A review of: “Collected papers of sir Harold Jeffreys on geophysics and other sciences: Volume 5, astronomy and geophysics.”

Abstract

Edited by Sir Harold Jeffreys and Bertha Swirles (Lady Jeffreys). Gordon and Breach Science Publishers, London, Paris, New York, xviii + 493 pages.     

A review of: “Collected papers of sir harold jeffreys on geophysics and other sciences: Volume 6, mathematics,probability and miscellaneous other sciences, 1977”

Abstract

Edited by Sir Harold Jeffreys and Bertha Swirles (Lady Jeffreys). Gordon and Breach Publishers, London, Paris, New York, xi + 622 pages, Price $70 (ISBN 0677 0322 OX).     

What Cassini-Huygens has revealed about Titan

In the Harold Jeffreys Lecture of the Royal Astronomical Society, given on 10 November 2006, Athena Coustenis examines the new world of Saturn's moon Titan, as revealed by the Cassini-Huygens mission.     

Monica Grady

The 2008 Harold Jeffreys Lecturer is Monica Grady, professor of planetary and space science at the Open University.     

Historical eclipses and Earth's rotation

F Richard Stephenson took a long view of Earth's rotation history in the 2002 Harold Jeffreys Lecture in October.     

Astronomy by microscope

Monica M Grady reviews the astronomy revealed in samples of solar system material that come to Earth as meteorites, in the Harold Jeffreys Lecture 2008.     

Mountain roots and the survival of cratons

What controls the deformation of the continents, the survival of ancient cratons and the roots of mountains? James Jackson explains in his Harold Jeffreys Lecture, 12 November 2004.     

A new bivariate Gamma distribution generated from functional scale parameter with application to drought data

Univariate and bivariate Gamma distributions are among the most widely used distributions in hydrological statistical modeling and applications. This article presents the construction of a new bivariate Gamma distribution which is generated from the functional scale parameter. The utilization of the proposed bivariate Gamma distribution for drought modeling is described by deriving the exact distribution of the inter-arrival time and the proportion of drought along with their moments, assuming that both the lengths of drought duration (X) and non-drought duration (Y) follow this bivariate Gamma distribution. The model parameters of this distribution are estimated by maximum likelihood method and an objective Bayesian analysis using Jeffreys prior and Markov Chain Monte Carlo method. These methods are applied to a real drought dataset from the State of Colorado, USA.     

Attenuation and Shock Waves in Linear Hereditary Viscoelastic Media; Strick–Mainardi,Jeffreys–Lomnitz–Strick and Andrade Creep Compliances

Dispersion, attenuation and wavefronts in a class of linear viscoelastic media proposed by Strick and Mainardi (Geophys J R Astr Soc 69:415–429, 1982) and a related class of models due to Lomnitz, Jeffreys and Strick are studied by a new method due to the author. Unlike the previously studied explicit models of relaxation modulus or creep compliance, these two classes support propagation of discontinuities. Due to an extension made by Strick, either of these two classes of models comprise both viscoelastic solids and fluids. We also discuss the Andrade viscoelastic media. The Andrade media do not support discontinuity waves and exhibit the pedestal effect.     

Earth tides

The main geometrical characteristics and mechanical properties of bodily tides are described, using the convenient elastic parameters of Love. The problem of the Earth's deformation is a problem of spherical elasticity of the sixth order. The importance of Earth tides in astronomy and geophysics is emphasized by their relation to the precession-nutation and tesseral tidal problems, the secular retardation of the Earth's speed of rotation due to the dissipation of energy in sectorial tides, the periodic variations of the speed of rotation due to zonal tides, the satellite orbit perturbations due to the Earth's potential variation, and the radial deformations in laser distance measurements. The possibility that dynamical effects would be produced in the Earth's liquid core was pointed out by Poincaré and developed by Jeffreys, Vicente, and Molodensky. An experimental confirmation is presented here. The role of the Earth tide phenomenon in gravimetry and oceanography is also described, as are the perturbing effects due to regional tectonic features. Instrumental developments are critical in the acquisition of precise data; the calibration problem is fundamental for a correct comparison with Earth models.     

