P waves are the fastest seismic waves, and consequently, the
first to arrive at any given location. Because of this fact, they
were initially referred to as the primary waves of an
earthquake. "Primary" was later shortened simply to "P".
P waves are longitudinal compressional waves.
Just like sound waves, they travel
through matter causing compression and dilatation in an orientation
parallel to their direction of propagation. As with sound, the speed
at which P waves travel depends upon the properties of the matter through
which they propagate. In general, the less dense the matter, the
slower the waves. Researchers have used observations of seismic waves
from known sources to create a velocity model --
a three-dimensional map of the variations in P-wave velocity
throughout the crust beneath southern California. The
velocity profile at left is a cross-section of such a model.
It shows that P waves typically travel through the rocks below
southern California at speeds between 5 and 7 kilometers per second,
though in sediment-filled basins (like the Los Angeles Basin),
they may move as slowly as 3 km/sec. That's still very fast;
the speed of sound in air is only 0.3 km/sec!
Because P waves are the first seismic waves to reach any given location
after an earthquake occurs, their arrival can be easily identified on
a seismogram, like the example at left. This record is read from left to
right, as time elapsed during the recording. You can see how the
trace (the dark line) starts out, at far left, as a flat line,
meaning all was "quiet". The first deflection of the trace from this
quiet mode represents the arrival of the P wave. Move the mouse pointer
over the image to see this arrival marked.
