The Three Economic Sectors. According to the three-sector theory, all economic activity can be classified into one of three sectors: the primary sector, the secondary sector and thetertiary sector.
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Seismic waves are sent through the earth during earthquakes, and those
energy waves are what cause the ground to shake as they travel through
it.Seismic waves travel at different speeds when they pass through
different types of material, so by studying seismograms, scientists can
learn a lot about Earth's internal structure.Body waves are seismic
waves that travel through Earth's interior, or its 'body.' Surface waves
are seismic waves that travel through Earth's surface.Surface waves are
important, but they don't provide much information about what happens
below the surface. For this, we need to study body waves so that we can
see what Earth's 'body' is like.
There are two types of body waves, called P waves and S waves. P
stands for primary waves because these waves travel the fastest and are
detected first. S stands for secondary waves because these are slower
than P waves, arriving second on the seismogram.
The seismic focus is the point in the earth's crust where an earthquake originates from.
