Abstract:In the light of the coda theory advanced by Aki et al.,this paper deduces a concrete formula for the lasting period ratio τH/τV of the horizontal and the vertical component of the local earthquake coda. After the approximate treatment, we get:
τH/τV=IH/IV(QH/QV)(1)/(4)·BH/BV
(1) The formula shows that the changes of τH/τV mainly reflect the focal body's difference in excitation and attenuation force to the coda wave from different directions in certain range caused by the anisotropy of the medium. The paper also discusses the physical mechanism of the τH/τV short and imminent anomalies, and rends to think that the occurrences of the anomalies are related to the appearance and closure of the cracks in the medium in earthquake preparatory stage.
The results obtained after processing the Mcnyuan earthquake Ms 6.4 of 1986 and the Haicheng earthquake Ms 7.3 of 1975 show that there appeared the short-period low value anomalies and imminent high value jump in various, degrees.In comparison with the maximum amplitudes, of the direct S wave recorded by Menyuan station before and after the 1986 Menyuan earthquake, it is found that AmH/AmV has the similar case with the anomalies of the coda τH/τV.
Through the analysis to the data of some earthquake cases, the paper finally gives the relation of the magnitude with the anomalous period as
M=0.657lnt+3.44.