DIFFERENCES OF CRUSTAL STRUCTURES IN NORTHEASTERN EDGE OF TIBET PLATEAU, ORDOS AND TANGSHAN EARTHQUAKE REGION IN NORTH CHINA-RESULTS OF DEEP SEISMIC SOUNDING

DIFFERENCES OF CRUSTAL STRUCTURES IN NORTHEASTERN EDGE OF TIBET PLATEAU, ORDOS AND TANGSHAN EARTHQUAKE REGION IN NORTH CHINA-RESULTS OF DEEP SEISMIC SOUNDING

STRUCTURAL PROPERTY OF THE CRUST AND THE UPPER MANTLE IN THE TANGSHAN EARTHQUAKE REGION——A METHOD OF COMPUTING BODY WAVE THEORETICAL SEISMOGRAM AND THE RESULT OF ANALYSIS OF 2-D INHOMOGENEOUS MEDIA

By analysing the short period geodetic data obtained in the 1976 Tangshan earthquake region before and after the earthquake, it can be demonstrated that precursory creep had occurred in the upper crust of the epicentral area prior to the main shock and the fault continued to slip after it but the rate fell off exponentially with time.

Strong earthquakes occurred frequently in North China area in history,but it showed tranquil since 2000.In order to analyze this phenomenon,we studied the joint area of Shanxi,Hebei and Inner Mongolia,the joint area of Hebei,Shandong and Henan province,Tangshan earthquake region, Yinshan Mountain and Yanshan Mountain earthquake belt with linear probability method that based on spectral analysis of earthquake.

The authors have processed the observed data in Tangshan earthquake region by the method proposed in this paper, and obtained the tomographic results of the middle and upper crust structures in this region.

The earth crust evolution under the North China basin since Cenozoic Era can be roughly divided into two stages: firstly, this basin was progressively extending since early Tertiary, with the result of wide development of normal faults and a series of depressions, where accumulated voluminous Cenozoic sediments. Then the extension rate of the earth crust was gradually decreasing at the end of Late Tertiary and eventually turned to be ENE-ward horizontal compression and vertical movement.The seismic.sounding...

The earth crust evolution under the North China basin since Cenozoic Era can be roughly divided into two stages: firstly, this basin was progressively extending since early Tertiary, with the result of wide development of normal faults and a series of depressions, where accumulated voluminous Cenozoic sediments. Then the extension rate of the earth crust was gradually decreasing at the end of Late Tertiary and eventually turned to be ENE-ward horizontal compression and vertical movement.The seismic.sounding shows that the Xiadian and Tangshan-Ninghe faults have deeply cut through the crust and the thickness of the crust is 36km at Tangshan and 38km at Xiadian.The gravity variation of the Tangshan earthquake region from March to July,1976 before mainshock was about 200 microgals. The depth of the density interface (or discontinuity) causing the gravity variation of the Tangshan area as conjectured from the Beijing-Leting gravity variation curve is about 36km. It has uplifted about 1m under Tangshan. The amount of density change at Tangshan was estimated from the density interface formula as 2.17 × 10-4g/cm3 and - 1.46 × 10-4g/cm3 at Xiadian. Tha gravity anomalies at Tangshan and Xiadian must be considered together as two integral sides of the same movement entity. It is supposed that the North China basin has been subjected to ENE-ward horizontal compression, which gave rise to the activation of the Tangshan deep fault and the upwelling of upper mantle hot material to increase the density.

In order to set up the relation between fault creeping and crustal deformation in the neighbourhood of it, in this paper, analytical expressions for quasi-static deformation field produced by a slipping f auit of arbitrary dip in a linear viscoelastic half-space are given in closed forms. The forms of slip-time function in the expressions are determined empirically, and the rock media are considered as generalized Kelvin's or Maxwell substances. The effect of viscoelasticity on the deformation field has been...

In order to set up the relation between fault creeping and crustal deformation in the neighbourhood of it, in this paper, analytical expressions for quasi-static deformation field produced by a slipping f auit of arbitrary dip in a linear viscoelastic half-space are given in closed forms. The forms of slip-time function in the expressions are determined empirically, and the rock media are considered as generalized Kelvin's or Maxwell substances. The effect of viscoelasticity on the deformation field has been discussed through numerical computation. By analysing the short period geodetic data obtained in the 1976 Tangshan earthquake region before and after the earthquake, it can be demonstrated that precursory creep had occurred in the upper crust of the epicentral area prior to the main shock and the fault continued to slip after it but the rate fell off exponentially with time. The slip parameters which fit the observed data are estimated and the implication of all these results are briefly discussed.

In order to study the propagation of waves in 2-D laterally inhomogeneous isotropic and absolute elastic media with curved interfaces, it is appropriate to calculate the wavefield using the solution of high frequency approximate expansion for elastic wave equation, under certain conditions.For the purpose of solving the system of twopoint ray tracing under certain initial conditions, the standard program (the method of Runge-Kutta) has been used. The geometrical spreading factor is determined by solving a system...

In order to study the propagation of waves in 2-D laterally inhomogeneous isotropic and absolute elastic media with curved interfaces, it is appropriate to calculate the wavefield using the solution of high frequency approximate expansion for elastic wave equation, under certain conditions.For the purpose of solving the system of twopoint ray tracing under certain initial conditions, the standard program (the method of Runge-Kutta) has been used. The geometrical spreading factor is determined by solving a system of two linear ordinary differential equations of the first order (the socalled dynamic ray tracing system) with a modified Euler's method. These are realized by the Seis 83 progam.In the present paper, the authors gave calculated with the Stis 83 program the observed data from two profiles (the Baigezhang-Fengnan-Fengning-Zhenglangqi profile and the Cang-zhou-Tianjin-Kelaqinzuyi profile) which pass through the epicentral region of the Tangshan earthquake where they intersect each other approximately perpendicularly. The structural property of the crust and the upper mantle in the Tangshan earthquake region and the relation between the Seismicity and the velocity distribution in the low-velocity layer of the crust are discussed.