摘要:

Present‐day true polar wander and the secular non‐tidal acceleration of the earth are usually attributed to post‐glacial rebound. In the models which relate this rebound to changes in rotation, the mantle is assumed to relax passively to the melted ice‐loads. The lithosphere is usually modeled as a highly viscous upper layer in these models, having viscosities which exceed mantle viscosities by several orders of magnitude.We propose that lithospheric processes unrelated to post‐glacial rebound and taking place under non‐isostatic conditions are also able to induce non‐negligible influences on the earth's rotation. Examples of such processes are mountain building and erosion, foundering flexure of oceanic basins and lithospheric snapbacking resulting from detachment of subducting slabs. Lithospheric and crustal rheologies and intraplate‐stresses are the dominant factors in these mechanisms, contrary to the mantle rheologies which are assumed to dominate the process of post

ISSN:0094-8276    

DOI:10.1029/92GL02957

年代:1993

数据来源: WILEY

摘要:

A variety of instruments (including borehole strainmeters, water wells, creepmeters, laser ranging and differential magnetometers) are used to monitor crustal deformation in areas that are prone to geologic hazards such as earthquakes and volcanic eruptions. In monitoring the deformation, one typically examines the data for either a change in rate, or a simple offset in the record. However, one needs to place a statistical confidence level that the detected signal differs from the background “noise”. Calculation of the statistical confidence level may be done using the formalism of thematched filter, whose output is the signal‐to‐noise ratio, ρ. Two ingredients are needed to form a matched filter: 1) The power density spectrum of the instrument and 2) the functional form of the signal that we desire to detect. Using the available crustal deformation data from the Parkfield, California network, the background noise for individual instruments as a function of frequency, f, is estimated using the traditional method of the power density spectra. Except for two‐color laser distance‐ranging data, the power spectra for most of the instruments have a frequency dependence of f−nwhere 2≤n≤3. The confidence level with which a hypothesized signal is present is determined directly from the signal‐to‐noise ratio, with the numerator being a function of the signal and the denominator being a function of the power spectrum. Using a creepmeter as an example, a 0.04‐mm change occurring over 1 hour, a 0.06‐mm occurring over 10 hours, or 0.20‐mm over 100 hours are all signals for which ρ=2 and therefore have only a 5% confidence that these sign

ISSN:0094-8276    

DOI:10.1029/92GL02718

年代:1993

数据来源: WILEY

摘要:

Over pressure of a volcanic blast has been measured for the first time by a lead‐plate blastmeter at Unzen Volcano, Kyushu, Japan. The over pressure of the volcanic blast on June 8 was 0.28 bar at 2700 m distance from the crater, and that was less than 0.06 bar at 4400 m distance. The estimated explosive yield of the explosion accompanied by the blast was 1.1 × 107kg TNT (5 × 1020erg) calculated based on the over pressure and distribution of the blast effect. The explosion energy is as much as 1/600 of that of the blast at Mount St. Helens on 18 May 1980. The lead‐plate of the blastmeter at 2700 m has also recorded the collision with some accretionary lapillis. The maximum particle velocity of the blast was estimated to be about 75 m/sec based on the trace of the collision. The significance of over pressure for the prediction of volcanic hazards is also discussed br

ISSN:0094-8276    

DOI:10.1029/92GL02720

年代:1993

数据来源: WILEY

摘要:

We propose a method for the non‐linear inversion of the velocity field from reflection profiles. The inverse problem is separated into a linear and a nonlinear domain. Linearized inversion is applied to the retrieval of the short wavelength features of the velocity or impedance field. This problem has a huge number of degrees of freedom but it can be solved by an efficient asymptotic migration‐inversion method. The low frequency part of the velocity field — the background — is inverted using a non‐linear genetic algorithm applied to an objective functional defined in migrated data space. Computer time is significantly reduced using this objective function instead of straightforward waveform fitting. We apply our method to the inversion of a 1‐D background velocity model from a reflexion profile of the North Sea. For this problem, we found that the genetic algorithm is more reliable and efficient than other velocity analy

ISSN:0094-8276    

DOI:10.1029/92GL02781

年代:1993

数据来源: WILEY

摘要:

