Ms_20 is the surface wave magnitude measured on the vertical component at around 20 s period in accordance with the IASPEI standards.



The Ms_20 amplitudes are calculated on vertical-component displacement seismograms corrected for the instrument response of a WWSSN_LP seismograph.

Station Magnitude

Ms_20 is the surface-wave magnitude at 20 s period based on the recommendations by the IASPEI magnitude working group issued on 27 March, 2013.

M_s = \log \left(\frac{A}{T}\right) + 1.66 \log(\Delta) + 0.3


A: WWSSN_LP corrected ground displacement in nm measured on the vertical-component seismogram as the maximum absolute trace amplitude of a surface wave at periods between 18 s and 22 s,

T: period of the surface wave in seconds.

The term Ms_20 is chosen in accordance with the IASPEI standard as of 2013. Alternatively, the term M_s (BB) may be used.

Network magnitude

By default, the trimmed mean is calculated from the station magnitudes to form the network magnitude. Outliers below the 25% and above the 75% percentiles are removed before the calculation.


Adjust the configurable parameters in global bindings in the Ms_20 section.