.. _global_ms_20:

#####
Ms_20
#####

Surface wave magnitude measured at around 20 s

Description
===========

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


Amplitude
---------

The Ms_20 amplitudes are calculated on vertical-component displacement seismograms
corrected for the instrument response of a :term:`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 (:cite:t:`iaspei-2013`).

.. math::

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

with

A: :term:`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 (:cite:t:`iaspei-2013`).
Alternatively, the term *Ms(20)* may be used.

* Amplitude unit in |scname|: **nanometer** (nm)
* Time window: 0 s - distance (km) / 3.5 km/s + 30 s
* Period range: 18 s - 22 s, configurable: :confval:`magnitudes.Ms_20.lowerPeriod`,
  :confval:`magnitudes.Ms_20.upperPeriod`
* Default distance range: 20 - 160 deg, configurable: :confval:`magnitudes.Ms_20.minDist`,
  :confval:`magnitudes.Ms_20.maxDist`
* Depth range: <= 100 km, configurable: :confval:`magnitudes.Ms_20.maxDepth`


Network magnitude
-----------------

By default, the trimmed mean is calculated from the station magnitudes to form
the :term:`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. Add
Ms_20 to the list of computed amplitudes and magnitudes in the configuration of
:ref:`scamp` and :ref:`scmag` and in :ref:`scesv` or :ref:`scolv` for visibility.
