唐山地震的某些前兆及上部地壳的向斜弯曲现象

EARTH-RESISTIVITY, GEOMAGNETIC ANOMALIES AND GROUND SURFACE SUBSIDENCE AND SYNCLINAL BENDING OF THE UPPER CRUST PRIOR TO THE 1976 TANGSHAN EARTHQUAKE

摘要:

1976年唐山强震发生在我国地电阻率、地磁观测、精密水准重复测量等多种观测网分布较密的地区,附近渤海沿岸还有数个长期记录的验潮站。震前地电阻率和地磁二种物理量出现了地理范围基本吻合、分布广阔的异常区。根据自然条件下岩(土)层压缩时电阻率变化的实验结果,认为只在1973~1976年间才出现的地电阻率区域下降,系地表浅层压密的结果,其压缩线应变的数量级约为3×10^-5。有意义的是基本在此同一区域内,1975~1976年间还观测到ΔZ和ΔF减小的效应。根据航空磁测测区内磁性地质体埋藏较深的资料及非线性压磁的实验,得知震前ΔZ、ΔF以及磁倾角减小的效应,系埋深8~25公里的磁性体非弹性体积膨胀的结果。震前精密水准测量发现,自1969年以来震中地区地面下沉,平均海平面资料表明,此下沉一直持续至地震发生。宁河(Δ=50公里)附近下沉幅度最大,可达8~12厘米。
综合震前出现浅层压缩、深层膨胀、地表下沉的现象,推论唐山震前震中区附近的上部地壳(0~25公里)发生了向针弯曲现象——一种活动褶皱。唐山地震的孕育及发生便可能与此活动褶皱不断增加其挠度以致破裂有关。

Abstract:

Prior to the M7.8 Tangshan earthquake of 1976, earth-resistivity caused by crustal strain and geomagnetism △Z=Z_a-Z_b showed anomalies over large areas of essentially coinciding geographic extent. According to experiments on the change of electric resistivity of rock (or earth)under artificial compression in situation, it is inferred that the regional decrease of earth-resistivity observed during the period of 1973-1976 was caused by the compaction of the upper layers of the crust near ground surface, amounting to a linear strain of about 3×10^-5.It is interesting to note that △Z was also observed to decrease during the period of 1975-1976. As compared with changes in △Z, the variation △F of the total magnetic intensity is smaller, showing that the magnetic dip Ⅰ also decreases. From the fact that magnetic bodies are buried rather deep as evidenced by aeromagnetic surveys, and from the results of experiments, on cemanent magnetization for samples stressed, it is inferred that the changes in △Z and △I are caused by inelastic expansion of magnetic bodies burred 8-25-km deep. The precision levelling over the epicentral area prior to the Tangshan earthquake showed that the ground surface had been subsiding since 1969. According to the mean sea-level at Qinhuangdao the subsidence of the land continued till the occurrence of the Tangshan Earthquake, being greatest near Ninghe (epicentral distance=50 kin)amounting to 82-12 cm.
By synthesizing the phenomena of ground surface subsidence, compression of the upper layers and expansion of the deep layers, a mechanical model is proposed:The upper crust (0-25km) of the epicentral Zone of the Tangshan earthquake underwent a synclinal bending(a kind of active folding). The occurrence of the Tangshan earthquake might be caused by continued accumulation of the bending strain of the syneline which resulted in fracturing or fault plane sliding. By this model the controversy between the source mechanism of the Tangshan earthquake and the mechanics of the Neocathaysian system can be explained.