Using empirical Green functions with regional and teleseismic surface waves, it is possible to resolve fault finiteness effects, in many cases uniquely defining the fault plane for relatively large earthquakes. The technique requires very little data processing and can be applied in near‐real time with the current distribution of seismic stations. The Landers strike‐slip earthquake was dominated by two sub‐events with predominantly north‐northwestward rupture. The second sub‐event was 1.5 times larger and rotated in strike by 12° counterclockwise relative to the first. The Cape Mendocino thrust event had a relatively smooth rupture that propagated to the southwest on a shallow dip

ISSN:0094-8276    

DOI:10.1029/92GL03032

年代:1993

数据来源: WILEY

摘要:

The effects of phase changes and strong internal heat generation may combine to bring about brief, but extremely intense episodes of rapid thermal convection in the Earth's mantle. Numerical calculations using realistic thermodynamic properties for the exothermic Olivine → Spinel and endothermic Spinel → Perovskite + Magnesiowustite phase transitions suggest the transition region of the Earth's mantle may act as a capacitor for subducting slabs. Slab material accumulates in the transition region until a threshold level of thermal buoyancy is reached, and is then rapidly discharged into the lower mantle. In my calculations, this occurs as a catastrophic burst of convection lasting ∼10 Myr, with elevated heat transfer rates lasting ∼100 Myr. Such episodes may be analogs to superplume activity which has been hypothesized to give rise to an intense episode of intra‐plate volcanism and stabilize the geodynamo against reversals. The topography on the two phase change boundaries is found to be negatively correlated, with the correlation becoming more negative during periods of rapid convection. Furthermore, the results of this study suggest the transition region of the Earth's mantle could be ∼250 K cooler on average, and consequently more viscous, than the surroun

ISSN:0094-8276    

DOI:10.1029/93GL00044

年代:1993

数据来源: WILEY

摘要:

Heat capacity of MgSiO3perovskite has been measured in the temperature range 140–295 K by differential scanning calorimetry. The Kieffer model approach has been used to calculate the heat capacity beyond the measured temperature range. The estimated entropy of MgSiO3perovskite is 57.2±1.0 J/mol.K at 298 K which is higher than that of ilmenite, in accord with the negative pressure‐temperature slope for the ilmenite‐perovskite transition. From the heat capacity thermodynamic Debye temperature of MgSiO3perovskite is evaluated to be 980±15 K at 298 K and 1030±20 K at temperatures above about 700 K. Thermodynamic Gruneisen parameter of MgSiO3perovskite at 30 GPa and above 1000 K is also estimated as 1.45±0.15. An adiabatic temperature profile in the pyrolitic lower mantle is calculated with the average gradient of 0.32±0.04 K/km. The temperature at the top of the D″ layer is estimated to b

ISSN:0094-8276    

DOI:10.1029/92GL02655

年代:1993

数据来源: WILEY

摘要:

We have successfully magnetised and demagnetised magnetic grains with microwaves. We show that the rapidity of the heating and cooling process significantly reduces the problem of mineral alteration that often occurs during conventional heating.

ISSN:0094-8276    

DOI:10.1029/92GL02782

年代:1993

数据来源: WILEY

摘要:

Interannual variations of precipitation in several regions around the globe are known to be influenced by sea surface temperatures. We describe a procedure that elicits the same frequencies of variations in the sea surface temperatures as in precipitation. The modes of variation in the two fields are phase locked, allowing us to identify regions in the oceans that vary in unison with changes in precipitation on different time scales. The method is illustrated by application to the northeast section of Brazil, the Nordeste, where flood and drought occur frequently.

ISSN:0094-8276    

DOI:10.1029/92GL02919

年代:1993

数据来源: WILEY

摘要:

Global temperature records over the past century are compared to estimated atmospheric loading of anthropogenic aerosol sulfate, in order to investigate the possible cooling effect of such pollutants. Between the 1940s and the 1980s south‐eastern Europe, China, and the northern Atlantic have experienced a summertime cooling. The first two of these regions agree well with those where anthropogenic sulfur emissions have increased during the same period. During winter, the two former regions experienced a substantial warming while the northern Atlantic and south‐eastern US exhibited a cooling. The results are qualitatively consistent with theoretical calculations which indicate that anthropogenic sulfate aerosols cause a substantial cooling. However, because of large natural variations in the temperature records, the statistical significance of this conclusion is

ISSN:0094-8276    

DOI:10.1029/93GL00111

年代:1993

数据来源: WILEY