Crustal phases in earthquake location

For crustal earthquakes of moderate or large size, the first reported phases at distant stations are usually the first predicted by the crustal model used. For smaller events, however, or for stations at larger distances, the first phase detected is often a later crustal phase of larger amplitude. This may be eitherPg which travels entirely in the upper crustal layer with a velocity of about 5.6 km/s, orP ^* with a path mainly in the lower crustal layer at a velocity of about 6.7 km/s. Many earthquake location programs do not take account of these phases, and treat their arrivals as if they were the earlier phasePn. At the International Seismological Centre we re-identify up to 200 crustal phases each month. This often results in significant improvement in position, the direct determination of depth or even the obtaining of a formal solution where none was possible before. We find that these re-identifications are needed in most continental areas of the world, and that the original crustal model of Jeffreys and Bullen still remains a useful standard.On leave from the Seismological Laboratory, Institute for Earth Physics, P. O. Box MG-2, 76900 Bucharest-Magurele, Romania     

Using beamforming for the global network location

The aim of this study is the design and trial of a novel sparse network beamforming (NB) technique to improve earthquake location. For this purpose bulletin phase arrival time data were processed via the use of complex exponents form and then used in an optimization via grid-search to search for the maximum semblance in hypocenter space. The use of the robust semblance statistic, provides reliable location results. The NB location results for a set of test events were compared to standard iterative ISC location procedure “iscloc” and its prototype the Jeffreys maximum likelihood estimator. For this purpose a data-base of 139 reference ground-truth events was extracted from the catalog of the International Seismological Centre (ISC) (97 GT5 events, assumed earthquakes, and 42 GT0 nuclear tests). The tuned NB procedure has shown excellent location results for events with a “good” ISC location and demonstrated the large epicenter deviations have been decreased in “bad” cases. Further developments of the algorithm would include allowances for 3D earth structure and a priori site-specific information.     

PKPdif波-内核低速层存在的地震学证据

通过对中国数字地震仪台网(CDSN)11个台1993—2012年间30个极远震记录的分析,识别出了PKP波入射到内外核界面(ICB)上的衍射波PKPdif波。PKPdif波在震中距120°左右时出现在PKIKP波之前,在震中距150°以后出现在PKIKP波之后,是一个长周期波。为了解释PKPdif波超前于PKIKP波的原因,设想在距地面约5 156.1～5 372.2 km的深度,即内外核界面下有可能存在一个PKIKP波的低速层(厚度约216.1 km)。该设想符合Jeffreys速度模型,给出的PKPdif波在震中距119.4°～176.1°间的走时表填补了目前使用的《IASPEI1991地震波走时表》的空白。讨论PKPdif波的运动学特征有助于对内外核界面物理性质的认识,有助于提高分析震相的水平和积累震相分析经验。     

Station corrections for the Kaapvaal seismic network: Statistical properties and relation to lithospheric structure

Station corrections for body wave travel times are required to compensate for lateral variations in the crust and uppermost mantle in the analysis of seismic travel times that are used to determine deep Earth structure by various methods, including tomography. Station corrections to be applied to P wave arrival times from teleseismic earthquakes recorded by the Kaapvaal seismic network were estimated by five different methods: (1) averaging, (2) computing the median, and (3) weighted averaging of residuals; (4) least-squares regression, and (5) weighted least-squares regression. The corrections display variations that are related to the tectonic features of southern Africa inferred from surface geology, clearly delineating the southern and central areas of both the Kaapvaal and Zimbabwe cratons as regions of early arrivals, and the area around the Bushveld complex by later arrivals. Use of a simple ray method for generating synthetic station corrections suggests that lateral variations in the top 230 km of the Earth can explain the observed pattern of variations in station corrections. A satisfactory way of compensating for the biasing effects of outliers in the individual estimates of station corrections is through adaptation of a method originally developed by Jeffreys, which involves ascribing weights to the observations that reduce the standard deviation on a single estimate of a station correction from 0.123 to 0.096 s. Methods (2), (3) and (5) avoid serious bias by outliers, although methods (3) and (5) are preferred, because they also provide information on the causes of outliers. The presence of some outliers cannot be explained by errors in the measurement process, but must be caused by timing errors at the stations during recording, and/or errors introduced during the process of constructing the archived data files from the field data.     

