Microseismicity and faulting geometry in the Gulf of Corinth (Greece)

During the summer of 1993, a network of seismological stations was installed over a period of 7 weeks around the eastern Gulf of Corinth where a sequence of strong earthquakes occurred during 1981. Seismicity lies between the Alepohori fault dipping north and the Kaparelli fault dipping south and is related to both of these antithetic faults. Focal mechanisms show normal faulting with the active fault plane dipping at about 45° for both faults. The aftershocks of the 1981 earthquake sequence recorded by King et al . (1985 ) were processed again and show similar results. In contrast, the observations collected near the western end of the Gulf of Corinth during an experiment conducted in 1991 ( Rigo et al . 1996 ), and during the aftershock studies of the 1992 Galaxidi and the 1995 Aigion earthquakes ( Hatzfeld et al . 1996 ; Bernard et al . 1997 ) show seismicity dipping at a very low angle (about 15°) northwards and normal faulting mechanisms with the active fault plane dipping northwards at about 30°. We suggest that the 8–12 km deep seismicity in the west is probably related to the seismic–aseismic transition and not to a possible almost horizontal active fault dipping north as previously proposed. The difference in the seismicity and focal mechanisms between east and west of the Gulf could be related to the difference in the recent extension rate between the western Gulf of Corinth and the eastern Gulf of Corinth, which rotated the faults dipping originally at 45° (as in the east of the Gulf) to 30° (as in the west of the Gulf).     

A comparison of archaeointensity results from Chinese ceramics using microwave and conventional Thellier's and Shaw's methods

Palaeo- and archaeointensity determinations of the geomagnetic field are limited by mineralogical alteration that may occur when samples are heated in the laboratory to produce a TRM ( Thellier & Thellier 1959 ; Shaw 1974 ). By using microwave excitation of the magnetic grains we have been able to create a TRM without significantly heating the bulk samples, thereby avoiding thermal alteration ( Walton et al . 1993 ; Shaw et al . 1996 ). When applied to Peruvian ceramics in a manner analogous to the standard Thellier method, the microwave technique was found to reduce the scatter dramatically in the archaeointensity results ( Shaw et al . 1996 ). In the present study we apply this microwave technique to a collection of Chinese ceramics covering the time interval 2700–7500  yr BP. These ceramics have already been investigated using adaptations of both Thellier's ( Coe 1967 ) and Shaw's ( Rolph & Shaw 1985 ) palaeointensity methods ( Yang et al . 1993a ). Although an acceptable agreement was found between those two methods, the equivalent virtual axial dipole moments (VADMs) were significantly lower than for the global model of McElhinny & Senanayake (1982 ). The present study, using the microwave technique with cooling-rate correction, has provided more consistent VADMs, and is much closer to the global model.     

Could the energy near the FCN and the FICN be explained by luni-solar or atmospheric forcing?

The observed time-series of precession/nutation show residuals with respect to an empirical model based on the rigid Earth theoretical nutations and a frequency dependent transfer function with resonances to the Earth's normal modes. These residuals display energy mainly in the frequency domain around 430 and 500 days in the inertial frame. In this frequency band, the energy is possibly related to two normalmode frequencies: the free core nutation (FCN) and the free inner core nutation (FICN). In this paper, we examine the possibility of obtaining this energy from the resonance effect induced by a luni-solar (or planetary) forcing, or by an atmospheric forcing at a frequency very close to these Earth free nutations. The amplification factor due to the resonance is computed from an analytical formula expressed in the case of a simplified three-layer ellipsoidal rotating earth (with an elastic inner core, a liquid outer core and an elastic mantle), as well as the empirical formula based on the analysis of VLBI observations. For the tidal forcing, the theoretical results do not show any resonance at the level of precision we have examined but it is still possible to find one frequency near the FCN or FICN frequencies which could be excited. In contrast, for the atmospheric pressure the level of energy needed could be obtained from the diurnal pressure, depending on the noise level of the Earth's global pressure. We also show that the combination of three waves can explain the observed decrease of energy with time. While the tidal potential amplitudes are too small, a pressure noise level of 0.5 Pa would be sufficient to excite these waves.     

