STUDY OF THE SYNTHETIC CALIBRATION FUNCTION OF BODY WAVE MAGNITUDE

The synthetic calibration functions of body-wave magnitude for Chinese stations are formulated in terms of P and 8 wave travel times from earthquakes in the Chinese region and its corresponding velocity distribution model in the crust and mantle and absorption band Q model for the earth (abbreviated ABM-Q model), discussing the effects of geometrical spreading of seismic waves and absorption by the medium in the earth. The results obtained demonstrate:(1)A factor T1-a in the synthetic calibration function is introduced through the frequency-dependence of quality factor Q. That isf(△, h, T) = g(△, h) + a(△, h)/T1-aWhere the parameter a characterizes the degree of frequency-dependence of the quality factor Q in the ABM-Q model, but here it represents the period-dependence of the calibration functions. This is a theoretical improvement on the calibration functions of Gutenberg and Riehter (1956) and of Nortmann and Duda (1982).(2)The term a (A, h) characterizing effect of absorption has higher compensation obviously in the upper mantle and the bottom mantle in the new calibration functions, in comparison with Nortmann and Dudas (1982). This is consistent, so far as known with the tendency of Q distribution with depth.(3)The tendency of underestimating magnitude by Gutenberg and Riehter (1956) calibration functions in the short period range and by Nortmann and Duda (1982) calibration function in the long period range is improved in the preliminary practical tests, so that the magnitudes determinated from different seismic wave periods are basically consistent.(4)The new calibration functions can be used as basic data in formulating the calibration function of body-wave spectral magnitude.