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The
AS-1 Seismograph –Magnitude Determination L. W. BraileÓ, November, 2002 Updated April
30, 2006 |
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Estimating Earthquake Magnitude from AS-1 Seismograms: Magnitude
is an estimate of the energy release or size of an earthquake. Magnitude estimates are calculated from the
amplitude of wave energy on a seismograph adjusting for the distance of the
seismograph station from the earthquake (seismic waves spread out and are
absorbed during propagation and thus generally become smaller at greater
distances from the earthquake epicenter) and the amplification of the signal by
the seismograph. Magnitude formulas
have been developed for different types of seismographs (usually dependent on
the frequency response of the seismograph) and different type of seismic
arrivals (body waves and surface waves).
Usually, the formulas are valid for a certain range of epicenter to
station distances or region (for example, Richter magnitude was developed for
neic.usgs.gov/neis/general/handouts/general_seismicity.html
www.seismo.unr.edu/ftp/pub/louie/class/100/magnitude.html
lasker.princeton.edu/ScienceProjects/curr/eqmag/eqmag.htm
http://www.eas.slu.edu/People/CJAmmon/HTML/Classes/IntroQuakes/Notes/earthquake_size.html
http://www.seismo.com/msop/nmsop/03%20source/source4/source4.html
Currently, in an effort
to reduce confusion about earthquake magnitudes and use the most reliable measure
of earthquake size, most magnitudes reported by the US Geological Survey (http://earthquake.usgs.gov)
are labeled “Magnitude” or “M” and are moment magnitude (sometimes
referred to as Mw) determinations (when available). However, traditional magnitude determinations
such as body wave magnitude (mb), surface wave magnitude (MS) and Lg wave
magnitude (mbLg) are also reported in USGS earthquake catalogs. Determinations of magnitude for these
magnitude definitions can be made using data from the AS-1 seismograph.
The procedure for
determining magnitude from seismograms recorded by the AS-1 seismograph is:
From
the AmaSeis software, determine the approximate arrival time of the
earthquake. If possible, pick the P and
S arrivals (filtering the seismogram may help in the identification of the S
wave) and estimate the distance using the travel time curve tool. Using the selection tool, zoom in on the P
wave (extract the early part of the seismogram; the time expansion tool at the
bottom left of the screen may also be useful for zooming in on the P arrival)
and determine the maximum amplitude (zero to peak, in counts) and the
approximate period of the P wave. Use
the largest amplitude of the P wave. The
P arrival may include energy that occurs during the first approximately 10 s of
the record. Often, a distinct secondary
P phase (such as pP or PP) will be present after the first P arrival. Estimate the period by measuring the time in
seconds between two successive peaks or troughs of the signal. A millimeter scale held up to the screen is
useful to this measurement. Next, using
the selection tool, extract the early (usually largest) part of the surface
wave signal (the surface waves will usually be distinguished by their much
lower frequency) on the seismogram and determine the maximum amplitude (zero to
peak, in counts) of the surface wave arrival that is near 20 s period. Note the period of the surface wave where
your amplitude measurement is made.
1.
Go to the USGS
earthquake search web site (neic.usgs.gov/neis/epic/epic.html)
or the IRIS earthquake search site (http://www.iris.washington.edu/, select Event Search under the Quick Links menu at
left side of screen) or, for recent events, to the USGS earthquake site (http://earthquake.usgs.gov,
select Latest Quakes and then NEIC Current Earthquake Information) or the IRIS
Seismic Monitor tool, www.iris.edu) and find
and record the “official” origin time, location (latitude and longitude; note S
latitudes and W longitudes are negative; and depth) and magnitude. Several magnitude estimates may be available
(for events that occurred at least a week earlier, the IRIS event search tool
can be used to find different magnitude estimates; for example, the primary
magnitude reported may be Mw (moment magnitude) which cannot be estimated from
AS-1 record, but mb and MS magnitude may be reported later). The IRIS sites also provide a brief
geographic description of the earthquake location that is often useful.
2. Go to the USGS travel time and distance calculation site (http://neic.usgs.gov/neis/travel_times/) and calculate the distance from your seismograph station to the earthquake epicenter by entering the latitudes and longitudes of the event and the station into the web site form. If you don’t know the latitude and longitude of your station, you can view a topographic map and use the tools to find a specific location at www.maptech.com (click on Online Maps select the Map Server, then enter city and state in the boxes and click go; select correct map from list if list of possible maps appears; choose DD.DD in coordinates window to the left of the displayed topographic map; place cursor on location of interest; read the latitude and longitude of the