Rotational remanent magnetization and the torque exerted on a rotating rock in an alternating magnetic field

Summary. Using an air turbine at rotation frequencies of between 1.5 and 275 revolutions per second (rps), the dependence of rotational remanent magnetization (RRM) on rotation frequency has been investigated for two igneous samples in and alternating field of 51 mT peak at 50 Hz. The same experimental arrangement has also been used to measure the dependence on rotation frequency of the torque exerted by the alternating field on the rock samples. The dependence of torque and RRM on peak field has also been measured at a rotation frequency of 112 rps and a linear relationship between RRM and torque has been demonstrated.
In an attempt to elucidate the way in which RRM arises, analytical and numerical models of the rock have been developed in order to calculate the torque curves and these agree quite closely with those observed experimentally. While the precise factor responsible for RRM has not yet been identified from the numerical model it is suggested that RRM may arise as a result of particle moments suddenly flipping into the field direction, and thus by virtue of their intrinsic angular momentum acquiring a transient component of magnetic moment antiparallel to the rotation vector describing the flip. This component, due to the hysteresis of the assembly of particles, will not then entirely disappear when the alternating field is removed. An estimate of the transient axial field which can be considered to deflect each moment towards the rotation axis during the flip yields a value of the order of 1 mT.     

利用封闭湖泊流域进行古降水量重建的历史、现状及未来

回顾了利用封闭湖泊流域进行古降水量重建的简单历史,重点论述了其理论与方法的特点及发展源流;在此基础上,结合作者近几年在青藏高原开展的这方面的研究工作,对季风区进行此项研究的可能方法、理论依据及其初步应用进行了分析,重点阐述了其存在的缺点与不足及未来的改进方式。     

Electromagnetic core–mantle coupling—I. Explaining decadal changes in the length of day

Measured changes in the Earth's length of day on a decadal timescale are usually attributed to the exchange of angular momentum between the solid mantle and fluid core. One of several possible mechanisms for this exchange is electromagnetic coupling between the core and a weakly conducting mantle. This mechanism is included in recent numerical models of the geodynamo. The 'advective torque', associated with the mantle toroidal field produced by flux rearrangement at the core–mantle boundary (CMB), is likely to be an important part of the torque for matching variations in length of day. This can be calculated from a model of the fluid flow at the top of the outer core; however, results have generally shown little correspondence between the observed and calculated torques. There is a formal non-uniqueness in the determination of the flow from measurements of magnetic secular variation, and unfortunately the part of the flow contributing to the torque is precisely that which is not constrained by the data. Thus, the forward modelling approach is unlikely to be useful. Instead, we solve an inverse problem: assuming that mantle conductivity is concentrated in a thin layer at the CMB (perhaps D"), we seek flows that both explain the observed secular variation and generate the observed changes in length of day. We obtain flows that satisfy both constraints and are also almost steady and almost geostrophic, and therefore assert that electromagnetic coupling is capable of explaining the observed changes in length of day.     

Magnetostatic and electrostatic torques within a planet

Several dynamic agencies control differential rotation between various regions of a rotating gravitating body such as a planet, namely advection of angular momentum (within fluid regions) and torques due not only to (a) viscous forces, (b) dynamic pressure forces and (c) gravitational forces, but also to (d) Lorentz forces (involving the flow of electric currents in electrically conducting regions), (e) magnetostatic forces (when magnetized material is present) and (f) electrostatic forces (due to the presence of electric charges). Torques due to (a), (b), (c) and (d) have already been treated in the literature, some extensively. It is of general theoretical interest to derive from first principles mathematical expressions for torques due to (e) and (f), even though they turn out to be quantitatively insignificant in the case of the Earth.     

