Table of Content

04 June 1980, Volume 2 Issue 2

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Brief Report

EVIDENCE OF PLATE ACTIVITY IN HIMALAYAS AND A MODEL FOR FORMATION OF QINGHAI-XIZANG PLATEAU

Pan Yusheng, Wang Yipeng, Chang Chengfa

1980, 2(2):  1-10. 

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Based on the data collected in the recent years by the scientific expedition in the Qinghai-Xizang region, the principal structural features of Himalayas and Qinghai-Xizang Plateau are discussed from viewpoint of plate tectonic theory.In the light of a lot of data, such as on the geological history, the sedimentary characteristics, the distribution of Gondwana sedimentary facies, ophiolite suite, melange and exotic blocks, the crustal deformation nature, the regularity of magmatic activity and paired metamorphic belt on both sides of Yarlung Zangbo River, this work deals with the pattern and nature of plate motion, makes a subdivision of Himalayan orogeny and gives a preliminary analysis of its character as well.A possible model on formation of Qinghai-Xizang Plateau by multi-underthrusting and multi-matching is presented and it is suggested that the Bangong Co-Dongqiao tectonic belt can probably serve as the northern boundary of Gondwana land. And we propose a new mechanism that the Yarlung Zangbo Jinag tectonic belt was opened in Mesozoic and closed in Early Tertiary.

THE RESULTS OF DEEP SOUNDING BY USING CONVERTED WAVES OF EARTHQUAKES IN THE BEIJING-TIANJIN- TANGSHAN REGION

Shao Xuezhong, Zhang Jiaru, Chen Xuehua, Zhang Siya

1980, 2(2):  11-20. 

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During the past 4 years (1975-1979) eight seismic profiles have been compiled by using the method of earthquake-generating converted waves in the Beijing-Tianjin-Tangshan region. Some details about the method of interpretating these data and the important results obtained are presented in this paper. This method is very useful in the regional investigations for earth’s crust and upper mantle.The comparison of the deep-seated structures and the distribution of earthquakes in the studied region indicates that all hypocentres of strong earthquakes just filled in some particular position of the granitic layer and that the Tangshan and Mafan strong earthquake regions are similar each other in the deep-seated structure characteristics, such as relative uplift of the granitic layer, intensive depression (concavity) of interfaces in upper mantle, sharp thickening of lithosphere and existence of deep-seated faults in the region under investigation.

THE FEATURES OF HIGHLY ELECTRICAL CONDUCTIVITY LAYER IN NORTH CHINA AND NORTHWEST CHINA REGIONS

Gu Qun Sun, Jie Shi Shulin, Shi Zhangsong

1980, 2(2):  21-29. 

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The magnetotelluric soundings reveal the existence of high conductivity layer in the middle crust and upper mantle in certain regions of North China and Northwest China. It has an important significance for studying the structures within crust and upper mantle, physical state of rocks, crustal seismicity and so on. This paper summarizes all the data of highly electrical conductivity layers in these regions and describes their distribution regularity and their conicidence with the tectonic characteristics and the temperature, gravity and wave velocity fields. Finally, the genetic mechanism of these layers and its relation to the continental shallow shocks are briefly discussed herein.

A DISCUSSION ON THE LOCATION OF 1970 TONGHAI EARTHQUAKE OF MAGNITUDE 7.7 IN YUNNAN PROVINCE

Hang Mukang

1980, 2(2):  30-30. 

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THE DEVELOPMENT OF PRECURSORY FIELD AND THE TECTONOMECHANICAL CONDITION IN THE HAICHENG-TANGSHAN EARTHQUAKE SERIES

Jiang Pu, Deng Qidong

1980, 2(2):  31-42. 

