(Institute of Geology, Seismological Bureau of China, Beijing 100029)

Under the current geodynamic system, the tectonic activity of the Chinese mainland plate is characterized by six first-class blocks (Qinghai-Tibet block, Gan Xin block, Northeast block, North China block, South China block and Southeast coastal block). The seismic activity and modern tectonic movement in Chinese mainland are influenced by the Tethys-Himalayan tectonic belt and the western Pacific tectonic belt. The force source of modern tectonic movement in western Chinese mainland mainly comes from the collision between Indian plate and Eurasian plate, while the southeastern and northeastern parts of Chinese mainland are mainly influenced by Philippine Sea plate and Pacific plate respectively. The situation in North China is more complicated. The western part of North China to the west of Taihang Mountain is mainly influenced by Tethys-Himalayan tectonic belt, and the eastern part of North China to the east of Tanlu belt is mainly influenced by the western Pacific tectonic belt. The central part of North China between them may be a transitional zone of mixed influence. The boundaries between blocks in continental plates are characterized by multi-stage diffuse deformation, and the relative motion amplitude between them is limited, which is very different from the relative motion and deformation mode between lithospheric plates. In the above blocks, the release of strain energy is mainly along the original structural weak zone. The most important structural weak zone is the ancient rift or the crustal necking zone of passive continental margin in the eastern Chinese mainland. In western Chinese mainland, the main boundary faults or central faults of some fold belts are still the main structural weak belts since Paleozoic. Large earthquakes are often distributed in bands along the above-mentioned structural weak zones. The statistical results of recurrence interval of intraplate major earthquakes show that the velocity of block movement and deformation in Chinese mainland intraplate is one or two orders of magnitude less than that of plate boundary, which is an important limitation of intraplate block kinematics model.

Earthquake; Tectonic geodynamics; Chinese mainland

1 Introduction

Since E.Argand first put forward the idea of Himalayan continental collision at the beginning of this century, the geodynamics research in Chinese mainland has experienced the efforts of several generations of Chinese and foreign scholars. So far, this is still a hot spot in the study of geodynamics in the world. Various scientific funds and international cooperation organizations compete for projects, and geoscientists from all over the world flock to Chinese mainland to occupy a place in the study of geodynamics.

This scientific charm of Chinese mainland first comes from its unique tectonic position in the global tectonic framework (Figure 1). From the perspective of global tectonics, Chinese mainland is located at the junction of two largest global-scale giant compressional tectonic belts in the world: the Tethys-Himalayan tectonic belt and the Pacific Rim tectonic belt. The Tethys-Himalayan tectonic belt represents the global convergence and collision of the north and south continents. It crosses Europe, Asia and Africa from west to east, and suddenly ends near east longitude 104 in Chinese mainland. The sudden termination of this giant structure here is obviously because it is blocked by the nearly north-south western Pacific tectonic belt, and its huge nearly north-south compressive deformation here must be coordinated with the structural deformation in eastern Chinese mainland under the influence of the nearly east-west plate convergence movement on the edge of the western Pacific in some way.

Figure 1 Schematic diagram of modern plates and intraplate movements in China and its adjacent areas.

Another important feature of Chinese mainland geology is its complex mosaic structure. Chinese mainland is different from the typical northern landmass (such as western Siberia and Russian) and the typical southern landmass (such as Africa, Australia and South America). It consists of some fragments of the northern continent, some fragments of the southern continent and several small pieces of land between the northern and southern continents. In the long process of assembly history, a large number of relatively tough fold belts of different ages have been formed around the relatively rigid ancient land mass.

The combination of soft and hard structures of Chinese mainland basement, coupled with the strong confrontation and coordination of the above two super-tectonic dynamic systems, will inevitably make its modern tectonic movement and deformation present unique complexity and diversity. It is under this tectonic background that a series of geodynamic phenomena that attract worldwide attention have taken place in Chinese mainland. For example, the rapid uplift, shortening, crustal thickening and eastward compression of the Qinghai-Tibet Plateau; The regenerative uplift of Tianshan and Altai Mountains and the rapid subsidence of Tarim and Junggar basins: the opening and migration of a series of Cenozoic rift basins in northern China; the sustained and slow uplift and eastward movement of South China block; The oblique collision between the Philippine Sea Plate in the east of Taiwan Province Province and the Eurasian Plate and the compressive shear deformation along the southeast coast of China are all related to the relative motion between the soft and hard blocks in Chinese mainland under the current geodynamic system. The study of these hot issues is not only of regional significance, but also of universal significance for understanding the tectonic behavior and deformation mechanism of the entire continental lithosphere of the earth.

Seismic tectonic analysis has always been an important method to study geodynamics. From the point of view of structural geology, earthquake is a rupture-dislocation event in the process of tectonic deformation of lithosphere. At present, there are increasingly mature methods of earthquake geology and geophysics to systematically study the relationship between earthquakes and structures, including the tectonic mechanical background of each earthquake, the process of source rupture and the temporal and spatial distribution of seismic activity in recent geological history. These research results are undoubtedly of great significance for understanding the current geodynamic process of the mainland, especially the relative movement and internal deformation of the continental internal blocks.

