TRAVEL TIMES AND BODY WAVE MAGNITUDE

The Q-charts used presently for magnitude determinations were obtained mainly from direct observations of ground motion amplitudes of several components of seismic waves e.g. PZ, PH, SH as functions of epicentral distance. After the compensation of the amplitudes of body waves for the radiation pattern at the focus, the amplitude variation is caused mainly by geometrical spreading. No lateral velocity heterogeneities are permitted. Indications are give that the effect of anelasticity upon the amplitudes is secondary for magnitude scales. Amplitude observations alone can serve for the definition of a magnitude scale applicable to events at only one particular focal depth. In order to assign the same magnitude to two earthquakes of identical size regardless of the focal depth, the velocity-depth and eventually anelasticity depth profile of the Earth must be known. A set of new Q-charts, obtained independently of direct amplitude observations, for PZ-, PH-, and SH-waves is presented. A refinement in the magnitude definition warrants the magnitude figures obtained with the new Q-charts to be uniform with regard to focal depth. Examples show the new Q-charts to decrease the scatter of magnitude determinations between stations.