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A earthquake series is referred as the events being successively occurred in a definite time and in the same seismic area. These events are characterized by the common developing process and an unified evolutional history of precursory field. We consider that the seismic precursory field is nothing but a regional field beyond a certain source region.For the North China fault block region can be recognized two earthquake series, Xingtai-Bohai and Haicheng-Tangshan series ever since the occurrence of Xingtai earthquake of 1966 to the year of 1978. The anomalies in the precursory field of this series started in 1972, after 1976 Tangshan earthquake it has been gradually becoming quiet. With regard to the developmental process of this field and its spatial distribution can be distinguished following five anomaly-development periods and relevant six stages.1. The primary period (A), during which the precursory field is developed from the stage in a scattered form (Ⅰ) to a divergent stage on a large scale (Ⅱ).2. Accelerated period (B) before Haicheng earthquake, during which the precursory field migrates northward entering into a convergent stage (Ⅲ) in the extent.3. Developing period (C), during which the precursory field enters into a rediver-gent stage (Ⅳ).4. Transitional or accelerated period before the Tangshan earthquake (D), during which the precursory field enters into a reconvergent stage(Ⅴ).5. Impending period before the Tangshan earthquake (E), during which the precursory field enters into a divergent stage (Ⅴ).It is suggested that the formation and development of the precursory field are closely related to the conjugated shear-rupturing network, and the distribution and development of anomalous points or zones are constrained by slipping of conjugated shear-ruptures and stress concentration at the various tectonic position. Consequently, the precursory phenomena would not be limited in the source area and its neighbouring regions, and the multi-anomalous regions (zone) are formed.Finally, the relationship of the regional precursory field with the earthquake prediction is studied. In general the regional precursory field does not imply only one event, perhaps an earthquake series, and the precursory extent has a certain relation to the magnitude of an event. To discriminate the location of one shock from the earthquake series it is necessary to pay attention to the spatial changes in the precursory field, such as the migration and convergence of anomalies. On the background of regional precursory field, the macroscopic anomalies in a great amount and concentrated form together with some abrupt anomalies can be taken as a precursory signal of a big imminent earthquake.

TANGSHAN EARTHQUAKE AND THE EARTHQUAKE RISK AREA

Ma Jin, Zhang Botao, Yuan Shurong

1980, 2(2):  43-54. 

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In this paper considering different viewpoints of seismogenic structures of Tangshan earthquake, the seismogenic mode of Tangshan earthquake has been studied using the finite element method. The characteristics of Tangshan earthquake are also described,A preliminary discussion has been made on identifying the earthquake risk areas on the basis of both regional stress direction and condition of stress release.

THE BASIC CONFIGURATION OF CRUSTAL STRUCTURE IN NORTH CHINA REGION AND ITS RELATION TO THE EARTHQUAKES FROM GRAVIMETRIC DATA

Wei Menghua, Shi Zhihong, Yin Xiuhua, Liu Zhanpo

1980, 2(2):  55-60. 

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Using gravity data the depths of Moho and Conrad discontinuities in North China region are calculated and thus the basic configuration of crustal structure in this region can be determined. The results obtained indicate that three seismic belts, namely, NW trending belt extended from the Bohai Bay through Beijing to Zhangjiakou, Hebei belt with the Mt. Taihangshan as a symmetric axis and Shanxi belt, located in the relatively uplifted portion of upper mantle in regard to their depths. Therefore, it is considered that isostatic adjustment exerts effect on the development of geological structures and the formation of their basic configuration to a large extent. Consequently, these processes control the seismic activity in North China region.

ANALYSIS OF TECTONIC CONDITION FOR 1978 HAICHENG EARTHQUAKE WITH MAGNITUDE OF 6.0

Xiang Hongfa, Ding Menglin, Song Fangmin

1980, 2(2):  61-69. 