Paleomagnetism of certain constituents of the Bay St. George sub-basin,western Newfoundland

Paleomagnetic studies have been made of certain constituents of the Bay St. George sub-basin. Specifically, results are reported from the Spout Falls Formation (Tournaisian), the Jeffreys Village Member of the Robinsons River Formation (Visean), and the Searston Formation (Namurian-Westphalian). The following magnetizations have been isolated: Spout Falls A (Tournaisian) with D = 343.5°, I = ?22.7°, k = 61.2, α₉₅ = 7.1° and the corresponding pole at 28.6°N, 139.5°E (4.5°, 8.5°); Spout Falls B (Kiaman) with D = 166.7°, I = 12.2°, k = 51.7, α₉₅ = 10.7° and the corresponding pole at 34.5°S, 42.7°W (5.5°, 10.9°); Jeffreys Village A (Visean) with D = 351.2°, I = ?27.3°, k = 54.0, α₉₅ = 7.6° and the corresponding pole at 26.5°N, 130.7°E (4.5°, 8.3°); Searston A (Namurian) with D = 161.7°, I = 11.7°, k = 107, α₉₅ = 7.4° and the corresponding pole at 33.9°S, 37.2°W (3.8°, 7.5°); and Searston C with D = 111.6°, I = ?13.8°, k = 28.8, α₉₅ = 14.5° and the corresponding pole at 19.6°S, 19.0°E (7.6°, 14.8°). After comparison with paleopoles of similar ages derived from eastern and western Newfoundland rocks, from constituents of the east coast basin and for interior North America, it is concluded that: (1) it is unlikely that any large scale relative motion took place since the Early Carboniferous between eastern and western Newfoundland; (2) it is unlikely that any north-south relative motion took place between the east coast basin and the Bay St. George sub-basin; and (3) the Bay St. George sub-basin results do not support the earlier proposed displaced terrane hypothesis of the northern Appalachians in as much as the motions during the Carboniferous are not supported. There is evidence of the northward motion of the Appalachians and North America as a whole during the Carboniferous. The magnetostratigraphic horizon marker in the Carboniferous separating a dominant normal and reversed magnetization on the older side and an entirely reversed (Kiaman) magnetization on the younger side may be placed in the Bay St. George sub-basin at the base of the Searston Formation.     

The D″ region

Two very different types of models are currently being proposed for D″, the lowest region of the earth's mantle: (a) those in which the P and S velocities vary smoothly down to the core-mantle boundary, without any extreme change in gradient; (b) those in which the velocity gradients decrease fairly abruptly at a height of 100 km or so above the core-mantle boundary, and maintain a value close to the critical gradient down to the boundary.Type (a) is represented by model UTD124A′ of Dziewonski and Gilbert (1972) and model B1 of Jordan and Anderson (1974). Both models are in good agreement with most travel time and free oscillation data. Their validity rests on the supposition, supported in part by theoretical studies, that data which suggest the presence of a low velocity zone in D″ result from distortion of seismic waves by the core-mantle boundary.On the other hand, slowness and amplitude data from short period P waves indicate a fairly rapid decrease in velocity gradient at a depth corresponding to an epicentral distance of about 92°, and it is very unlikely that these data can be interpreted as interface phenomena. The measured P and S times at distances beyond about 96° also indicate reduced velocities in D″. The suggestion that the measured velocities are in error as a result of interface effects is weakened by the fact that the results are apparently not wavelength-dependent.Type (b) is represented by model B2 of Jordan (1972), Bolt's (1972) model, and a new model designated as ANU2. All models have high density gradients indicative of inhomogeneity in the region. Model B2 fits the oscillation data reasonably well, but has an unjustifiably low S velocity at the core-mantle boundary. In Bolt's model the P and S velocities at the top of D″ are based on the models of Herrin et al. (1968) and Jeffreys (1939), whereas in ANU2 the values are taken from Hales and Herrin (1972) and Hales and Roberts (1970b). The velocities at the core-mantle boundary in Bolt's model and ANU2 are based on observations of “diffracted” P and S. Both of these models were designed to produce flattening of the P curve at about 92°. Both may require some modification in order to be compatible with free oscillation data.     

