By linear regression and orthogonal regression methods, comparisons are made between different magnitudes (local magnitude M L , surface wave magnitudes M S and M S7 , long-period body wave magnitude m B and short-period body wave magnitude m b ) determined by Institute of Geophysics, China Earthquake Administration, on the basis of observation data collected by China Seismograph Network between 1983 and 2004. Empirical relations between different magnitudes have been obtained. The result shows that: 1 As different magnitude scales reflect radiated energy by seismic waves within different periods, earthquake magnitudes can be described more objectively by using different scales for earthquakes of different magnitudes. When the epicentral distance is less than 1 000 km, local magnitude M L can be a preferable scale; In case M<4.5, there is little difference between the magnitude scales; In case 4.5M S , i.e., M S underestimates magnitudes of such events, therefore, m B can be a better choice; In case M>6.0, M S >m B >m b , both m B and m b underestimate the magnitudes, so M S is a preferable scale for determining magnitudes of such events (6.08.5, a saturation phenomenon appears in M S , which cannot give an accurate reflection of the magnitudes of such large events; 2 In China, when the epicentral distance is less than 1 000 km, there is almost no difference between M L and M S , and thus there is no need to convert between the two magnitudes in practice; 3 Although M S and M S7 are both surface wave magnitudes, M S is in general greater than M S7 by 0.2~0.3 magnitude, because different instruments and calculation formulae are used; 4 m B is almost equal to m b for earthquakes around m B 4.0, but m B is larger than m b for those of m B ≥4.5, because the periods of seismic waves used for measuring m B and m b are different though the calculation formulae are the same.
In order to investigate the physical mechanism of non-synchronism of seismo-electromagnetic radiation precursors, we have made shear fracture and frictional sliding tests of rock samples by loading them in biaxial compression to model the activity of tectonic fault zones in seismogenic region. By installing antennae of different frequency responses and acoustic transducers around the fracture surface, the signals of electromagnetic radiation and acoustic emission in different directions and of different frequencies produced by rock samples during their shear fracturing and frictional sliding have been recorded by an automatic, high-.speed and continuous observation system. Some implications given by the experimental results are as follows.(1) During the process of shear fracturing and frictional sliding of rock samples under biaxial compression, large amounts of electromagnetic signals are produced; their frequencies range from several hundred Hz to several thousand Hz.(2) Signals received by antennae in different directions and of different frequencies are non-synchronous. They differ in amplitude from one another, with the signal received by antennae nearest to the fracture surface being the maximum.(3) Electric signals and magnetic signals do not appear synchronously; among them, electric signals appear more frequently and are of larger amplitude.The authors hold that electric signals (E) and magnetic signals (M) are of different genetic mechanisms: electric signals from inside the rock sample are produced by the piezoelectric effect of rock-forming crystals and net electric charge produced by the newly created surfaces due to rock fracture, whereas magnetic signals are produced by the high-speed motion of charged rock fragments and the ionization of ambient air excited by electrons emitted by rock in fracturing. Analysis shows that in some moderately strong earthquakes the electric and magnetic signals also showed the phenomenon of non-synchronism: the electric signal was earlier than the magnetic signal. If the same station observes simultaneously the electric field (underground) and magnetic field (air), the efficiency of earthquake prediction by seismo-electromagnetic radiation precursors would be raised effectively.
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