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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 90.504E, 36.014N (USGS); 90.504E, 36.014N (Earthquake research Institute, Tokyo University); 90.8E, 36.58N (Center of China National Digital Seismograph Network) and 90.9E, 36.2N (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, 1999Wu, 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 CONTINENTAL
EARTHQUAKES

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

4) Mathematical and physical simulation for strong continental earthquakes

5) Ambulatory seismic experiments

6) Prediction of strong earthquakes and seismic risk assessment for special areas.

The aims of the subproject,Temporal and spatial evolvement of active tectonics in Mainland Chinaare:

To delineate the active continental blocks within China based on field investigation, satellite images, geological maps, crustal deformation, geophysical fields etc. And to describe the temporal and spatial evolvement of the active blocks and their boundaries based on investigation of some typical faults and zones, such as the dynamics and kinematics of faults, the relationship between faults and paleoseismicity (Wu, et al, 1999b; Yi, et al, 2002; Xu, et al, 2001; Tian, et al, 2002; Guo, et al, 2000, Xu, Wen, 2003; Yuan, et al, 2002; Ran, et al, 2001).

Here the first concept of the hypotheses of active continental blocks is defined as: ACTIVE CONTINENTAL BLOCKS are the active tectonic blocks cut and enclosed by variant Late Quaternary active rifts. And inside it, the tectonic activity is relatively stable, and its boundary zone is relatively tectonically active, its deep end controlled by variant detachment faults. The second hypothesis of active continental blocks is its criteria. Activity of the boundaries is focused to late Quaternary active rifts, and geophysical anomalies or significant grads belts, and crustal deformation zones or grads belts. The integrity of the blocks is considered as the relatively integrate lithosphere structures, similar tectonic history, similar crust deformation and movement, and the similar seismicity, geophysical anomalies and geodynamic background (Zhang and Zhang, 2000; Zhang, et al, 2002d).

The aims of the subproject, Deep tectonic environment for strong earthquakes in mainland of Chinaare

1) Inversion of the Rayleigh wave (10-100s) group velocity for the crust and upper mantle with CDSN data;

2) Tomography with Pn velocity based on records from China (Wang, et al, 2001a);

3) Geophysical profiles in the NE part of Tibetan Plateau (Li Songlin et al, 2002);

4) Seismic exploration in the eastern Tibetan Plateau and adjacent area (Wang, et al, 2003a; Wang, et al, 2003c)

The aims of the subproject, Crust movement and kinematics of active blocks and their boundaries are

1) To describe the kinematic characteristics of the crust in the North, Northwest, Southwest China based on GPS data (Wang, et al, 2001b; Jiang, et al, 2000; Ren, 2002; Zhang, et al, 2002a; Zhang Peizhen et al, 2002b);

2) To analyze the deformation before and after some recent moderate earthquakes, such as the Zhangbei event (MS=6.2, 1998.01.10).

The aims of the subproject, Mathematical and physical simulation for strong continental earthquakesare

1) To get characteristics for rock medium and seismogenic environment by analyzing the seismogenic structures in typical continental areas of China (Ma and He C., 2001; Ma, et al, 2002; He, et al, 2002a);

2) To investigate the mechanism of strong earthquakes by modeling and simulation based on the kinematics and dynamics of active blocks, the physical and rheological features of rocks in different depths (Bai, Lin, 2003; Zang, et al, 2002);

3) To do numerical simulation of regional earthquake-released energy based on the results from the research on the active blocks (Yu, et al, 2003, Chen, Willemann, 2001).

The aims of the subproject, Ambulatory seismic experiments are is to build ambulatory seismic and precursory network in:

1) Northern part of North China (Deng, 2000; Wang, Stammler Klaus, 2002);

2) Western part of the Yunnan and Sichuan Province (Qin, et al, 2001; Xu, et al, 2002);

3) Western part of the Xinjiang Uygur Autonomous Region (Zhang, et al, 1999; Zhou, Chen, 2002).

And the work included in this subject, will be based on the investigation of active tectonics, geophysical observation and seismicity analysis. Main work had been done in ambulatory seismic experiments includes: S-wave splitting, immediate foreshocks, nucleation, clustering, stress triggering, time dependent failures, etc.

The aims of the subproject,Prediction of strong earthquakes and seismic risk assessment for special areasare to build data bases and GIS-based seismic risk assessment system for the Capital area with the results from the former 5 sub-projects and the collected basic data (Zhang, Xie, 2001; Fu, Liu, 2003; Yu, Gao, 2001; Gao, et al, 2002).

