Difference between body wave (Mb) and surface wave (Ms) magnitudes

The difference is one of known discriminants between explosions and earthquakes. It is supposed that surface waves of earthquakes are stronger than of explosions due to asymmetry of sources.

See more details in E Kremenetskaya, V. Asming, Z. Jevtjugina and F. Ringdal. Study of Regional Surface Waves and Frequency-dependent Ms: mb Discrimination in the European Arctic. Pure appl. geophys. Vol. 159, 2002, p. 721-733. (See Publications)

We used digitized seismograms of Apatity broadband analog seismic station (to do it we developed a program for semi-automatical digitizing of scanned seismograms and their conversion to CSS 3.0 format). We analyzed a set of events which included Novaya Zemlya test site nuclear explosions, peaceful nuclear explosions, earthquakes and an event of unknown nature occurred on August 1, 1986 near Novaya Zemlya.

This event was carefully studied. Marshall (Marshall, P.D., Stewart, R.C. & Lilwall, R.C. Geophys. J. Int. 98, 565-573 (1989)) had located it and obtained its depth 30 km. Thus he identified the event as earthquake. Our location resulted in 0 depth.

Digitized recordings have been passed through a set of 5 bandpass filters: 0.03-0.04 Hz, 0.04-0.06 Hz, 0.06-0.08 Hz, 0.08-0.1 Hz, 0.1-0.2 Hz.

For each band we measured a maximal amplitude of Rayleigh wave and calculated Ms.

Ms(Ti) = Log(Ai/Ti)+1.66× Log(D), where i is index of frequency band, Ai is amplitude, Ti - period (corresponding to the center of frequency band) , D is distance to the station. In addition, we computed average Ms.

We have taken Mb magnitudes from international seismic bulletins ISC. Results are shown in figure below.

Ms-Mb for different kinds of events (left top), Ms(T)-Mb plots with respect to T for all the events (left bottom), map of the events with propagation paths (right top).

It is easy to see that for shorter periods curves Ms(T)-Mb for earthquakes and explosions have cross-sections. Nevertheless the curves are separated for longer periods. Average Ms-Mb for explosions and earthquakes are strictly separated. The unknown event 01.08.1986 lies in explosions area.

Our conclusion is that Ms-Mb criterion works well for our region but it is necessary to check Mb magnitudes by our own data.