CONTINENTAL DYNAMICS AND CONTINENTAL EARTHQUAKES
ZHANG Dongning1) , ZHANG Guomin2) and ZHANG Peizhen3)
1) Institute of Geophysics, China Seismological Bureau, Beijing 100081, China
2) Center for Analysis and Prediction, China Seismological Bureau, Beijing 100036, China
3) Institute of Geology, China Seismological Bureau, Beijing 100029, China
On June 28th, 1976, a large earthquake hit Tangshan, China, 240 000 people were killed by that great shock. Scientists in China had shown great interest to intra-continental earthquakes for its great disaster. Now seismologists noticed that continental dynamics is the basic knowledge to the research work of intra-continent earthquake.
The meaning of East Asian continent in geoscience is the vast area between West Pacific Ocean subduction zone, India-Eurasian collision zone and Tianshan-Baykal Lake seismic zone. Almost all geodynamical phenomena in the earth can be found in this region, such as ocean-continent subduction, continent-continent collision, intra-continent earthquakes, ultra-high pressure (UHP) metamorphic belt, magma and volcano actives. Most of these are highlight question, and the key of the research on continental geodynamics. The aims of the Project of East Asian Continental Geodynamics are probing the dynamic source and process of tectonic deforming in east Asia, discussing the dynamic mechanism of strong earthquakes, offering the scientific suggestion to protecting the nature and Ecosystems, resource exploration and nature disasters mitigation (Chen Yuntai, Chief scientist of the Project of East Asian Continental Geodynamics, 1999, Introduction of special issue for Project of East Asian Continental Geodynamics, Acta Seismologica Sinica, 21(5)).
Because of the importance of seismological research, and also the importance of putting difference disciplines together into an orderly, functional, structured whole, in 1998, organized by China Seismological Bureau, and involved Chinese Academy of Science, and some universities, a large project, the Mechanism and Prediction for Strong Continental Earthquakes, from 1998 to 2003, which was included in the National Key Project of Science and Technology Development Program, was funded by the Ministry of Science and Technology. The fund for this project is over US$ 5 000 000.The aims of the Mechanism and Prediction for Strong Continental Earthquakes project were to understand the mechanism of continental earthquakes, and to provide geological, geophysical and kinematic background for earthquake prediction. This project is based on continental tectonics of the Mainland China, and hypotheses of continental active blocks, and new-developed monitoring techniques(Zhang Guomin, Chief scientist of the Project of The Mechanism and Prediction for Strong Continental Earthquakes, and Zhang Peizhen, 2000).
On November 14, 2001 (at 09:26:14.7 UTC), a MW=7.8 earthquake occurred near to the Kunlun Mountain Pass at the border of Qinghai, Tibet and Xinjiang. The magnitude given by the Center of China Digital Seismograph Network (CCDSN) is MS=8.1.
This earthquake is the largest one striking the Tibetan Plateau since the August 15, 1950, Zayu, Xizang, MW=8.6 earthquake (known as Assam earthquake in western literature) and the November 18, 1951, Damxung, Xizang, MW=7.5 earthquake. The great earthquake occurred on the east Kunlun fault, which is a quite active left-lateral strike-slip fault. The surface rupture zone can be found is more than 400 km. The location results of main shock given by difference seismograph networks are 90.504¡ãE, 36.014¡ãN (USGS); 90.504¡ãE, 36.014¡ãN (Earthquake research Institute, Tokyo University); 90.8¡ãE, 36.58¡ãN (Center of China National Digital Seismograph Network) and 90.9¡ãE, 36.2¡ãN (China Digital Seismograph Network).
Being located near to the Hoh Xil National Natural Preserving Region, the earthquake caused few damages and no fatalities. The seismological studies of this earthquake as well as its aftershock sequence provide a good opportunity to understand the nature of earthquakes (Wu Zhongliang, 2002. Review of the field monitoring of 2001 Kunlun Mountain Pass MS=8.1 earthquake, Annual Review of Earthquake Monitor and Prediction, Institute of Geophysics, CSB, 2002; Department of earthquake monitoring and prediction, CSB, 2002)
I. THE PROJECT OF EAST ASIAN CONTINENTAL GEODYNAMICS
In the last few years in 20th century, scientists proposed a project, focused on the research of East Asian continental geodynamics. This work is supported by the Climb Project 95-S-05 sponsored by Ministry of Science and Technology of the People's Republic of China, and includes some sub-project:
1) 3 dimensional deep velocity structure of the East Asian continental lithosphere;
2) High-resolution 3 dimensional velocity structure of the main deforming areas in the interior East Asian continent.
