Body wave magnitude at teleseismic distances
mb is the standard body-wave magnitude. Compare also with the mB magnitude.
mb is defined on the amplitude of the first few cycles of the P-wave, typically a time window of 20 s - 30 s. Only the first few cycles are used to minimize the effects of radiation pattern and depth phases, which result in complicate waveform signatures. In SeisComP mb amplitudes are measured on vertical-component displacement seismograms in a 30 s time window after simulation of a WWSSN_SP short-period seismometer. Amplitudes are used from stations with epicentral distances between 5° and 105° (configurable). The methods for measuring amplitudes are configurable in the global bindings.
The general formula is
with A as the displacement amplitude in micrometers, T as the dominant period of the signal in seconds, Q as a correction term for depth and distance. mb is usually determined at periods around 1s in adaptation to the use of the World-Wide Standard Seismograph Network (WWSSN) short-period stations. A scatter in the order of +/- 0.3 for the station magnitudes is usual. Typically, mb is determined for stations with distances larger than 5° to have a distinct direct P-wave phase. A correction term for the distance has to be determined empirically, which is quite complicate for distances smaller than 20°. This reflects the complexity of the body waves that traverse only in the upper mantle. mb saturates at about magnitude 5.5 to 6.0 because the maximum amplitudes of larger earthquakes occur at lower frequencies than the frequency range between 0.7 Hz - 2 Hz used for the magnitude calculation.
By default, the trimmed mean is calculated from the station magnitudes to form the network magnitude. Outliers beyond the outer 12.5% percentiles are removed before forming the mean.