Study of calibration function for surface wave magnitude of DK1 seismographs

The calibration function for surface wave magnitude of DK1 seismograph is deduced by using least squares method of error theory based on the records of DK1 seismograph from 1987 to 1993 and the MS magnitude defined by International Seismological Center (ISC) as basic standard, which takes the form σ DK1 ( Δ )=(1.66±0.034)lg( Δ )+(3.2±0.11) (10° ≤ Δ ≤ 130°, T =8～20 s) From its physical essence, the calibration function for estimating surface wave magnitude by DK1 seismograph can be also deduced by means of the wave propagation theory in layer media, if the dispersion of seismic wave, absorption of media and the effect of the linear filter consisting of earth media and DK1 seismographs on the amplitudes and periods of surface wave are taken into account and it is also assumed that the maximum amplitude is response to Airy?s phase, which takes the form σ DK1 ( Δ )=1.661lg( Δ )+3.3 (10°≤ Δ ≤170°) No correction value is needed when using the calibration function deduced in this paper. Examination on surface wave magnitude of 315 events by 36 prompt earthquake stations and DK1 prompt network shows there exits no systematical error between surface magnitudes of MS (DK1) and MS (ISC) given by ISC and the average error is approximately zero.

Comparison between different earthquake magnitudes determined by China Seismo-graph Network

By linear regression and orthogonal regression methods, comparisons are made between different magnitudes （local magnitude ML, surface wave magnitudes Ms and MsT, long-period body wave magnitude mB and short-period body wave magnitude mb） 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 1000 km, local magnitude ME 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, mB〉Ms, i.e., Ms underestimates magnitudes of such events, therefore, mB can be a better choice; In case M〉6.0, Ms〉mB〉mb, both mB and mb underestimate the magnitudes, so Ms 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 Ms, 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 ME and Ms, and thus there is no need to convert between the two magnitudes in practice; ③ Although Ms and Ms7 are both surface wave magnitudes, Ms is in general greater than Ms7 by 0.2～0.3 magnitude, because different instruments and calculation formulae are used; ④ mB is almost equal to mb for earthquakes around mB4.0, but mB is larger than mb for those of mB〉4.5, because the periods of seismic waves used for measuring mB and mb are different though the calculation formulae are the same.

Discussion on the Relationship between Different Earthquake Magnitude Scales and the Effect of Seismic Station Sites on Magnitude Estimation

Based on the earthquake catalog reported by the Chinese digital seismic network in recent years,we select the earthquakes with both surface wave magnitude and local magnitude and fit them into a relationship between the two magnitudes.The systematic difference is found from the formula which has been used for 30 years.Because of a large dynamic range and wide frequency range of the current digital observation system,in addition to a larger number of stations and earthquakes being used compared to before,the relation obtained in this paper seems more reliable.Our calculation shows that there is no significant difference before and after magnitude conversion so we suggest the abandonment of magnitude conversion.The site response of a station consists of amplification at different frequencies.The amplification is equal to about 1 and changes little with frequency at stations located on basement rock,and it is greater than 1 at low frequency ranges and less than 1 at high frequency ranges at stations located on sediment layers.The difference between magnitudes from single station located on sediment layer and the average magnitude from the whole network increases from negative to positive with period.It seems that there is no fixed station correction factor and the station correction method does not work to improve the accuracy and magnitude estimates.