3) The tectonic stress field of the East Asian continental lithosphere.
4) The deforming, deep structure and the rheology of the Benioff-zone of the West Pacific Ocean.
5) The deep structure of the Himalayan collision zone and the seismicity of Tianshan- Baykal lake seismic zone and its geodynamical implication.
6) Rebuilding of the Eurasian plate evolution history, the simulation of the India plate ¨CEurasia plate collision process, and the geodynamical simulation of some earthquakes.
7) The mechanisms and tempo-spatial rupture process of some typical large earthquakes in East Asian continent.
8) The active faults and the crust movement in Chinese mainland.
9) The multi-discipline study of the crust stress field in Chinese mainland from the seismic, stress measure and fault slip scrape data.
Tens of seismologists and geologists had been engaged, so it is impossible to review in detail all in this project. Instead, only some interesting results are shown in this section of the China IUGG National Report.
Three dimensional velocity structures of entire eastern Asian continent, Tibetan Plateau and its vicinity, Dabieshan UHP metamorphic belt, Tengchong volcanic area, were constructed from the data of broad-band seismograph stations and exploration (Yin, et al, 1999; Zeng, et al, 2000, Ding, et al, 2001; Wang, et al, 1999a, Wang, et al, 2001a, 2002a; He, et al, 2002b; Ruan, et al, 2002; Lou, et al, 2002).
By seismological and numerical simulation methods, the tectonic stress field regime of the entire eastern Asian continental lithosphere, and some regional tectonic stress field patterns in it, were studied (Wu, et al, 2002; Sun, et al, 2002; Xu, et al, 1999a; Xu, 2001; Xie, et al, 1999; Cui, Xie, 1999, 2001). Also the deforming, deep structure and rheological properties of the Eurasian plate boundary were studied (Jing, Ning, 2001; Ning, Zang, 2001; Zang, Ning, 2001a; Zang, et al, 2001b; Zhou, et al, 2002).
On the research work of Chinese mainland seismicity, active faulting and the crustal movement characteristics (Wu, et al, 1999a; Wu, et al, 2001a), the collision model of the Indian plate-Eurasian plate was given (Teng, et al, 1999). Ning and Zang (2001) also studied the generation of deep focus earthquakes in subduction zones. Possible dynamical processes of the plate collision boundary, intra-continent deep tectonic environment, and focal dynamical process were simulated (Wang, et al, 1999b; Cai, et al, 1999; Liu, et al, 2002).
From the data of mobile seismic array and China Digital Seismograph Network (CDSN) Chen Yuntai's research group had got some large earthquakes' moment tensor solutions of focal mechanism in eastern Asian continent (Zhou, et al, 1999a; Zhou, et al, 1999b; Mozaffari, P. et al, 1999; Xu, et al, 1999a; Xu, et al, 1999b), spatial and temporal rupture process (Xu, Chen, 1999£»Wu, et al, 1999c; Chen, Xu, 2000), and some research of basic seismological theory (Wu, 1999d; Wu, et al, 1999b, 2001b).
II. THE MECHANISM AND PREDICTION FOR STRONG CONTINENTALEARTHQUAKES
The goals of the Mechanism and Prediction for Strong Continental Earthquakes project in 5 years are:
1) To develop a prediction theory for strong continental earthquakes based on the hypotheses of active continental blocks (Zhang, et al, 2002a; Zhang, 2003);
2) To delineate the regions with high probability for strong earthquakes in next 10 years and limit the areas as small as possible;
3) To do ambulatory seismic experiments in order to push time prediction for strong earthquakes;
4) To provide seismic risk maps for special areas.
The subprojects included by this project are:
1) Temporal and spatial evolvement of active tectonics in Mainland China
2) Deep tectonic environment for strong earthquakes in Mainland China
3) Crust movement and kinematics of active blocks and their boundaries