# Comparison between different earthquake magnitudes determined by China Seismograph Network

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## Abstract

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 4.5<*M*<6.0, *m* _{B}>
*M* _{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 (6.0<*M*<8.5); In case *M*>8.5, 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.

## Key words

local earthquake magnitude surface wave magnitude body wave magnitude## CLC number

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