In recent ten years, China has cooperated with the United Nations International Plan for Disaster Reduction 10, and carried out extensive work in earthquake zoning, major projects and urban earthquake risk analysis, involving a series of basic research on earthquake structure. A large number of research results are the new basis for us to further understand the geodynamic process in Chinese mainland today. Based on the data of seismic activity, focal mechanism, paleoearthquake, surface rupture and deformation zone accumulated and collected in seismic zoning and engineering earthquake work in recent years, this paper makes a reanalysis of the seismic structural characteristics of Chinese mainland. On this basis, we briefly discuss some problems that people are concerned about in the study of geodynamics in Chinese mainland from the perspective of seismic tectonics.

2 Seismic tectonic zoning and intraplate blocks in China.

The spatial distribution of earthquakes used to be an important basis for determining the boundaries of modern lithospheric plates. Similarly, the blockiness of modern tectonic movement within the continental plate is also reflected in the spatial distribution of earthquakes. However, due to the scattered distribution of intraplate earthquakes, the situation is more complicated, and the research methods should be different. For lithospheric plates, generally, according to the distribution of giant seismic zones, the plate boundaries can be clearly divided, while for intraplate blocks, besides considering the spatial distribution of earthquakes, it is also necessary to analyze the regional characteristics of seismic structures, that is, seismic structural zoning is needed first.

According to the spatial distribution of earthquakes and the regional characteristics of earthquake structures. We divide China into the following 10 seismic tectonic belts (Figure 2): Gan Xin seismic tectonic belt, Qinghai-Tibet seismic tectonic belt, Himalayan seismic tectonic belt, northeast seismic tectonic belt, North China seismic tectonic belt, South China seismic tectonic belt, southeast coastal seismic tectonic belt, central and western Taiwan Province seismic tectonic belt, eastern Taiwan Province seismic tectonic belt and South China Sea seismic tectonic belt.

In the above-mentioned 10 seismic tectonic belt, there are two seismic tectonic belts, namely Himalayan seismic tectonic belt and eastern Taiwan Province seismic tectonic belt, which correspond to Himalayan plate collision belt and eastern Taiwan Province plate collision belt respectively. In addition, there are two seismic tectonic belts, namely, the central and western Taiwan Province Province seismic tectonic belt and the South China Sea seismic tectonic belt, which can be regarded as the transition between the plate edge and the intraplate tectonic belt. The other six seismic tectonic zones have the nature of intraplate seismic tectonic zones.

Comparing the positions of these six intraplate seismic tectonic zones with the distribution of Precambrian crystalline basement, it can be seen that most intraplate seismic tectonic zones are centered on one or two Precambrian ancient landmasses and generally surround the fold zones since Paleozoic. For example, the seismic tectonic area in North China is centered on the famous Sino-Korean platform; The northeast earthquake zone is centered on Songnen block and surrounded by Paleozoic fold belt. The seismic tectonic area in South China is centered on the west of Yangtze Platform and surrounded by Paleozoic fold belt on the east. The southeast coastal seismic tectonic area is extended with Cathaysian ancient land block as the core; Gan Xin seismic tectonic area is composed of Tarim platform, junggar block and Paleozoic fold belt between them. The situation of Qinghai-Tibet seismic tectonic region is special, mainly composed of fold belts in different periods since Paleozoic, but also mixed with a series of smaller ancient landmasses, such as Qaidam block, Qiangtang block, Gangdise block and Songpan-Bikou block. Each of the above-mentioned seismic tectonic areas has its own unique characteristics of modern tectonic stress field, crustal deformation, seismic energy release mode and block movement direction. Therefore, under the modern tectonic movement system, it should be regarded as a first-class block in the Chinese mainland plate.

Fig. 2 Distribution of epicentre and seismic tectonic zoning in China.

ⅰ-Gan Xin seismic tectonic area; Ⅱ-Qinghai-Tibet earthquake tectonic area; Ⅲ-Himalayan seismic tectonic region; Ⅳ —— Northeast first-class seismic tectonic area; ⅴ —— Seismic tectonic area in North China; ⅵ-South China seismic tectonic area; ⅶ —— the first-class seismic tectonic area along the southeast coast; ⅷ —— Seismotectonic area in the central and western Taiwan Province Province; ⅸ —— the seismic tectonic area in the east of Taiwan Province Province; ⅹ —— South China Sea Seismic Structure Area

The boundaries of these intraplate blocks generally follow the suture lines of pre-existing fault zones or ancient blocks, but they do not necessarily coincide with the boundaries of previous tectonic units.

