Answer:
While earthquake location can be determined fairly
rapidly, earthquake size is somewhat more problematic.
This is because location is mainly based upon
measurements of the time that seismic waves arrive at a
station. Magnitude, on the other hand, is based upon
the amplitude of those waves. The amplitude is much
more variable than the arrival times, thus causing
greater uncertainty in the magnitude estimate.
For larger earthquakes, the problem is compounded by the
fact that the larger the earthquake, the lower the
characteristic frequency of the seismic waves. This
means that surface wave arrivals, which contain lower
frequency energy than the body waves, must be used to
determine the magnitude. For a great earthquake,
several hours of data must be recorded in order to
accurately determine the magnitude.
Thus, accurate estimates of the magnitude can follow an
accurate estimate of the location by several hours. In
the case of the M 9.0 Sumatra-Andaman Islands
earthquake, the standard methods were inadequate for
measuring the very low frequency energy produced and had
to be modified. This delayed the final determination of
the magnitude until the next day.
