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STRUCTURE AND PHYSICAL PROPERTIES OF THE EARTH'S INTERIOR

ZANG Shaoxian1 and  ZHOU Huilan2

1Department of Geophysics, Peking University, Beijing 100871, China

2Graduate School, Chinese Academy of Sciences, Beijing 100039, China

Progress in the study on the structure and physical properties of the Earth's interior has been made in the last 4 years. Zang et al. (1999) have reviewed the studies on the structure, geodynamics and the physical properties of the earth's interior in China during 1994-1998. This review will summarize the work done by Chinese geophysicists on the structure and physical properties of the Earth's interior from 1999 to 2002. The contents are mainly on the structure and physics of the earth's interior beneath the lithosphere and therefore may be limited. Related fields can be found in other reviews, including the reviews on the seismic wave propagation (Zhang and Chen, 2003), on the lithosphere structure and geodynamics (Xu and Shi, 2003) and on the experimental research of rock mechanics and tectonophysics ( Ma and Ma, 2003).  

 

I.  STUDTY ON THE STRUCTURE OF THE EARTH'S INTERIORS USING SEISMIC TOMOGRAPHY

In the past four years, the structure of the earth's interiors is an important research field in China. The main method is seismic tomography, including seismic tomography of body and surface waves. The areas of study are continent of China and its adjacent region.

The seismic tomography of surface waves is one of the methods used to study the structure of the earth's interiors in China. Some progresses have been made in the recent years. Zhu et. al.(2002) used Rayleigh waves to carry out tomographic inversion and obtained the group velocity distribution in East Asia (70°-145°E,10°-55°N)for a period range from 10s to 120s. The results show that there exists a deep root under the Tarim Basin, and the South-North seismic belt appears to be a region with high group velocity gradient. In the vicinity of Chiang Mai, Thailand, a low velocity block with a scale of 1000km can be seen; around the Philippine Sea and the Japan Sea, there is a low velocity belt of about 400km wide. Cao et.al.(2001) carried out the partitioned waveform inersion with long period data from stations of CDSN. The images of shear velocity structure of the crust and upper-mantle (0-430km) show that clear differences exist in the structure of the lithosphere and asthenosphere between South China Sea and its adjacent regions. Chen and Chen (2002) presented a new systematic and efficient algorithm to calculate the modal solutions of multi-layered ocean-Earth model. Their algorithm distinguishes itself as terseness of formulation, efficiency in numerical computation, and stableness at high frequencies, thus, thoroughly solving the problem of loss-of-precision at high frequencies. Teng et.al.(2002) determined the pure -path dispersion of Rayleigh waves crossing Southeast China and its continental margin by applying the matched-filter frequency time analysis technique to the mid-long period data from CDSN. Based on this, the 3D shear wave velocity structure to the depth of 200km was studied. The Moho is generally 30-40km deep in the continent region of Southeast China and becomes shallow gradually eastward to the depth of 25-28 km in its continental margin. The depth of the low velocity layer in the upper mantle in the continent of Southeast China is 60-150km and varies greatly from one place to another. With the dispersion curves of fundamental-mode Rayleigh waves, Li et.al. (2001) studied the lateral velocity variation of the shear wave in Eastern China and vicinal sea areas (98°-150°E, 5°-50°N) located in the junction zone of Eurasian and Pacific plates. He et.al. (2001;2002) obtained the 3D S wave velocity structure of the crust and upper mantle of Chinese mainland and its vicinities by genetic algorithm with smoothness constraint. The S wave velocity images are shown on two latitudinal sections along 30o N and 38o N, two longitudinal sections along 90o E and 120o E, and four horizontal slices at different depths. Wu et.al (2001) obtained the S wave velocity within the depth of 0-100km beneath digital seismic stations of Yunnan Province from teleseismic receiver function modelling. Zhu et.al(2002) carried out the high resolution surface wave tomography in East Asia and West Pacific marginal seas(60°-160°E, 20°S-60°N). The results indicate that from the upper crust to the depth of 70km, the high velocity is displayed in the region of eastern part of East Asia and West Pacific marginal seas. Extremely low velocity is in the Tibet and its surrounding areas. There is a low velocity anomaly chain along the convergence belt of Tethys from Mediterranean Sea, Turkey, Iran, Himalayan orogens and Burma to Indonesian Islands. And at the depths of 85 to 250km, a longitudinal low velocity anomaly belt appears in the eastern part of East Asia and West Pacific marginal seas, while a high velocity is displayed in the western.

