First Motion 5
First Motion 5 : Natural Disaster. The first half-cycle of a body wave, usually the direct P wave. On a seismogram, the first discernible displacement of the record trace caused by the arrival of a P wave at the seismometer. The direction (polarity) of first motion, which for the P wave is either away from source (called compression) or towards it (called dilatation), is assumed to be conserved along the ray path from the focus to a given station, given that corrections have been made for any polarity changes due to reflection of the wave at any boundary between source and receiver. First motions of P arrivals are often used for determining earthquake focal mechanism. However, when the seismic signal arrives in the presence of seismic noise the proper polarity of the first motion may be distorted and difficult to recognize
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