Table of Content

06 June 1990, Volume 12 Issue 2

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Brief Report

CONTRIBUTIONS OF PROF. XU YU-JIAN TO THE GEOLOGICAL SCIENCES FOR 50 YEARS

Editorial Board and Editorial Office of SEISMOLOGY AND GEOLOGY

1990, 12(2):  97-97. 

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THE SURFACE RUPTURES OF DANXUNG (TIBET) EARTHQUAKE (M=8) IN 1411

Wu Zhang-ming, Shentu Bing-ming, Cao Zhong-quan, Deng Qi-dong

1990, 12(2):  98-108. 

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A great earthquake(M=8)occurred at southwest Danxung,Tibet Autonomous Region,on September 29,1411. A surface ruprure zone as long as 136 km was formed during the event. It consists of two subparallel secondary fractures which are different in their strikes and modes of motion. The maximum vertical displacement is 8-9m.,while the maximum horizontal one 11-13m. The surface rupture is a discontinues fracture characterized by tensile shearing. The geometry and characteristics of the displacement of the rupture zone are summarized,and the seismogenic structure of the 1411 earthquake is discussed in this paper as well.

THE QIONGHAI CRUSTAL TILT OBSERVATION SYSTEM AND THE SEISMIC ACTIVITY IN WESTERN SICHUAN PROVINCE,CHINA

Zhang Chong-li, Zhang Cun-de Xiang Jia-cui, Li Tie-ming, Zhang Da-wei

1990, 12(2):  109-120. 

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In this paper,we introduce the measurements of crustal tilt using the Qionghai Observation System and a preliminary approach to the relation between the crustal plane tilt observed by the System and seismic activity in the region around the Qionghai Lake,at Xichang.The results show that the observed accuracy may reach the magnitude of 10^-7-10^-8 radian. The processes of the crustal stress accumulation and release are characterized by the activity of moderate earthquakes (M≥5.0) whose epicenters are located within a range away from the Qionghai Lake,and are reflected to a certain extent by time dependent processes of the crustal tilt observed by the System.The short-term and impending precursors of moderate earthquakes are similar to those of the great ones,featuring of trending and abrupt anomalies.

PRINCIPAL CHARACTERISTICS OF VERTICAL CRUSTAL DEFORMATION IN SOUTHEAST CHINA

Hu Hui-min, Shen Yong-jian

1990, 12(2):  121-130. 

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The vertical crustal deformation map of southeast China has been compiled on the basis of available data from repeated precise levelling (the first and second orders levelling). According to the characteristics of vertical deformation,southeast China can be divided into 3 regions. The northern and southern regions are dominated by uplift,while the central region by subsidence. The basic characters of the recent tectonic movement in the area and the variation of regional tectonic stress field have also been analysed and discussed in detail,based on surveying data across faults.

THE CRITERIA AND SEISMOGENIC PROBABILITIES OF STRONG SHALLOW EARTHQUAKE

Wang Sheng-zu

1990, 12(2):  131-140. 

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According to the multi-criterion principle for identifying potential hypocenter,it is suggested that the occurrence of strong earthquake should satisfy not only the quasi-static strength condition,but also those of sudden release of enough strain energy. All these conditions can be expressed by the sudden-stress-drop criterion,the total-energy criterion,and the strength criterion.A set of probabilistic expressions of these criteria and the method of estimating general seismogenic probability are established in this paper,and examined by the calculations of 28 earthquake cases in China.

RECONSTRUCTION OF THE BASEMENT VELOCITY IMAGE BASED ON HEAD WAVE

Xue Guang-ying, Feng Rui

1990, 12(2):  141-148. 

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Seismic tomography has been applied to seismology as a new technique. The application of the technique,however,is sometimes limited by the incomplete projection and parameter coupling of the models used in geosciences. In this paper,a method of reconstructing the velocity image of sedimentary basement or Moho discontinuity based on head wave data is developed.This method can also be used to correct further the depth of hypocenter. In the computation,the algebraic reconstruction technique (ART) is adopted and modified. In order to improve the computation stability and convergence of ART and SIRT algorithms,a reasonable weighting factor is suggested. The corresponding resolution and variance of different algebraic reconstruction algorithms are also analysed through numerical estimation.Using P₈ wave data from Profile Ⅰ and Ⅱ of deep seismic sounding in Hainan Island,the velocity image of the basement in the region is reconstructed. The result shows that two low velocity zones are well consistent with the important Chunjiang-Zhonghe and Ganchong-Mutang surficial fault zones,indicating that the two fault zones cut through the sedimentary basement. In addition,the segmentation features of the fault zones are well reflected on the basement velocity image.

THE OBSERVATION OF MAGNETOTELLURIC AND ELECTRICAL CHARACTERISTICS OF THE CRUST-UPPER MANTLE ALONG HUBEI XIANGFAN-FUJIAN LUOYUAN PROFILE

Deng Qian-hui, Liu Guo-dong, Liu Jin-han, Zhang Mu-shen, Tang Jie, wang Guan-yong

1990, 12(2):  149-158. 