How can the ISC location procedures be improved?

For many decades the International Seismological Centre (ISC) has used a well defined procedure to locate seismic events using first P-onsets and the Jeffreys-Bullen tables ([Jeffreys, H., Bullen, K.E., 1940. Seismological Tables. British Association for the Advancement of Science, Gray Milne Trust, London, 50 pp]) as the travel-time reference. However, during the last two decades, more accurate spherical Earth models have been published and enhanced computer capabilities make it easier to implement more sophisticated data inversion schemes. Several features that may improve the location procedure at the ISC were systematically tested using the location program HYPOSAT. The investigated features were the influence of

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the usage of the spherical Earth models JB, PREM, IASP91, SP6, and AK135;

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the usage of later onsets;

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travel-time corrections for local crustal structure based on CRUST 5.1;

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different weighting of the residuals;

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reducing the amount of defining data at a late stage of the inversion process.

Application of different combinations of these factors led to a reduction of the location errors for the 156 test events, of which the epicenter is known with an accuracy of less than 5 km. However, no clear rule of common factors to achieve this result could be defined. Most promising is the application of AK135 as model for travel-time calculations, applying crust specific station corrections and corrections for the reflection points of surface reflections, a combined usage of surface and core reflections, and removing data which have large residuals or do not much contribute to the solution for the last iterations.     

Influence of natural radioactivity on creep rate

Steady-state creep in the earth's mantle is controlled by the diffusion coefficient and by the distances traveled by defects between sources and sinks. In this paper, we investigate the role played by additional vacancies created by natural radioactivity. According to recent studies, bombardment of solids by energetic particles can act as a perturbation. The resulting strain rate is proportional to the rate of vacancy creation and, contrary to ordinary creep mechanisms, is only weakly dependent on the temperature and the energy necessary to create those vacancies; it is independent of the grain size. The strain rate is of the same order of magnitude as the diffusion mechanism but smaller than the dislocation-climb rate by two to three orders of magnitude. Consequently, the strain rate produced by natural radioactivity does not play a major role in the observed rate of deformation, although it could constitute a lower limit to the creep rate and act as a regulatory mechanism. Furthermore, being less strongly affected by temperature and the vacancy-formation energy than are the other mechanisms, it is also less strongly affected by depth.     

混合被动控制优化设计及性能研究

本文将调谐液体阻尼器(TLD)和黏弹性阻尼器(VED)同时作用于结构,构成混合被动控制系统。通过对两类阻尼器分别进行优化设计并考虑两者间的相互影响,在充分发挥两者各自优良控制性能的同时,克服了VED大量使用导致控制系统整体造价过高的问题。算例分析表明,混合控制可以得到令人满意的整体减震效果,同时大大节约了VED用量,提高了控制系统的综合经济性能。     

上海地震台阵与台网的定位及视慢度研究

阐述了台阵的地震定位原理和视慢度在台阵与台网定位中的一些重要特性，并比较了相同位置上的上海地震台阵与台网对同一地震的定位结果。最后，对上海台队与台网导致定位误差的横向速度变化的位置和范围给出了矢量校正的结果并进行了初步讨论。