.. _global_mb:

##
mb
##

Body wave magnitude at teleseismic distances

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

mb is the standard body-wave magnitude.
Compare also with the :ref:`mB magnitude <global_mb_bb>`.


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 |scname| mb amplitudes are measured on vertical-component displacement seismograms
in a 30 s time window after simulation of a :term:`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

.. math::

   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 |scname|: **nanometers** (nm)
* Time window: 30 s
* Default distance range: 5 - 105 deg, configurable: :confval:`magnitudes.mb.minDist`,
  :confval:`magnitudes.mb.maxDist`
* Depth range: no limitation, for depth < 0 km, depth = 0 km is assumed

.. note::

   In 2013 the IASPEI commission (:cite:t:`iaspei-2013`) recommended a minimum distance of
   20 deg. However, :ref:`scautoloc` requires mb amplitudes by default for
   considering a pick.
   For maintaining consistency, 5 deg is therefore kept as the default
   for :confval:`magnitudes.mb.minDist`.


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

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