In the past four years, seismic tomography of body waves has also been developed. Combined with the studies on dynamics in the continent, good progresses have been made.

Xu et.al.(2001a) reconstructed the 3D velocity of the crust and upper mantle beneath orogenic belts and adjacent basins of the northwestern continent of China using seismic tomography of body waves. High velocity abnormalities are observed beneath orogenic belts, and low velocity abnormalities are observed in the basins and depressions that are obviously related to unconsolidated sediments. A low velocity boundary exists in the middle crust between eastern and western Tianshan Mountains. The orogenic belts and the northern Tibetan Plateau have a Moho deeper than that of the basins and depressions. The top depth of upper mantle asthenosphere varies from place to place. It seems shallower under the northern Tibetan Plateau, Altay and Qilian Mountains, and deeper under the Tarim and Tianshan regions. Hot mantle probably rose to the bottom of some orogenic belts along tectonic boundaries. Based on the results of seismic tomography Xu et.al. (2001b) presented the possible colliding types between orogenic belts and adjacent blocks in northwest Chinese continent. Several tectonic patterns, such as the embedding, subducting, detaching and lateral inserting of continental lithosphere, are shown between the Tianshan and Tarim regions. Sharp deep boundaries exist between the Tibet and its northern geological provinces, showing the upper mantle's flowing northward. The authors inferred that the lithosphere of the Tibet has been flexed or broken in the moving northward which is resisted by the rigid Tarim block. However, the shallower asthenosphere in the north of the Qilian mountain seems to be a free boundary, which makes the upper mantle substances beneath the plateau transport northward much further. Lei and Zhou (2002) studied the 3D velocity structure of P wave in the upper mantle beneath southwestern China and its adjacent areas (10°-36°N, 70°-110°E) down to the depth of 400 km, using data of P wave arrival times selected from the bulletins of ISC, China and NEIC. The lateral velocity heterogeneity is obvious till 400 km though it attenuates with the increasing depth. In the vertical velocity profiles along latitude 16o N and 24o N, the collision and extrusion of India plate to Eurasia plate is displayed, and a remarkable velocity difference from India plate to Eurasia plate is shown. In the vertical profile along longitude 90o E, the subducting of India plate northward beneath Eurasia plate is also obvious. Xu et al. (2000a) found that a subducted ancient block had been preserved beneath the Moho of the Dabie-Sulu orogenic belt based on the Seismic tomography. They inferred that the Yangtze block subducted northward beneath the Sino-Korean block and broken off at the depth between 170 km to 200 km during 200 -190 Ma. Xu et al.(2000b) found a significant lateral heterogeneity of velocity structures exists in the crust and uppermantle beneath the Dabie orogenic belt. Beneath the southern and northern Dabie tectonic units, the Moho depresses and a north-dipping high-velocity block which corresponds to the ultralhigh-pressure metamorphic rocks is deveoped in the crust. Liu et al. (2000) showed a slab-like high velocity anomaly down to the depth of 250 km beneath the western Yunnan Tethyan orogen by seismic tomography,  and demonstrated it is a part of the subducted plate of Yangtze continental segment after the closure of Paleotethys.

II.  ANISOTROPY IN THE UPPER MANTLE IN CHINA AND ITS ADJACENT AREAS

With the digital seismic data, some progresses have been made in the study on anisotropy in the earth's interiors.

