STRUCTURE AND PHYSICAL PROPERTIES OF THE EARTH'S INTERIOR
ZANG Shaoxian1） and ZHOU Huilan2）
1）Department of Geophysics, Peking University, Beijing 100871, China
2）Graduate 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 Q