Earthquake mbvs MsRelations and Source Multiplicity

A relation Ms = 1·07mb − 0·23 has been obtained from about 900 earthquake magnitude values reported by CGS during May 1968 to January 1971. Hyderabad observatory seismograms were examined and it was found that all earthquakes lying above the line Ms = 1·07mb + 0·27 are complex multiple events and the ones lying below the line Ms = 1·07 mb − 0·73 are simple events. For complex earthquakes clear later phases are observed on short-period seismograms following the initial P-phase. On the long-period seismograms the initiation is some times later than on short-period seismograms, often characterized by slow build up of energy and is followed by later unaccountable larger phases within a few tens of seconds. Simple earthquakes have considerably small energies in surface waves and long-period body waves. Variation of source time function associated with the increase of magnitude only does not seem to explain the exceptionally high Ms found for some earthquakes and multiplicity of the source is suggested to be the cause of this. The difference in the observed Ms values and the ones obtained corresponding to the observed mb values could be used as a measure of source complexity.