Abstract：Reliability should be the key in precursor observation. Usually, we just compare the measurements and variation with the theoretic earth tide to judge whether the data are reliable. However, the way can’t determine the consistency of data and the longterm trend should be affirmed firstly in precursor analysis. According to the elastic mechanics theory and the observation insitu, it is shown that for a 4 components strain instrument, their component directions spread with 45 degree angle, the sums of measure values of unit 1(1#) and 3(3#) should be the same with that of unit 2(2#) and 4(4#), and the sum value is also equal to 1.3 times of the volume strain. We can calculate a value η by use of the 4 components strain measurements, its absolute value is the more close to 0, the more reliability of the observation. This way may provide a new evidence in estimation of observation reliability. In order to get more reliable observation, contrasting analysis can be held in those observatory with two type of strain instruments.Otherwise for those observatory with just a component strain instrument, self test , that 1#+3# should be equal to 2#+4#, may also be used to verify the reliability.