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The similarity in the seismogenic structure, rupture mode and the focal stress field indicates that the earthquake of magnitude 6.0 occurred in Haicheng on May, 18, 1978 can be regarded as a strong aftershock of the 1975 Haicheng earthquake with magnitude 7.3. After main earthquake there have not been intensive activities of aftershocks. But they toke place much later, it may be explained by the disrupture of the locked section of NNE running active structure.The authors of this paper re,fer the strong shocks taken place within 10 years after the main shock to be the late strong aftershocks. By the synthetic analysis of the geotectonic features and the nature of the whole seismic activity series of the Haicheng earthquake, it is shown that the multi-locking structures must be a necessary background and the locations for generating the late strong aftershocks. Its anomalous precursory, however, was less obvious and appeared only along the direction obliquely aerossing the principal structure in area concerned. Some characteristics of the late strong aftershocks and relevant strike-slip mechanism are slao discussed herein.

THE NEWLY ACTIVATED SEGMENT OF TANCHEN-LUJIANG FAULT BELT NEAR ZHANGSHANZHA, XINGYI COUNTY, JIANGSU PROVINCE

Zhang Jingwen, Peng Gui, Jiao Wenqiang, Li Guiying, Zhao Xitao

1980, 2(2):  70-70. 

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CHRONOLOGY OF LATE QUATERNARY DEPOSITS ALONG THE COAST OF THE BOHAI BAY

Peng Gui, Zhang Jingwen, Jiao Wenqiang, Chen Yijian, Li Daming, Li Guiying, Zhao Xitao

1980, 2(2):  71-78. 

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Radiocarbon datings indicate that the marine deposits at the depths of 23-40m along coast of Bohai Bay were formed sonn after the slope of the peat development (32000 -22900yr. BP), which can be comparable with the sub-interglacial periods of the last glacial epoch(about 44000-29000yr. B P) and is agreeable with the high sea level period over the whole world in the late stage of the Late Pleistocene (about 35000-23000yr. BP).The lacustrine-swamp peaty deposits or puddly deposits of various thickness have a wide distribution at the depths of 12-16 m beneath the ground surface and 5-6m depth below the Bohai sea floor with 20-30m water depth. In the light of the radiocarbon datings the peaty deposits concerned can be largely placed in Early Holocene (about 10000-8000yr. BP) and named as Lijiabao. Formation, it can be correlated with the Purangdian Formation of Liaonan region. Above the peaty deposits are in succession developed a series of marine sediments. From the radiocarbon datings of shelly bed and oyster-bearing bed in offshore and epicontinental sediments from the underlying peaty deposits and marine sediments, especially from the top of the latter, the marine sediments may be formed in Middle Holocene (about 8000-2500yr. BP) and named as Hou-chengzhuang Formation which can be correlated to the Dagushan Formation of Liaonan region. Most of recent deposits near ground surface are of fluvial facies, and the peaty deposits are also found in the swamp or the ancient river course. From the radiocarbon datings it can be suggested that the marine sediments beyond the Ⅱ chenier ridge along the western coast-of the Bohai Bay and the continental sediments within the Ⅱ chenier ridge both have the age less than 2500yr. belonging to the Late Holocene and correlating to the Zhuanghe Formation of Liaonan region. It is referred to the Qikou Formation.The lithological changes happened in this region and the datings of Holocene deposits indicate that there has been a transgression in Holocene period and sea water arrived at the studied region 7000 years ago. Later on the transgression reached its maximum in the region 6000-5000 years ago when many regions such as Leting, Luannan, Ninghe, Tianjin, Cangzhou, Huanghua and Haixing along the coast of the Bohai Bay are submerged or affected by sea water. About 4000 years ago there was started a regression forming successively the Tongju-Qianmiaozhuang (4700-4000yr. BP), the Zhang-guizhuang -Changzhuang (3800-3000yr. BP), the Baishaling-Nigu-Qikou(2500-1500yr. BP) and the Chengtougu-Dagu chenier ridges (600-500yr. BP).

STATISTICAL ANALYSIS OF FACTORS CAUSING LIQUEFACTION OF SAND DURING TANGSHAN EARTHQUAKE

Zhu Shulian

1980, 2(2):  79-80. 

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