唐山地震前地壳的异常隆起及无震蠕动

ON THE ANOMALOUS CRUSTAL BULGE AND ASEISMIC CREEP PRIOR TO THE 1976 TANGSHAN EARTHQUAKE

  • 摘要: 根据唐山地区震前11期水准复测资料,对本区的地壳形变的时空变化特征进行了分析.结果表明:唐山地震前1968年-1969年震中区发生了幅度为50毫米、持续时间2年的异常隆起,并在唐山及宝坻附近的唐山断层和蓟运河断层显示出应变积累与无震蠕动叠加的作用.采用了均匀应变积累及弹性位错理论分别计算了唐山、宝坻附近测点由于各处断层蠕动引起的垂直位移理论值,按残差均方根最小的原则确定历年应变积累及蠕动断层参数的最佳值.1969年-1975年唐山地下发生蠕动的断层走向为北东47、倾角87、倾向南东的右旋正断层,断层长度为8公里、宽度6公里、断层上界深度为2公里、走滑与倾滑错距分别为104厘米及8厘米,平均应变积累速度为0.910-7/年.同时得到断层走向、倾向蠕动速度分别为18.6厘米/年及1.4厘米/年.另外由蓟运河断层结果得到.宝坻附近蠕动断层的规模比唐山小,但蠕动速度比唐山大.本文分析了上述异常隆起及蠕动断层与唐山地震的关系,认为地壳的异常隆起是孕震的早期显示,而无震蠕动可能是浅源地震的又一前兆特征.唐山地震经历了长期应变积累-体积膨胀-蠕动-形变反向(或蠕动速度减慢)-发震的过程.

     

    Abstract: Based on the observed data of 11 repeated surveys in the Tangshan area prior to the 1976 earthquake, an analysis has been made for the temporal and spatial variations of erustal deformation within this region. The result indicates that before the earth-quake, from 1968 to 1969 a surface bulge of amplitude 50mm, lasting for two years, took place in the epicentral region. Near Tangshan and Baodi, on the Tangshan fault and the JiYunhe fault, superposition of accumulated strains and aseismic creeping are shown.By adopting the theories of uniform strain accumulation and ,elastic dislocation, theoretical values of vertical displacements caused by creeping at the various parts on the faults respectively, for the observation points near Tangshan and Baodi have been calculated and by the principle of least residual root mean squares to determine the optimal values of the parameters of strain accumulating and creeping fault.From 1969 to 1975, the creeping fault underneath Tangshan strikes 47 NE, dipping 87toward SE, right-lateral normal fault in character. Its length is 8 km, width 6 km, depth of burial 2 km, strike and dip slips amount, to 104 cm and 8 cm respectively, average rate of strain accumulation; 0.910-7/year. At the same time, the creep rate of the fault in the strike and dip directions are found to be 18.6 cm/year and 1.4 cm/year respectively. Besides, from the result on the JiYunhe fault, the creeping fault near Baodi is smaller in dimension than near Tangshan, but its creeping rate is greater than near Tangshan.In this paper, it is also discussed in connection with the relation of the above-mentioned anomalous erustal bulge with the occurrence of the Tangshan earthquake. Anomalous erustal deformation is considered as the early indication of the development of the earthquake, while aseismie creeping probably may be another precursory feature of a shallow earthquake. The occurrence of the Tangshan earthquake had come through a long process of development of strain accumulation-volumetric dilatancy-inverse erustal deformation (or decrease of creeping rate).

     

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