Gravitational torque on the inner core and decadal polar motion

A decadal polar motion with an amplitude of approximately 25 milliarcsecs (mas) is observed over the last century, a motion known as the Markowitz wobble. The origin of this motion remains unknown. In this paper, we investigate the possibility that a time-dependent axial misalignment between the density structures of the inner core and mantle can explain this signal. The longitudinal displacement of the inner core density structure leads to a change in the global moment of inertia of the Earth. In addition, as a result of the density misalignment, a gravitational equatorial torque leads to a tilt of the oblate geometric figure of the inner core, causing a further change in the global moment of inertia. To conserve angular momentum, an adjustment of the rotation vector must occur, leading to a polar motion. We develop theoretical expressions for the change in the moment of inertia and the gravitational torque in terms of the angle of longitudinal misalignment and the density structure of the mantle. A model to compute the polar motion in response to time-dependent axial inner core rotations is also presented. We show that the polar motion produced by this mechanism can be polarized about a longitudinal axis and is expected to have decadal periodicities, two general characteristics of the Markowitz wobble. The amplitude of the polar motion depends primarily on the Y ¹₂ spherical harmonic component of mantle density, on the longitudinal misalignment between the inner core and mantle, and on the bulk viscosity of the inner core. We establish constraints on the first two of these quantities from considerations of the axial component of this gravitational torque and from observed changes in length of day. These constraints suggest that the maximum polar motion from this mechanism is smaller than 1 mas, and too small to explain the Markowitz wobble.     

Iceocean mass balance during the Late Pleistocene glacial cycles in view of CHAMP and GRACE satellite missions

During the last glacial cycles, global sea level dropped several times by about 120 m and large ice sheets covered North America, northern Europe and Antarctica during the glacial stages. The changes in the iceocean mass balance have displaced mantle material mainly via viscous flow, and the perturbation of the equilibrium figure of the Earth by glacial isostatic adjustment is still observable today in timedependent changes of gravitational and rotational observations. Contemporary iceocean mass balance from volume changes of polar ice caps also contributes to secular variations of the Earth's gravitational field.
In the near future, several satellite gravity missions will significantly improve the accuracy of the observed timedependent gravitational field. In view of the expected improvements in the observations, we predict glacially induced perturbations of the gravitational field, induced by Late Pleistocene and contemporary ice volume changes, for a variety of radial mantle viscosity profiles. We assess the degree of uncertainty for the glacially induced contributions to gravitational and rotational parameters, both in the spectral and the spatial domain.
Predictions of power spectra for the glacially induced freeair gravity and geoid anomalies are about one order of magnitude lower than the observed values, and uncertainties arising from different plausible viscosity profiles are around 0.150.4 mGal and 0.21.5 m, respectively. Uncertainties from different ice models are of secondary importance for the predicted power spectra. Predicted secular changes in geoid anomalies in formerly glaciated areas are mainly controlled by the viscosity profile and contemporary ice volume changes. We also show that the simple threelayer viscosity profiles currently employed for the majority of postglacial rebound studies represent a limited subset for model predictions of the timedependent gravitational field.     

The Passive Influence of the Oceans upon the Rotation of the Earth

A general theory is developed which allows the exact numerical computation of the static equilibrium response of a non-rotating spherically symmetric Earth model covered by thin oceans with geometrically irregular coastlines to the action of an imposed static tidal or centrifugal potential. The theory is self-consistent, and takes into account the gravitational self-attraction of the oceans and the elastic-gravitational response of the Earth model to both the applied potential and the equilibrium oceanic tidal load on the surface. The results are used to determine the influence of an equilibrium pole tide on the free period and the associated rotational eigenfunction of the Chandler wobble. If the pole is globally well represented by this equilibrium approximation, its effect is to increase the Chandler wobble period by 27·6 days. It is shown that a fully self-consistent theory of the rotation of an Earth model with oceans predicts that changes in spin and wobble will be coupled, and that the Chandler wobble should, as a result, be accompanied by an associated periodic change in the length of day. The consequences of spin-wobble coupling are explored quantitatively, and found to be slight.     

中国县域粮食产量与农业劳动力变化的格局及其耦合关系

本文构建了中国粮食产量与农业劳动力数量变化的耦合关系模式,初步探讨了县域粮食产量与农业劳动力比重变化的耦合过程。利用全国1991年、2000年和2010年3期分县截面数据,分别建立了粮劳弹性系数和农业劳动力转移效应系数用以分析中国县域粮食产量与农业劳动力变化的耦合关系。研究表明：① 中国农业转型发展过程中,县域粮食产量与农业劳动力数量变化的耦合关系呈现多样化并存的格局,1991-2010年中国县域农业劳动力数量减少而粮食产量增加、农业劳动力数量增加而粮食产量也增加和农业劳动力数量减少的同时粮食产量也减少的县域数量占县域总数的比重分别为48.85%、29.11%和19.74%;3种耦合类型分别集中于传统农区、西部欠发达牧区和农牧交错区、东南沿海农业快速转型区。② 中国县域粮食产量与农业劳动力比重的耦合曲线呈现阶梯式波动变化,且二者耦合关系的离散趋势不断加强。③ 粮劳弹性系数和农业劳动力转移效应系数的变化表明农业劳动力变化在促进粮食增产中的作用在不断下降。④ “胡焕庸线”是中国粮食产量与农业劳动力变化耦合特征的重要分界线,塑造了二者耦合格局的主体结构。     