Different from the plate boundary, the seismic activity of the block boundary in the plate shows obvious dispersion in many paragraphs, and the intensity of seismic activity is also very uneven. According to the intensity and distribution characteristics of seismic activity, the boundaries of primary blocks in the plate can be divided into three types:

(1) Linear fast moving boundary. For example, on the northern boundary of the Qinghai-Tibet block, a large-scale strike-slip movement occurred along the Altun fault and the piedmont fault of Qilian Mountain, and earthquakes were densely distributed. The block boundary in this plate is similar to the plate boundary, and the relative motion rate between blocks on both sides of the boundary is relatively high, which can reach the order of 1cm/a at the maximum.

(2) Diffusion boundary. For example, in the eastern margin of the Qinghai-Tibet block and the western section of the boundary between the North China block and the South China block, earthquakes are distributed in a wide band along many faults, and the relative motion between blocks may generally have a considerable amplitude, but the displacement does not occur along one or two main faults, but is realized by a distribution deformation with a considerable width.

(3) Weak motion boundary. For example, the boundary between North China and Northeast China, the eastern part of the boundary between North China and South China, and the boundary between South China and Southeast coastal areas are weak in seismic activity and relative motion between blocks.

These characteristics of seismic activity at the boundary of intraplate blocks show that the relative motion of blocks within continental plates is quite different from that between plates in terms of activity intensity and mode.

3 First-order block motion model of Chinese mainland plate

Under the present geodynamic system, all intraplate blocks in Chinese mainland are undergoing relative movement and internal deformation adjustment in different ways.

At present, there are nearly 100 Holocene prehistoric earthquake relics discovered and dated by field seismic geological investigation in China [6]. The repetition of prehistoric earthquake events has been confirmed by careful trench exploration in many places, and the recurrence interval of large earthquakes has been estimated by using various dating methods such as 14C, thermoluminescence and ESR.

From the recurrence interval of prehistoric earthquakes listed in table 1, it can be seen that the recurrence interval of large earthquakes in Qinghai-Tibet block and its surrounding areas is generally1000 ~ 2000 a; The recurrence interval of Gan Xin block earthquake is about 2000～3000 a；; The recurrence interval of large earthquakes in North China block is generally 2000 ~ 5000 a or longer, which is one or two orders of magnitude different from the seismic belt at the plate edge, and it is only100 ~ 200 a. This fact, together with the dispersion and weakness of the boundary movement within the plate mentioned above, shows that the relative movement speed and scale of the blocks within the continental plate are limited. Driven by the surrounding plates, there is a certain range of relative motion between Chinese mainland blocks, which can be used to adjust the motion between plates. However, it seems controversial whether there is a continental escape with an annual rate of horizontal movement as high as cm as some foreign scholars think.

Table 1 Repeat Interval of Prehistoric Earthquake Events in Chinese mainland

Judging from the recurrence interval of major earthquakes, it can be considered that the horizontal movement of intraplate blocks with an annual rate of cm in Chinese mainland is very rare. Generally, the boundary of the first-order block in the plate and the main active faults in it have a horizontal movement rate of millimeter level, which is high in the west and low in the east. At the same time, slow or extremely slow intraplate fracture activities below millimeter level are quite common in eastern Chinese mainland. It should be pointed out that "slow" or "extremely slow" here is only relative to the activity rate of the plate edge. These "slow" or "extremely slow" intraplate fracture activities can also cause destructive earthquakes, leaving various structural traces, but their recurrence period is relatively long and the time nonlinear characteristics are more complicated. This is the difficulty of intraplate earthquake prediction and engineering seismic safety evaluation.

6 conclusion

The progress of geodynamics research depends largely on the development of observation technology. In a sense, it can even be said that what kind of observation technology will have what kind of geodynamics.

Although people have made many important progress in deep exploration, geophysical data interpretation, space technology application, geochemistry and geological dating technology in recent ten years. However, it should be noted that, on the whole, our ability to explore the depths of the earth and trace back geological history is still quite limited. There are many "blind areas" and "fuzzy areas" in observation ability. In this case, many inferences and explanations at present (including some understandings put forward in this paper) are only of stage significance, and some may be proved to be based on misunderstandings with insufficient information in the future.

In the next twenty or thirty years, how much progress can be made in geodynamics research does not depend entirely on the efforts of geoscientists, but largely on the technical support that the overall scientific and technological level of mankind can provide to geoscientists. However, as a geoscientist, we should not just wait for the development of other disciplines to bring us new "technical swords", but should actively look for them in other disciplines' arsenals, actively follow the technological development frontiers of other disciplines, or add our own "creativity" to gather a new generation of "experts" and "Mo Xie" in earth science.

Acknowledgement This paper was supported by the National Natural Science Foundation (No.:49572 155) and the key projects of Seismological Bureau of China (No.:85-07-0 1 and 95-05-02). The author thanks Ding Guoyu, Wang, Deng Qidong, Shi, and others for their support and help to the seismic geological work for many years, and thanks Mr. Qian Xianglin of Peking University for his enthusiastic suggestions on the regional structure of China and the crystalline basement of the mainland. In addition, there are Zhou Yongdong, Yang Wenlong and Zhang Hua. Everyone has participated in this work to varying degrees, and I would like to express my sincere thanks here.

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