Tasks of the project The Mechanism and Prediction for Strong Continental Earthquakes are giving some quite important maps, such as: Map of the active rifts over China, Map with active blocks over China, Map of crustal structures over China, Map with motions of each block over China, 10-year seismic risk map over China, and giving some physical models, such as Dynamic models of the active blocks to explain some of the strong continental earthquakes, risk assessment system for special areas.

III.  MONITORING OF THE Nov.14, 2001 KUNLUN MOUNTAIN PASS MS=8.1 EARTHQUAKE

After the Nov.14, 2001 Kunlun Mountain Pass MS=8.1 earthquake, the Institute of Geophysics of the China Seismological Bureau (IGCSB), sponsored by the CSB sent a team to the field. The near-real-time monitoring was conducted for 5 months since then, obtaining valuable high-quality data. The observation system includes 11 broadband seismological stations and a field data center.

The velocity frequency response of the instrument is flat from 20 Hz to 20 s according to its transfer functions. Seismic signal is transmitted from each station to the data center through a wireless short-wave transmission system.

The elevation of that place is from 3000m (at Golmud) to 4 700m (at the Kunlun Mountain Pass). Also the weather change and temperature are fierce during the wintertime. To implement the monitoring task, seismic array was chosen as a solution to the difficulties. It is worth pointing out that in the western part of China, a large portion of land is covered by desert or mountain ranges, being almost inaccessible for seismological observation. In other countries/regions, as well as in the polar regions, the situations and/or difficulties are much similar. In this case, small aperture seismic array, which was used previously for the purpose of nuclear test monitoring, may be a pretty good solution to the difficulties in these regions.

The Kunlun Mountain Pass earthquake has attracted an increasing attention among seismologists and geologists. There have been several papers and/or research letters published reporting the study of this earthquake (Xu, Chen, 2002; Yang, et al, 2002; Zhang, et al, 2002c; Lin, et al, 2002; Xu, et al, 2002; Lu, et al, 2002; Qiao, et al, 2002). Further study on this earthquake as well as its aftershock sequence will play an important role in the development of Earth sciences and in the reduction of seismic disaster.

IV.  DISCUSSION

1. Chinese Continental Active Blocks Map and Some Puzzles

Though some results had got in last three years, but puzzles are still existing: some scientists in China and abroad suspicion the hypotheses of active continental blocks, they want more evidences and tests of the hypotheses of continental active blocks. And some questions are still puzzle to seismologists and geologists, such as:

1) The relationship of the active blocks and the active plates;

2) The correlation between the active blocks and the active rifts

3) The boundary, basement and driving force of the active blocks

4) How to identify the activity of the continental active blocks

5) The relationship between the active blocks and the strong continental earthquakes (Fig.1, Zhang, et al., 2003)

Now scientists in this project need help in the following 3 aspects:

1) To know more about the geophysical features on the boundaries and the basement of the active blocks;

2) To explain the driving mechanism of the continental active blocks;

3) To obtain the 10-year risk map with new methods.

 

 

Fig.1.  Chinese continental active blocks and the strong earthquake distribution (Zhang, et al, 2003)

2.  Deep Tectonic Environment of the 2001 Kunlun Mountain Pass MS=8.1 Earthquake

Re-location work of aftershocks of the 2001 Kunlun Mountain Pass MS=8.1 earthquake shows that the depth of hypocenters of aftershocks was confined from 5 km to 28 km. It was also shown that all events were almost in on the same nearly vertical fault plane striking the EW direction, with left-lateral strike-slip.

The whole fault broken is more than 400 km (Yang, et al, 2002; Zhang, et al, 2002d). Occurring at the border of the Hoh Xil-Kunlun Mountain block and the Qiangtang block, this earthquake reflects the differential motion of these two blocks and fills in a gap of seismic moment release built by previous earthquakes (Ren, et al, 1999). The majority of aftershocks distributed more than 400 km long in an elongate area, which is in an approximate agreement with the distribution of surface cracks or breaks as revealed by geological investigation (Xu, et al, 2002c).

In order to improve understanding of the detailed structure of the Kunlun fault, an exploration was conducted at a site along the 400-km long rupture zone generated by the MW=7.8 Kunlun earthquake of Nov. l4th, 2001. The explanations for the field exploration show that the width of the rupture zone is greater than 100m at a depth of 5 40 m, which is much greater than the width of 10-20 m on the surface. Simulation of the trapped waves indicates that the width of the rupture zone is about 150 m (Wang, Ding, et al, 2003b).

Acknowledgment The authors thank to Dr. WU Zhong-liang and Dr. LI Li for providing helpful information about some research projects.

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