Liu et al. (2001) applied the Butterworth band-pass filter to S-wave data recorded at 8 stations in China mainland and analyzed S-wave splitting at different frequency bands. The results show that the delay time and the fast polarization directions of S-wave splitting depend upon the frequency bands. There is an absence of S-wave splitting at the station of Urumqi (WMQ) for the band of 0.1-0.2 Hz. With the frequency band broadening, the delay time of S-wave splitting decreases at the stations of Beijing (BJI), Enshi (ENH), Kunming (KMI) and Mudanjiang (MDJ); the fast polarization direction changes from westward to eastward at Enshi (ENH), and from eastward to westward at Hailar (HIA). The variations of delay time with bands at Lanzhou (LZH) and Qiongzhong (QIZ) are similar, and there is a coherent trend of fast polarization directions at BJI, KMI and MDJ, respectively. In this paper initial interpretations to the results of frequency band-dependence of S-wave splitting were also presented. Based on the theory about the shear wave propagation in the laminar anisotropic media and observations of the shear wave splitting, using signal identification methods with high precision like wavelet analysis, Liu et al. (2001) obtained the image of the anisotropy strength and polarization direction beneath twenty stations in China and its surrounding areas after the waveform analysis for ScS wave in 136 earthquakes.Combined with the previous results concerned, the characteristics and origin of the upper mantle anisotropy were discussed.  Jiang et al. (2001) studied the characteristics of shear wave anisotropy in Tibetan plateau and its neighboring areas. They showed that the anisotropic direction of the upper mantle above the depth of 200 km is mainly affected by the movement direction of the upper mantle material; In a long period in geohistory, the crust and lithosphere of different terrains shared continual movement, and the corresponding main direction of anisotropy was determined by the shear stress exerted on the upper mantle, probably inconsistent with the strike of mountains and upper crust structures. The strongest anisotropy along the margin of high velocity terrains often relates to the partial melt material deep in the mantle. Near the strike-slip faults on the edges of these terrains, the anisotropic direction is consistent with the strike of the fault systems. Ruan and Wang (2002a) studied the shear wave splitting for SKS and the corresponding parameters by fitting the theoretical transverse component with the observed one. The results show that the fast orientation in Yunnan area is north-northeast in general and the time delay between fast and slow splitting shear waves is 0.5-2.0 s. As the transitional zone between Tibet and the block of South China, the orientation of fast shear wave polarization in Yunnan area indicates that the subduction of India plate into Eurasian plate is the fundamental background of earth dynamics. Furthermore, the authors deduced that the anisotropy of the upper mantle is mainly in the lithosphere rather than the whole upper mantle. Based on the detailed derivation of seismic wave velocity in weak anisotropy medium, Ruan and Wang (2002b) introduced some calculation methods using Pn phase to upper mantle anisotropy, illustrated the inversion approaches of upper mantle anisotropy using SKS and ScS phases, and analyzed their advantages or disadvantages and interrelations. 

Furthermore, study on the cause of the anisotropy of the inner core has started. According to the theory on crystal growth, Liu et al. (2000) discussed the origin of the seismic anisotropy in the earth inner core basing on the observation of differential rotation between the solid inner core and the molten outer one, and pointed out that the c-axes of the hcp iron which constitutes the inner core is aligned along the inner core's rotation axis and results in observed seismic anisotropy. 

 

III.  QUALITY FACTOR Qb

Quality factor is an important parameter to characterize the viscoelasticity of the media. By applying the improved method of multi-filtration to the data selected from the vertical component of long period Rayleigh surface wave for two station paths, Li et al. (2000) obtained the group velocity and amplitude spectrum and then the attenuation factor for each paths. By Talentola inversion method, local attenuation factor was obtained. And then the 3D Qb image in the crust and upper mantle in the eastern Chinese continent was inverted. The results show that there is correlation between the seismic activity and Qb structure in the crust and upper mantle in North China. The Yangtze block seems to collide with and subduct into the North China block from the southern border of the Qinling in the southern Shaanxi Province. Hu and Duan (2000) applied the frequency variation filtering technique to extract the fundamental surface wave signal from the records based on long period Reyleigh surface wave data from CDSN. From fundamental surface wave signal recorded by the two stations located in the same great circle path with same epicenter, the interstation Green's function was calculated, and the attenuation coefficient of fundamental Rayleigh surface wave with the periods of 10-98s through Sino-Korean paraplatform was determined. Based on these, the inversion of Qb value of crust and upper mantle in Sino-Korean paraplateform was obtained. The results show that in north China the Qb value is 250, 450 in Yellow Sea, and the Qb value of the upper mantle in Sino-Korean paraplatform, which more similar to that in tectonically active regions, is obviously lower than that in stable regions.

