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79 lines
2.6 KiB
ReStructuredText
79 lines
2.6 KiB
ReStructuredText
.. _global_md:
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##
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Md
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##
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Duration magnitude plugin
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Description
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===========
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The duration magnitude is based on coda duration measurement.
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It's usually valid for small earthquakes up to magnitude 4 to 5.
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First used in 1972 by Lee et al., Duration magnitude (Md) or Coda duration
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magnitude plugin estimates Richter magnitude of local earthquakes by using
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signal duration on vertical components of seismographs.
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Estimations are quite stable for local earthquakes ranging from magnitude
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Md 0.0 to 5.0.
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Amplitude processing
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--------------------
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Duration magnitude is usually computed on short period seismometers by searching
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the time at which the amplitude of the signal is close to pre-earthquake amplitude.
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Since it's mainly used for small earthquake whose signal is at rather high frequency,
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it's usefull to highpass filter broadband seismomters (select sismo type 6 and a
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Butterworth filter "3,1.5").
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Or a better solution is to deconvolve the signals and reconvolve with a widely used
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short-period instrument : the 1Hz eigen-frequency L4C (select sismo type 9).
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If you have the full responses in your inventory and have activated them
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(amplitudes.enableResponses set to true), you will be able to use also accelerometers.
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The plugin then searches for the maximum amplitude of the signal, which should be
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the S-wave and then computed mean amplitude of one-second time windows.
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As soon as a one-second time window mean amplitude vs pre-earthquake amplitude
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reaches the configured SNR ratio, the process is stopped.
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The middle of the one-second time window is assumed to be the end of the Coda and
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the time difference between Coda time and P arrival time is stored as Coda duration.
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Magnitude processing
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--------------------
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Once amplitudes calculated by the AmplitudeProcessor and a Coda has been found,
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the generic formula is applied and the duration magnitude is computed
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for a given station, if it fits the criteria (max depth, max distance).
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.. math::
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mag = FMA + FMB \times \log10(period) + (FMF \times period) + (FMD \times epidistkm) + (FMZ \times depth) + STACOR
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Plugin
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======
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The Coda duration magnitude plugin (Md) is installed under :file:`share/plugins/md.so`.
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It provides a new implementations of AmplitudeProcessor and MagnitudeProcessor.
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To add the plugin to a module add it to the modules configuration, either
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:file:`modulename.cfg` or :file:`global.cfg`:
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.. code-block:: sh
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plugins = ${plugins}, md
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Basically it can be used by modules: :ref:`scamp`, :ref:`scmag`, :ref:`scolv`.
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More information
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----------------
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Description of the formula can be found in Hypo2000 manual from USGS website.
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`<http://earthquake.usgs.gov/research/software/#HYPOINVERSE>`_
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