# Ms_20¶

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

## Description¶

### Amplitude¶

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 1.

with

A: WWSSN_LP corrected ground displacement in units of 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 1. Alternatively, the term Ms(20) may be used.

### Network magnitude¶

By default, the trimmed mean is calculated from the station magnitudes to form the network magnitude. Outliers below the 12.5% and above the 12.5% percentiles are removed before the calculation.

### Configuration¶

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

## References¶

1(1,2)

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