 

IV.  SUBDUCTION ZONE

Subduction zone is still a research area which drew big attention from Chinese geophysicists, some new progresses have been made in the last four years.  The study on the physical properties of the subduction zone is one of the important fields. Ning and Zang (2001) obtained the P-wave velocity structure of subduction zone by using the finite-element method with the quasi-static model. Numerical simulation shows that high velocity anomalies exist in most depths of subduction zones with the maximum velocity at the depths of about 400 km and 550 km, which is consistent with the wave structure determined by seismic tomography. It is also shown that negative velocity anomalies existed at some depths which appear near the depths of 400 km and 720 km and the range of anomalies is dependent on the subducting conditions. The largest anomaly is -10%. Zang and Ning (2001) calculated the negative buoyancy caused by the subducting slab and discussed the influence of metastable olivine on the negative buoyancy. They found that low temperature and high density make the negative buoyancy increase with the depth while h<400 km and h>740 km. Cool materials of subducting  slab exist as low density and low pressure phase in the depth range of 100 km beneath the  660 km discontinuity. In the depths from 400 to 660 km, the existence of metastable olivine makes the recruitment of negative buoyancy decrease with the depth. Metastable olivine hinders the slab's ability to penetrate through the 660 km discontinuity. Zang et al. (2001) calculated the viscosity structures of subducting slabs based on the thermal structures of subduction slabs and phase transformation process. The upper most layer of the slab has a high effective viscosity which reaches 1034 Pa·s, while the lower layer has a relatively low effective viscosity that decreases obviously below the kinetic phase boundary of olivine to wadsleyite and the effective viscosity reaches a minimum of 1022 Pa·s. Small areas with higher effective viscosities exist above the depth of about 700 km in subducting slabs. The 1% and 99% isogram of spinel proportion delineate tortuous belts with low effective viscosity width varying from 1 km to 5 km which would affect the geodynamic behavior of subducting slabs. 

The study on the stress state is another important field. With the 2-D elastic FEM, Mao et al.(2002) studied the distributions of the stress fields produced by the volume change of the phase transformation, temperature difference, density difference of subducted slab and the force on the boundary. The simulation result shows that the stress field produced by the factors can be computed with elastic model, but it could not be used to explain the distribution of deep seismicity and other methods should be considered. Liu et al. (2002) studied the characteristics of the stress fields in deep subducting slabs by using viscoelastic plain strain finite element method. The result shows that there emerge two regions with great shear stress just below the olivine-spinel phase transition zone, which encompass the low viscosity zone below the lower tip of metastable wedge. Further, the directions of the main compressional stress of these two regions are all along the dip direction of the slab and their characteristics are in accordance with the directions of main compressional stress of the deep earthquakes. While in the area where metastable olivine exists, the stress state at the transition zone from olivine to spinel does not coincide with those from seismic study. 

The subduction of the continental crust is a very interesting problem. Shi and Fan (2001) studied the subduction of the continental crust dragged by the adjacent subducting oceanic lithosphere with 3-D FEM. The computational result shows that the subducting oceanic lithosphere can drag adjacent continental sliver with a width up to 150km to the depth of 100km and produce ultra high pressure metamorphism (UHPM) belts and mature continental-continental collision does not allow continental crust subduction to UHPM depth.

The study on collision between India and Eurasia plate is a hot field for Chinese geoscientists. Zeng et al. (2000) put forward a concept of multiple crustal subductions and used it to interprete the collision process between India and Eurasia plate. They believed that the crustal subduction occurred at Yaruzampbo suture, and stopped at a depth of 80-100km; then, it migrated to the south, other new crustal subductions started successively at MCT (main central thrust) and MBT (main boundary thrust), respectively, and stopped at a depth of 80 to 100km too. Then the inserting Indian crust has been split apart from its upper-most mantle, multiple crustal subductions are performed and the upper part of upper-most mantle may be subducted deeper into the Eurasia mantle. 

The generation of deep focus earthquakes in subduction zone is a field in studying. Ning and Zang(1999) studied the stress distribution in subduction zone with the numerical simulation method. Comparing with seismological evidences and results of laboratories, it is proposed that earthquakes occurred below 400 km are probably controlled by anti-crack mechanism.

