for mapping the temporal and spatial variability of the current. Two and a half years of data were used to compute the statistical structure of the variability, leading to a better understanding of the role of variability in the dynamics of the current. The statistics included the probability distribution functions for each component of the current, the time‐lagged auto correlation functions of the variability, and the Reynolds stress produced by the variability. The results demonstrate that stress is correlated with bathymetry. In some areas the distribution of negative stress indicate that eddies contribute to an acceleration of the mean flow, strengthening the hypothesis that baroclinic instability makes important contributions to strong oceanic cur

ISSN:0094-8276    

DOI:10.1029/92GL01287

年代:1992

数据来源: WILEY

摘要:

Simulations were carried out using the two‐dimensional version of a non‐hydrostatic numerical model in order to calculate the mesoscale response to flow across the Scandinavian mountain range. The initial conditions were deduced from an upstream sounding of 27 January 1992 during the EASOE campaign. Over and to the east of the mountain crest regions of mesoscale extent developed which experienced cooling up to 7 K. Air parcel trajectories passed these areas in approximately 10

ISSN:0094-8276    

DOI:10.1029/92GL01160

年代:1992

数据来源: WILEY

摘要:

Ground‐based spectroscopic observations from Kitt Peak in Arizona showed no enhancement of stratospheric HCl or HF when the plume of the Pinatubo volcano was overhea

ISSN:0094-8276    

DOI:10.1029/92GL01112

年代:1992

数据来源: WILEY

摘要:

On August 20 1991, unusual, hazy clouds were encountered by commercial jet aircraft flying over continental SE Australia. Image data from the AVHRR on board the NOAA satellites have been used to discriminate the clouds from water/ice clouds. The clouds were again observed over Australia by lidar eight days later, presumably after a complete circumnavigation in the zonal winds of the southern mid‐latitudes. The lidar measurements revealed a strong scattered signal from a layer situated near 12 km above the surface. The satellite and lidar data indicate that the clouds were of volcanic origin and we suggest that they originated from the Mt Hudson eruptions in Chile. We have used a numerical model to simulate the transport of the volcanic clouds and find good agreement between the projected position of the clouds and observation

ISSN:0094-8276    

DOI:10.1029/92GL01122

年代:1992

数据来源: WILEY

摘要:

The 1991 Antarctic springtime ozone decline, as measured by the Total Ozone Mapping Spectrometer (TOMS), was similar to those of earlier deep ozone hole years, 1987, 1989, and 1990. The minimum total ozone value was recorded on October 5, 1991 at 108 Dobson units near the South Pole. This was 8 DU lower than in any of the earlier years. Four of the last five years have exhibited an extensive, deep ozone hole. The area of the hole was about the same as in 1987, 1989, and 1990. The recovery of the low total ozone values occurred in mid‐November as the polar vortex broke u

ISSN:0094-8276    

DOI:10.1029/92GL01113

年代:1992

数据来源: WILEY