Magnitude Thresholds

The United Nations Conference on Disarmament Group of Scientific Experts (GSE) conducted its Third Technical Test (GSETT-3) during 1995. This test of methods for monitoring the Comprehensive Test Ban Treaty involved continuous transmision of full waveform digital data to the Experimental International Data Center (EIDC) from more than 40 carefully selected low noise seismic stations, many of them arrays.

Questions have arisen about a possible bias of body wave magnitudes computed at the EIDC as compared with values computed by other agencies using globally distributed stations. The ISC uses amplitudes reported by the EIDC, and this has significantly increased the number of events for which the ISC Bulletin can include a body wave magnitude. Users of ISC magnitudes should be aware, however, of the potential for a bias in mb values computed for 1995 and later versus those for earlier events.

Even with the EIDC amplitudes available during 1995, analysis of the magnitude frequency distributions indicates a completeness threshold greater than 4 in many regions. Distributions for 1994 indicate even higher thresholds in some regions, and comparisons of the distributions for the two years show a striking improvement during 1995 in many cases.

A paper discussing the thresholds was presented at the 1998 General Assembly of the European Seismological Commission.