In recent years, besides the research on the subduction zone itself, the interaction of the subduction zone with discontinuities is also a subject of study. As related to the study on mantle discontinuities, it will be reviewed in the section of "mantle discontinuities".

 

V.  STUDY ON THE MANTLE DISCONTINUITIES

With the accumulation of digital seismological data, the study on the mantle discontinuities has been an important field in the last four years. 

The study on methods for picking up the weak seismic phases is an important part of the study on the mantle discontinuities, because the mantle discontinuities are studied mainly using the secondary phases with weak amplitudes, so effective method is an essential prerequisite. Wei et al. (2000) studied the effects of low velocity zone (VLZ) on the PdS converted phase related to the 660km discontinuity. There are oscillation relations of both the incident angle and the relative amplitude with epicentral distance. While the epicentral distance is greater than 33o, the oscillatory decreases with the epicentral distance and the PdS can be distinguished easily. Zhou and Zang (2001) put forward the rectilinearity-polarization method for picking up PdS phases by modifying the linearity filtering method (Vinnik, 1977; Paulssen, 1988) and introduced the cubic angle that is the angle between observed and theoretical vibrating directions of SV wave. The converted phases are picked out efficiently and objectively when this new method is used. Zang and Zhou (2002) analysed the functions and procedures of the N-th root slant stack method on picking up the secondary phases related to mantle discontinuities and pointed out that the method can increase the SNR (signal-to-noise ratio) and make the signal clearer,so it is useful for obtaining signal travel times. At the same time, they found that the deflection of discontinuities can affect the travel time and slowness of conversion phases and suggested a modifying method to correct the effects. Zhou and Zang (2001) developed the N-th root slant stack method by introducing focal depths modification and the method can be used to process the data recorded by one single station from many earthquakes to study the mantle discotinuities beneath the station. The modification broadens the application of the N-th root slant stack method.

Study on the depths and configuration of mantle discontinuities with different methods has increased gradually in recent years and becomes an important part of the study on structure of the Earth's interior. Zhou and Zang (2001) analysed the digital data recorded at stations MDJ and HIA in northeast of China with the rectilinearity-polarization filtering method and the N-th root slant stack method and studied the mantle discontinuties beneath the two stations. The results obtained from the two methods are basically coincident with each other and show that there are possible discontinuities existed at the depths of 140, 350, 570, 740 and 1080 km, except for those discontinuities at the depths of 220, 410, 520 and 660 km. The layered structure between 660 and 840 km under MDJ station is more complicated than that under HIA station. It may show the effect of subducting slab. Yang and Zhou (2001) estimated the depths of mantle discontinuities in the upper mantle beneath China and its adjacent area. They processed the 3 components digital teleseismic data recorded at 18 stations using the receiver function method. The average depth of the 410 km discontinuity is 403 km and the discontinuity appears obvious lateral variation with the depth ranges from 390 km to 416 km. The average depth of the 660km discontinuity is 663 km and the discontinuity has also a lateral variation in its depth from 653 km to 672 km. Zhou et al. (2002) studied the topographies of mantle discontinuities beneath Izu-Bonin with the data provided by the SCSN, NCSN and PNSN at western USA, GRF and GRSN at Germany and there are some possible mantle discontinuities existed at 170, 220, 300, 410, 660, 850 and 1150 km. Beneath the Izu-Bonin region, the 410 km discontinuity is uplifted, and the 660 km discontinuity is depressed and its topography appears regionalized difference which may be affected by the subducting depths of subduction slab. Jiang et al.(2002) studied the mantle discontinuities beneath the Okhotsk Sea and the result shows that there are possible discontinuities near the depths of 150, 280, 410, 520, 660 and 900 km, the 410 km discontinuity is uplifted, while the 660 km discontinuity is depressed,the 660 km discontinuity beneath the northern part of the region is affected obviously by the subducting slab.

