Magnitude calibrating functions for a multidimensional homogeneous system of reference stations

Calibrating functions play a substantial role in the determination of earthquake magnitudes. Their shape and character is closely connected with the internal structure of the Earth, the distribution of velocities and attenuation of seismic waves in the Earth's body. These facts have been considered during the establishment of the Eurasian Homogeneous Magnitude System (EuHMS). The paper summarizes the procedures used for constructing the new complete set of calibrating functions for body and surface waves (vertical and horizontal components of P, S and L waves from medium period instruments, and vertical component of P waves from short period instruments).The calibrating functions were derived in the distance range between 20° and 100°, being based on several thousands of homogeneous observations for each wave type. The application of a HMS gives a possibility to derive the magnitude calibrating functions with a considerable degree of reliability due to the method of simultaneous optimization of the calibrating functions and the systems of station corrections. The magnitude calibrating functions for EuHMS represent the first complete set of calibrating functions, which are derived by the same standard method and initial observational material. A comparison with the existing calibrating functions shows that the application of σ and Q functions, presently used in the seismological practice for magnitude determinations, may lead to errors exceeding of magnitude unit at certain epicentral distances, specific for each wave type. It appears that the internationally recommended Q functions are not admissible for magnitude determinations from short-period P waves.     

“Spectral ω2 model” and variable absorption coefficient

Summary Aki's spectral ² model[1] is used to estimate the amplitude field at epicentral distances up to 1000 km in the case of an absorption coefficient of seismic waves which varies with distance. Velocity spectra, the periods of the maximum of these spectra and the maximum velocity amplitudes are obtained. The results are compared with[2] and it is shown that the divergence of the amplitude field decreases if a variable absorption coefficient is used. The theoretical results, obtained in this case, approach the results seen in practice. The possibility of determining the limits of epicentral distances, at which the amplitude field may be considered as linear, is given.     

Determination of the homogeneous magnitude system magnitudes in seismological practice

Routine determination of network magnitudes by means of the Eurasian homogeneous magnitude system (HMS) is described and tested on a selection of recent shallow earthquakes. The HMS earthquake magnitudes appear to be more accurate and more reliable than the conventional earthquake magnitudes used in the present seismological practice. A preliminary investigation of correlations between magnitudes determined from different wave types shows a possible dependence of the correlation on the type of focal mechanism of earthquakes.     

Single and multiple dislocation models of extended earthquake sources

Summary A short review of simple theoretical models for extended earthquake sources visualized as single and multiple dislocations is presented. Formulas for the duration of the radiation t _i ^s from the starting point of the destruction up to the region forming the amplitude maxima of body waves are given. A method for determining some basic parameters of extended earthquake sources (length and depth of the dislocation, azimuth of the first dislocation and the angles between every two consecutive dislocations) from the seismogram data for t_i ^s is presented. The procedure is applied to yield some practical results.     

On the divergent character of amplitude curves for different magnitude levels

Pure and Applied Geophysics - The basic principles of the magnitude scales, especially for short epicentral distances, are considered. Theoretical and experimental evidence about the divergent...     

Optimisation of amplitude curves of seismic P-, S- and L-waves in the homogeneous magnitude system of the Eurasian continent

Amplitude curves for seismic P-, S- and L-waves have been constructed on the basis of the homogeneous magnitude system of the Eurasian continent. The optimum amplitude curves are compared with the standard amplitude curves used in seismological practice.