Abstract
Kamiyama and Yoshida proposed a method to estimate the nonstationary variation of the stiffness from the time histories of strain and stress by using the complex envelope. They applied the method to the strong motion records of the 1995 Hyogoken Nanbu Earthquake. In this paper, the method is theoretically and numerically investigated and it is shown that in general the method does not give a precise value of the stiffness. It is, however, shown that under a condition that the stain does not include low frequency component and the stiffness does not include high frequency component, the method evaluates a correct value. It is also shown that even if the condition is not rigorously satisfied the method can give a good approximation of the true value of the stiffness.