 

VI.  EXPERIMENTAL STUDIES

During the four years, some experimental studies concerning the physical properties of the earth's interior have been carried out gradually. Zhao et al.(1999; 2001) investigated the creep behavior in Ni2GeO4 and (Mg,Ni)2GeO4 using synthetic polycrystalline aggregates. At temperatures ranging from 1223-1523K and confining pressure of 330 MPa, two deformation mechanisms were identified and characterized by a flow law with the stress exponent equal to 2.9±0.1 which suggests deformation was accommodated by dislocation creep and a flow law which suggests deformation was accommodated by grain-boundary diffusion (Coble) creep respectively. When creep data for olivine and spinel are normalized and extrapolated to Earth-like conditions, spinel (ringwoodite) has a strength similar to olivine in the diffusion creep regime at coarse grain size. However, when grain size reduction occurs, spinel can become weaker than olivine due to its high grain-size sensitivity. Zhao et al. (2001) also studied effect of Fe content on the water solubility in San Carlos olivine single crystals. Experiments were conducted under a pressure of 300 MPa and at temperatures from 1000-1300oC. The result shows that the water solubility in San Carlos olivine single crystals increases with the increase of the Fe content, and increases with the increase of the temperature if the Fe content is the same.

The conductivity of rock changes with temperature is an important research area in experimental studies. Liu et al.(2001)measured the conductivity of granite, basalt and pyroxene peridotite under the condition of high temperatures ranging from 563 to 1173K and high pressures ranging from 1.0 to 2.5 GPa. The data indicate that the conductivity increases with the increasing temperature. In the range of 563-1173 K, the conductivity has a variation of about 3 to 5 orders of magnitude. And the conductivity of these three rocks has an abrupt change or a large variation near a given temperature. For example, at the confining pressure of 1.5 GPa and the temperature of about 973 K, the conductivity of the pyroxene peridotite has a mutation. So does the basalt at the confining pressure of 2.5 GPa and the temperatures of about 710 to 830 K. These variations may result from partial melting. Zhu et al. (1999; 2001) measured the electrical conductivity of serpentine at temperatures ranging from 300 to 870oC and pressures ranging from 1.0 to 3.0 GPa and under the condition of temperatures from 220-780oC and pressures from 2.5 to 4.0 GPa respectively. The results show that the electrical conductivity of serpentine increases significantly after dehydration of serpentine.

The variation of velocity of the seismic waves and its cause are also important topics for laboratory studies. Xie et al. (2000) measured the elastic properties of the serpentinite at the confining pressure of 1.0 GPa and high temperature during the process of dehydration. The ultrasonic velocities of the serpentinite decreases sharply with increasing temperature as it is higher than 640oC, while the amplitude of the ultrasonic wave increases clearly. The analysis of data shows that these two phenomena are related to the dehydration of the serpentine in the rock. Gong et al.(2000) measured the compressional sound velocity for enstatite of polycrystalline specimens at pressures ranging from 40 to 140 GPa using the optical analytical techniques under shock loading. The data indicate that enstatite is stable throughout the conditions of the lower mantle. The velocity of P wave is 0.5% lower and the wave velocity of S wave is 2% higher than that of PREM respectively. The results show that the lower mantle is mainly composed of perovskite (Mg1-x,Fex)SiO3 and only a small amount of (Mg1-x,Fex)O is allowed in it. Zhou et al.(1999) measured the compressional wave velocities in a trachybasalt at pressure of 2.0 GPa and temperature up to 1350oC. The observation of the thin sections of the run products indicates that, corresponding to the variation of the compressional wave velocity in the trachybasalt, the phase transition has taken place. The relationship between the change of the compressional wave velocity and the hydrous mineral dehydration, solid-solid phase transformation and partial melting has been discussed based on the experimental results.

Quality factor (Q) is an important parameter to characterize the viscoelastic properties of the material. Ye et al. (2001) calculated the Q values of bar samples from the measured waveform using waveform inversion technique and studied the method itself. The results show that the logarithm dispersion caused by attenuation is working for the bar wave, but will not affect much the waveform even while the Q value is small. Therefore, it will not do obvious effect on the credibility of inversion calculation.

Gu et al. (2000) studied the aquiferous effect of the olivine at high temperatures and high pressures. The data indicate that water enters not only the void of the mineral but also its lattice. Compared with dry sample, the wet ones have characteristic optical property and infrared absorption bands, and their specific gravity drops off about 0.02.

