TANGSHAN 1976 EARTHQUAKE AND THE OPTIMUM OBSERVATION STRATEGY FOR SEISMOMAGNETISM

Based on the theory of three-dimensional seismomagnetic modelling proposed by the author,Hastie and Staeey,a computation for the Tangshan 1976 earthquake is undertaken. The piezomagnetic anomalies of seven models with different orientations,dip angles and depths of the fault at different magnetic latitudes are compared. The results indicate that seismomagnetic effect is strongly dependent on the focal mechanism of the earthquake. It appears that strike-slip fault with a dip angle of 45-60 provides the optimum observation condition where the maximum piezomagnetic anomaly is as high as 12 nT. The worst condition for seismomagnetic observation appears when the fault is located at the magnetic equator,or orients magnetic E-W,or with a focal depth of 10 km o>r more. Under one of such circumstances the corresponding piezomagnetic anomaly can hardly be detected. In most of the models,anomaly of the vertical component is slightly higher than that of the total field strength and reaches an extreme value of three fold of the latter when the fault is located at the magnetic equator. Computation for the Tangshan earthquake shows that the maximum anomaly is about 3-4 nT and the ratio of the linear extension of the area with an anomaly exceeding InT to the fault length is only 1.04-1.10. Since the piezomagnetic field is a weakly induced field by an earthquake,the fact that no obvious magnetic precursor to this earthquake had been measured seems to 'be due to the low sensitivity of the magnetometers involved and their large epicentral distances.