Asymptotic behaviour of decay rates of internal waves in a rotating stratified spherical shell

Summary. Boundary layer techniques are used to examine the dissipative decay of an internal oscillation that is a member of the inviscid spectrum of normal modes for a rotating fluid shell stratified under a radially directed gravitational field. A formula is derived for the decay factor on the so-called homogeneous spin-down time-scale. Estimates are obtained for the size of the decay factor as a function of wavelength, a function of the frequency and a function of a parameter A which measures the ratio of the stratification strength to the rotation strength. It is shown that all modes decay on the spin-down time-scale. The results are interpreted in the context of a model for the Earth's fluid core. It is observed that the presence of regions of unstable stratification may increase the decay rate for oscillations at frequencies less than twice the rotation frequency.     

Tracking a non-dipole geomagnetic anomaly using new archaeointensity results from north-east China

A collection of ceramics and samples, collected from north-east China with ages ranging from 1000 to 7000 years, have been investigated using a modified version of the Shaw palaeointensity techniques (Shaw 1974; Rolph & Shaw 1985) in which only partial NRMs and TRMs (PNRMs and PTRMs) with blocking temperatures (T_b) above 300 C are used after pre-selection of samples by mineral magnetic analysis. A secular variation curve obtained from this study is quite consistent with previous results from other areas of China (Wei et al. 1987; Tang et al. 1991), as well as with the global model of McElhinny & Senanayake (1982). Comparison of the Chinese results with contemporaneous results from Greece (Aitken et al. 1989) has allowed us to track the movement of a large non-dipole anomaly as it drifts westwards.     

A pseudo-Thellier relative palaeointensity record, and rock magnetic and geochemical parameters in relation to climate during the last 276 kyr in the Azores region

In the pseudo-Thellier method for relative palaeointensity determinations (Tauxe et al. 1995) the slope of the NRM intensity left after AF demagnetization versus ARM intensity gained at the same peak field is used as a palaeointensity measure. We tested this method on a marine core from the Azores, spanning the last 276  kyr. We compared the pseudo-Thellier palaeointensity record with the conventional record obtained earlier by Lehman et al . (1996 ), who normalized NRM by SIRM. The two records show similar features: intensity lows with deviating palaeomagnetic directions at 40–45  ka and at 180–190  ka. The first interval is associated with the Laschamps excursion, while the 180–190  ka low represents the Iceland Basin excursion (Channell et al. 1997). The pseudo-Thellier method, in combination with a jackknife resampling scheme, provides error estimates on the palaeointensity.
  Spectral analysis of the rock magnetic parameters and the palaeointensity estimates shows orbitally forced periods, particularly 23  kyr for climatic precession. This suggests that palaeointensity is still slightly contaminated by climate. Fuzzy c -means cluster analysis of rock magnetic and geochemical parameters yields a seven-cluster model of predominantly calcareous clusters and detrital clusters. The clusters show a strong correlation with climate, for example samples from detrital clusters predominantly appear during rapid warming. Although both the pseudo-Thellier palaeointensity m _a and fuzzy clusters show climatic influences, we have not been able to find an unambiguous connection between the clusters and m _a .     

Observation of Coriolis coupled modes below 1 mHz

We present observations of spectral energy at toroidal mode frequencies in vertical seismic recordings of the 1998 Balleny Islands earthquake. Since toroidal modes on a spherically symmetric, nonrotating Earth have horizontally polarized particle motion these observations call for an explanation. We first rule out local and instrumental effects as being responsible for the verticalcomponent signal of the toroidal modes ₀ T ₃ (0.59 mHz) and ₀ T ₄ (0.77 mHz). The global effects that we consider are general heterogeneous mantle structure, ellipticity of figure and rotation. We find that rotation through Coriolis coupling of loworder spheroidal and toroidal oscillations is the dominant mechanism.     

