摘要
Based on the principle formula for the four-component strainmeters, we can directly obtain the specific plane strain, shear strain and azimuthal angle of the principal strain, and the maximum and minimum principal strains calculated afterwards are the indirect result. The problems of practicality of the sensitivity coefficients A and B of plane strain and shear strain are then discussed. Based on this idea, we analyzed the observation data of several four-component borehole strainmeters near the epicenter of the Yiliang M_S5.7 earthquake in 2012 and the Ludian M_S6.5 earthquake in 2014 in the Zhaotong area, Yunnan Province. The results show that the analysis based on the perspective of plane strain and shear strain has an obviously better effect than that based on the component readings, and can directly peel off the respective abnormality of the plane strain and shear strain. In addition, the correlation coefficient curves between measured data of two plane strains show significant anomalies which often occur several days before and during the earthquake.
Based on the principle formula for the four-component strainmeters, we can directly obtain the specific plane strain, shear strain and azimuthal angle of the principal strain, and the maximum and minimum principal strains calculated afterwards are the indirect result. The problems of practicality of the sensitivity coefficients A and B of plane strain and shear strain are then discussed. Based on this idea, we analyzed the observation data of several four-component borehole strainmeters near the epicenter of the Yiliang M_S5.7 earthquake in 2012 and the Ludian M_S6.5 earthquake in 2014 in the Zhaotong area, Yunnan Province. The results show that the analysis based on the perspective of plane strain and shear strain has an obviously better effect than that based on the component readings, and can directly peel off the respective abnormality of the plane strain and shear strain. In addition, the correlation coefficient curves between measured data of two plane strains show significant anomalies which often occur several days before and during the earthquake.
引文
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2Strictly speaking,the surface strain should be 1/2(εI+εII),and the maximum shear strain is 1/2(εI-εII).However,in the field of ground strain observation, for the sake of convenient description, 1/2 is often conventionally omitted.
3It is only when the components are evenly distributed that the test formula is so simple. Gladwin(1984)changed the three-component to four-component in the end of the 20th century. He just inserted the fourth component in the original three-component pattern,perpendicular to the original one. In this case,the data self-checking formula is complex,and the weights of the four components are not uniform. The formulas for calculating shear strain and azimuth angle are also complicated and not practical enough.
4 Since the beginning of the 21st century,Ishii Hiroshis drillhole strain instrument has been improved from three components to four components,which is called “two-group and two-component”. Four components are divided into two groups,and the angle of the components in the group is 90°. The cross-checking relationship is established. But the angle between the two groups of components deviates from 45°by 5°. When calculating the shear strain and azimuth angle,the more complex formulas that formula(13)and formula(14)have to be used.
