mb

mb is the teleseismic body wave magnitude

Description

Compare also with the mB magnitude.

Amplitude

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.

Station Magnitude

The general formula is

mb = \log \left(\frac{A}{T}\right) + Q(h,\Delta) - 3.0

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.

  • Amplitude unit in SeisComP: nanometers (nm)

  • Time window: 30 s

  • Default distance range: 5 - 105 deg, configurable: magnitudes.mb.minDist, magnitudes.mb.maxDist

  • Depth range: no limitation, for depth < 0 km, depth = 0 km is assumed

Note

In 2013 the IASPEI commission 1 recommended a minimum distance of 20 deg. However, scautoloc requires mb amplitudes by default for considering a pick. For maintaining consistency, 5 deg is therefore kept as the default for magnitudes.mb.minDist.

Network magnitude

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.

Configuration

Adjust the configurable parameters in global bindings in the mB section or use global.cfg as MLv. Add mb to the list of computed amplitudes and magnitudes in the configuration of scamp and scmag and in scesv or scolv for visibility.

References

1

IASPEI magnitude working group (2013). SUMMARY OF MAGNITUDE WORKING GROUP RECOMMENDATIONS ON STANDARD PROCEDURES FOR DETERMINING EARTHQUAKE MAGNITUDES FROM DIGITAL DATA, Link to PDF document