first Motion 4
first Motion 4 : Natural Disaster. On a seismogram, the first discernible displacement of the record trace caused by the arrival of a P wave at the seismometer. Upward motion of the ground at the seismometer indicates an expansion in the source region, downward motion indicates a contraction. When the seismic signal arrives in the presence of seismic noise the proper polarity of the first motion may be difficult to recognize
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