极地冰雪化学组成分析:毛细管离子分析与离子色谱的比较

与传统的离子色谱相比较，讨论了目前分析化学的前沿技术毛细管离子分析技术，它具有分析快速、高效、分离度好、样品预处理简单、用样量少、维护方便、自动化程度高等特点，完全可替代目前在分析极地冰雪中所通用的离子色谱仪，成为极地冰雪化学分析强有力的新的分析工具     

New seismological constraints on differential rotation of the inner core from Novaya Zemlya events recorded at DRV, Antarctica

Novaya Zemlya nuclear test records at the seismic station DRV, Antarctica, are analysed in order to obtain further constraints on a possible differential rotation of the inner core with respect to the mantle. These data allow the sampling of the inner core along a nearly polar path in very stable conditions over more than two decades, from 1966 to 1990. The PKP (BC)– PKP (DF) traveltime residuals, which reflect the inner-core anisotropy and/or heterogeneities sampled along the path, exhibit a great stability through time. A computation of the residuals that are expected for various differential rotation rates and the same rotation axis as the mantle has been performed using the worldwide residual catalogue of Engdahl et al . (1997) for summary rays that include the time as an additional parameter in data stacking. Comparison of data and predictions shows that an eastward differential rotation with a rate as large as 3°  yr⁻¹, as suggested by some authors, is not possible, but an eastward rotation at 1°  yr⁻¹ or lower cannot be rejected.     

山区流域坡面流的一种近似解

在山区流域坡面流近似为运动波的条件下,利用Ｇｏｖｉｎｄａｒａｊｕ．ｅｔａｌ．提出的坡面流分离变量形式,导出了坡面流的偏微分基本方程．根据坡面净雨历时与汇流平衡时间的关系,得到了不同净雨历时的坡面流漫流过程．     

Seismic image of a CO2 reservoir beneath a seismically active volcano

Mammoth Mountain is a seismically active volcano 200 000 to 50 000 years old, situated on the southwestern rim of Long Valley caldera, California. Since 1989 it has shown evidence of unrest in the form of earthquake swarms (Hill et al. 1990), volcanic 'long-period' earthquakes (Pitt & Hill 1994), increased output of magmatic ³He (Sorey et al. 1993) and the emission of about 500 tonnes day ⁻¹ of CO₂ (Farrar et al. 1995; Hill 1996; M. Sorey, personal communication, 1997), which has killed trees and poses a threat to human safety. Local-earthquake tomography shows that in mid-1989 areas of subsequent tree-kill were underlain by extensive regions where the ratio of the compressional and shear elastic-wave speeds V_P/V_S was about 9 per cent lower than in the surrounding rocks. Theory (Mavko & Mukerji 1995), experiment (Ito, DeVilbiss & Nur 1979), and experience at other geothermal/volcanic areas (Julian et al. 1996) and at petroleum reservoirs (Harris et al. 1996) indicate that V_P/V_S is sensitive to pore-fluid compressibility, through its effect on V_P . The observed V_P/V_S anomaly is probably caused directly by CO₂, and seismic V_P/V_S tomography is thus a promising tool for monitoring gas concentration and movement in volcanoes, which may in turn be related to volcanic activity.     

Research on the Urban Influence Domains in China

Through research on the gravity model, the paper studies the geometric characteristics of urban influence domain and the principles for the change of urban influence domain along with the evolution of distance decay index, calculates the distribution of the gravitational field by using ARCGIS, establishes a spatial cluster system for the megalopolis in china, delineates urban influence domains by dissolving spatial features, and compartmentalizes China into 13 economic regions based on the megalopolis clusters and urban influence domains combining with the physical and economic locations. Major conclusions are: the distribution of urban gravitational field is the manifestation of regional unbalanced development; the spatial structure and characteristic of urban system can be studied through the distribution situation of urban gravitational field; the urban influence domains in China have not formed the mosaic structure of a standard regular hexagon; the economic region with weak urban gravitational field may be compressed by the region with stronger urban gravitational field.