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84 lines
2.9 KiB
ReStructuredText
.. _global_mb:
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##
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mb
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##
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Body wave magnitude at teleseismic distances
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Description
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===========
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mb is the standard body-wave magnitude.
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Compare also with the :ref:`mB magnitude <global_mb_bb>`.
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Amplitude
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---------
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mb is defined on the amplitude of the first few cycles of the P-wave,
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typically a time window of 20 s - 30 s. Only the first few cycles are used to
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minimize the effects of radiation pattern and depth phases, which result in
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complicate waveform signatures.
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In |scname| mb amplitudes are measured on vertical-component displacement seismograms
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in a 30 s time window after simulation of a :term:`WWSSN_SP` short-period
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seismometer. Amplitudes are used from stations with epicentral distances between
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5° and 105° (configurable). The methods for measuring amplitudes are configurable
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in the global bindings.
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Station Magnitude
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-----------------
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The general formula is
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.. math::
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mb = \log \left(\frac{A}{T}\right) + Q(h,\Delta) - 3.0
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with A as the displacement amplitude in micrometers, T as the dominant period of
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the signal in seconds, Q as a correction term for depth and distance. mb is
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usually determined at periods around 1s in adaptation to the use
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of the World-Wide Standard Seismograph Network (WWSSN) short-period stations.
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A scatter in the order of +/- 0.3 for the station magnitudes is usual.
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Typically, mb is determined for stations with distances larger than 5° to
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have a distinct direct P-wave phase. A correction term for the distance has to
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be determined empirically, which is quite complicate for distances smaller than 20°.
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This reflects the complexity of the body waves that traverse only in the upper
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mantle. mb saturates at about magnitude 5.5 to 6.0 because the maximum amplitudes of larger
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earthquakes occur at lower frequencies than the frequency range between 0.7 Hz - 2 Hz
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used for the magnitude calculation.
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* Amplitude unit in |scname|: **nanometers** (nm)
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* Time window: 30 s
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* Default distance range: 5 - 105 deg, configurable: :confval:`magnitudes.mb.minDist`,
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:confval:`magnitudes.mb.maxDist`
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* Depth range: no limitation, for depth < 0 km, depth = 0 km is assumed
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.. note::
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In 2013 the IASPEI commission (:cite:t:`iaspei-2013`) recommended a minimum distance of
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20 deg. However, :ref:`scautoloc` requires mb amplitudes by default for
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considering a pick.
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For maintaining consistency, 5 deg is therefore kept as the default
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for :confval:`magnitudes.mb.minDist`.
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Network magnitude
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-----------------
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By default, the trimmed mean is calculated from the station magnitudes to form
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the :term:`network magnitude`. Outliers beyond the outer 12.5% percentiles are
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removed before forming the mean.
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Configuration
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-------------
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Adjust the configurable parameters in global bindings in the mB section or use
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:file:`global.cfg`
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as in :ref:`global_mlv`. Add mb to the list of computed amplitudes and magnitudes
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in the configuration of
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:ref:`scamp` and :ref:`scmag` and in :ref:`scesv` or :ref:`scolv`/:ref:`scesv`
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for visibility.
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