 

VII.  OTHERS

Besides the six topics mentioned above, other studies on the structure and physics of earth's interior have also been carried out in the past four years. Only some of them will be stated briefly here because they are dispersed and concern with different disciplines. The others will be mentioned in other reviews.

In recent years, the study on the viscosity of the earth's interior (including the lithosphere) is one of the research areas in China. However, little work has been done on the viscosity of the mantle. Under the constraint of the observed secular drift of the earth's pole, Yang (2001) estimated the mean viscosity of the lower mantle as (0.5-1.7) ´1022 . Taking into account the effect of three main rheological mechanisms, namely, friction sliding, brittle fracture and creep in the lithosphere, Zang et al. (2002) calculated the 3-D structures of the viscosity in the lithosphere in North China (105o-124oE,30o-42oN). The results show that the viscosity in the lithosphere have layering characteristics. Under the strain rate of 10-15 s-1, the upper part of the upper crust is in the brittle region and the lower parts of the upper crust may be in the ductile region dominated by creep; the middle crust can be in the brittle region dominated by brittle fracture, or the upper layer of brittle fracture and lower layer of creep ductile; the lower crust almost is in the creep region dominated by creep. The effect viscosity at the bottom of thermal lithosphere is about 1020 .

Using gravity data to study the earth's interior is also an important research area in China. Lei and Xu (2002) introduced the tri-frequency spectrum method with clearly geometrical and geophysical meaning for the resolution of the parameters of the earth free core mutation(FCN). The observational results of FCN parameters obtained from this method with the tide data at three superconducting gravity stations are accordant with those from VLBI.  Lei et al. (2002) investigated the Earth's normal modes excited by Peru Ms 7.8 earthquake on June 23, 2001 with the superconducting gravimeter C032, and obtained all base normal modes from 0S0 to 0S32 and the splittings of 0S2 and 0S3. Those results show a good agreement with the HB1 model.

In a three-dimensional spherical geometry frame, Ye and Hager (2001) used different viscosity models to investigate the generation and distribution characteristics of global heat flow on the basis of exploring thermal effects of density anomaly and plate driven mantle flows. The result of a mantle viscosity model in which viscosity in the lower mantle is 30 times more than more that in the upper mantle appears to fit data better. The rapid variation of the temperature in the lithosphere and the layer D'' is also shown, which is consistent with the results from other methods.

Sun et al. (2002) investigated the mantle unsteady flows in an incompressible and isoviscous spherical shell using algorithms of the parallel Lagrange multiplier dissonant decomposition method (LMDDM) and the parallel Lagrange multiplier discontinuous deformation analyses (LMDDA). Some physical fields about mantle flows are calculated.

It is very important to use incomplete elastic coefficients to study mantle convection and continent dynamics. On the basis of wave equation in medium with linear rheological property, Wang and Di (2000) deduced two methods and corresponding theoretical formulae which can determine the parameters of medium with linear rheological property from observed seismic wave velocities and amplitudes. These coefficients can determine the non-elastic characteristics of the earth medium with linear supposition.

It is one of the classical problems in earth's electromagnetism that continuation of the observatory data of magnetic field into the conducting region. Ma (1999) built the coupling vector equations governing the poloidal, toroidal and potential fields in 3-D inhomogeneous conducting mantle. Considering the limitation on variable scale of inhomogeneities in global mantle from study of the Earth's deep interior, a perturbation theory of gradually lateral variation was presented. It is unnecessary that the 1-D spherical symmetric distribution of electric conductivity as zero-degree approximation. Serving as an example of solvability of the zero-degree approximation, it was demonstrated how the gradually lateral variation of electric conductivity affect the anti-diffusive problem of poloidal field in Earth's mantel.

Zhang et al. (1999) discussed the tidal motion equations of the Earth for the anisotropic medium in upper mantle. Based on the earth parameters  of the isotropic and anisotropic medium given by Dziewonski, the Love  numbers and load Love numbers are calculated using a classical Runge-Kutta  numerical integration method. The results demonstrate that the influence of  the anisotropic medium in upper mantle on the Love numbers is very small (0.06%),  but relatively large on the load Love numbers (2.5%). The results show simultaneously  that the load Love numbers with lower and middle order are sensitive to the characteristics  of the upper mantle medium.

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