Using the AmaSeis seismogram viewer and downloading - QuarkNet

Using AmaSeis… Page 20 of 69
111.19 km distance along the Earth’s surface) is then given to the left of the seismogram (from the
distance scale). The horizontal scale is time in minutes after the earthquake.
3.7 Measuring amplitudes in AmaSeis and calculating magnitude: The amplitude of arrivals
on extracted seismograms can be measured using the mouse cursor and the AmaSeis software.
Positioning the mouse on the peak of an arrival displays the amplitude in a window in the lower
right hand corner of the screen as illustrated in Figure 27. If the amplitude of the “zero line”
(the seismogram amplitude prior to the first arrival) is approximately zero amplitude, then the
amplitude displayed is the amplitude of the arrival. If the “zero line” is not near zero amplitude,
the difference between the arrival (peak) amplitude and the zero line amplitude is the amplitude
of the arrival. Amplitudes can be measured at any location on the seismogram. However, to
calculate the magnitude of an earthquake, the amplitudes of the P-wave arrival, the 20-second
surface wave arrival, or the amplitude of the mbLg wave arrival can be measured and used to
calculate magnitude estimates. Magnitudes can be calculated using the online AS-1 magnitude
tool MagCalc at:
http://web.ics.purdue.edu/~braile/edumod/MagCalc/MagCalc.htm.
For the example seismogram shown in Figure 27, the MagCalc AS-1 magnitude tool calculated
a body wave magnitude of 6.3 from the measured amplitude of the AS-1 seismogram. The
official body wave magnitude (mb) reported by the US Geological Survey was 6.1. More
information, including additional examples, about measuring amplitudes and calculating
magnitudes using AmaSeis and the MagCalc tool can be found at the MagCalc link, at:
http://web.ics.purdue.edu/~braile/edumod/as1mag/as1mag3.htm, and at:
http://web.ics.purdue.edu/~braile/edumod/MagCalc/AS1Results.htm.
Magnitude information (as well as location, origin time and other information about an
earthquake) can be found on the Internet as described in section 3.12. The epicenter to station
distance is also required to calculate the magnitude. This distance can be determined using the
online tools described in section 3.13 or estimated from the S minus P arrival times (for
seismograms with distinct S arrivals) as described in section 3.6.