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Magnetotelluric measurements were made at 23 sites along more than thousand kilometres long profile from Xiangfan,Hubei Province to Luoyuan,Fujian Province.The profile crosses through different geological tectonic units.The results of MT clearly reflect the characteristics of electrical structure of the crust and upper-mantle,such as shape and boundary of tectonic units,position of fault,development of the high conductive layer in the crust and upper-mantle etc.The MT-results can provide some useful informations for understanding of deep structural background,and inquiring into the relationship between the deep and shallow structures in the region.

THE INVESTIGATION ON MAGNETOTELLURIC DATA OF THE JUAN DE FUCA PLATE

Zhao Guo-ze, T. Yukutake, Y. Hamano, H. Utada, J. Segawa, J. H. Filloux, L. K. Law, T. White, A. D. Chave, P. Tarits

1990, 12(2):  159-167. 

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As a part of the EMSLAB project,the principal purpose of the study is to provide the constraints on electrical structures of the lithosphere and asthenosphere beneath Juan de Fuca plate and its vicinity.We have carried out two-dimensional modelling of the magnetotelluric data provided by EMSLAB group. One-dimensional method has been used in inversion,and then finite clement method used in two-dimensional modelling. The known superficial features are determined at various stages of the modelling. In order to confirm the uniqueness of the model,we have changed some important parameters of the model and assessed the increase of misfit.The model with best-fitting data shows that the lithosphere of the Juan de Fuca plate is a resistive layer with a thickness of about 40km,which is thinner than that of the older plate. The thickness of the asthenosphere is about 60km,underlain by less resistive layer overlapping a conductive half space.

DISCUSION ON THE MODERN TECTONIC STRESS IN WESTERN HUBEI PROVINCE BASED ON SEISMIC WAVE

Xue Jun-rong, Li Rong-chuan

1990, 12(2):  168-172. 

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In this paper,by using the structural shear stress formula for recorded seismic wave of small earthquakes,the shear stress valuesin in western Hubei Province,are estimated.It is found that the stress level is as low as 1 bar to 50 bar in this region.

DETERMINATION OF THE STRATIFICATION PLANE OF CENOZOIC VOLCANIC ROCKS AND ITS TECTONIC IMPLICATION

Cheng Guo-liang, Sun Yu-hang, Zhang Min-jie, Xu Xi-wei, Han Zhu-jun

1990, 12(2):  173-177. 

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The mathematical expression of the original horizontal plane can be obtained by coordinate transformation,provided that the following conditions are fulfilled:1. The block where the volcanic rocks outcroped has not been displaced in north-south direction; 2. The statistical average vector of the remanent magnetization follows the axial geocentric dipole magnetic field. In that case,it will enable us to get more reliable amounts of the rotation of the block around the vertical axis,and that of the tilting of the strata around the horizontal axis,based on paleomagnetic method. In addition,it will be possible to apply paleomagnetic method to the study of the tectonic movement of volcanic rocks with ambiguos attitute.

A NEW EVIDENCE FOR RECENT CRUSTAL SUBSIDENCE OF THE SOUTHERN COAST OF HANGZHOU BAY——THE DISCOVERY OF BURIED ANCIENT TREES IN SHAOXING CITY,ZHEJIANG PROVINCE

1990, 12(2):  178-178. 

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RADIOCARBON AGES OF SEDIMENTS IN ZHANGSHANZHA GROUND FISSURES AND THEIR GEOLOGICAL SIGNIFICANCE

Peng Gui, Jiao Wen-qiang

1990, 12(2):  179-182. 

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In this paper the authors discuss the reliabiliy of radiocarbon ages of black clays filled in Zhangshanzha ground fissures and the ages of calcareous concrete outcroped on both sides of the fissures. In addition,the stratigraphical division of late Quaternary,the ages and formation mechanism of the ground fissures have been discussed as well.

THE LATEST NWW-TRENDING TECTONIC ZONES AND TECTONIC BACKGROUND OF LARGE EARTHQUAKE IN FUJIAN-GUANGDONG-TAIWAN COASTAL AREA

Bi Fu-zhi, Yuan You-shen, Fan Guo-sheng

1990, 12(2):  183-192. 

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The latest active tectonic zones,consisting of several NWW-trending uplifts and denressions. have been formed gradually in Fujian-Guangdong-Taiwan coastal area since Taiwan orogeny. These tectonic zones along with the known NE-NNE-trending uplift-depression zones have constituted a regional mosaic structure framework.In addition to plate collision and subduction,as well as the upwarping of upper mantle materials,the principal features of the region,such as the large amplitude of the fault throw or the upheaval of Moho surface,the great thickness of low velocity layer,crustal thinning,significant differential movement of the earth surface in the last 2000—3000 years and the NWW-trending tectonic activity in Taidong offshore,also provide important tectonic background for the occurrence of large earthquake.