The May 2 earthquake (now called the Duvall earthquake because of its
proximity to the small town of Duvall on the Snoqualmie River)
was located about
25 miles northeast of Seattle or 6 miles east-northeast of Duvall. It
was a shallow earthquake, centered at a depth of only a few miles beneath
the earth's surface. A magnitude estimate of 4.8 was initially released about
ten minutes after the earthquake by the
University of Washington. This preliminary magnitude was based on automatic
processing by a computer recording system picking the
duration of shaking on sensitive instruments. The U.S.
Geological Survey issued a magnitude estimate of 5.4 about an hour
after the earthquake. This is known as a body wave magnitude and is
based on the amplitude or height of the initial signal (the P wave) on a
seismogram. About the same time manual analysis of the local seismograms
at the University of Washington determined a local magnitude of 5.3.
A magnitude of 5.2 was determined by Oregon State University a couple of hours
later by analyzing the complete wave forms of the earthquake as recorded by
broadband seismographs. This is known as a moment magnitude and is
considered by seismologists the best type of magnitude to measure the
relative amount of energy released by the earthquake. A similar analysis
the next day at the University of Washington on a subset of data
used by Oregon State determined a moment magnitude of 5.1. None of these
different magnitudes are exactly the same as the original "Richter"
magnitude which was based on the peak amplitude of the earthquake on a
standard seismograph. But all of the the magnitude scales roughly agree
with Richter's scale so that a magnitude 5 earthquake, regardless of
scale, is a moderate earthquake capable of doing minor damage in the
immediate epicentral area.
Text adapted from report by Lori Dengler of Humbolt State University
