Arrival Patterns cont.
The shadow zone.
Why then no direct P wave arrivals between 103o and 143o?
This can be explained by refraction. Because the earth is density zoned,
internal layers will refract waves more than the external layers. The last
direct P waves to skim off the core will exit at 103o. A portion
of these P waves, however, will cross the mantle/core boundary and be refracted
inward. After these waves cross the core, they recross the core/mantle
boundary on their way out, refract outward this time, and resurface at
143o. Because of this double refraction, there is no possible
way for P waves to cross this part of the earth, and therefore, there are
no direct arrivals in this shadow zone.
Wave arrival patterns also give us information as to the internal density
of the earth. We know from exploration and direct measurements that the
specific gravity of the surface materials of the earth is near 3.0. We
also know that on the average the whole earth's specific gravity is closer
to 5.5. We therefore know that the interior must be denser than the surface.
Suppose then that we compare the arrival times of the two types of waves
(surface and body) at two points that are the same distance from the epicenter.
If the earth were homogeneous, both sets of waves should arrive at expected
times and, if we are correct for all factors other than density, they should
arrive at the same time. But, in fact, body waves arrive earlier than expected,
indicating not only that internal materials are more dense (which we knew
already, although it is always reassuring to have one's calculations confirmed),
but also that density increases with depth.
When we look at body wave behavior, there is an abrupt increase in velocity
for all waves anywhere from 3 to 35 miles down. This tells us that there
is an abrupt change in composition, or in materials, or both at that depth.
Such an abrupt change in wave behavior is called a discontinuity.
The upper layer, above this seismic discontinuity, is called the crust,
and the layer below it, in which there are no major discontinuities till
we reach the next major discontinuity at the core, is called the mantle.
From this information we can therefore conclude that the earth is density
zoned into three layers, the crust, mantle and core,
separated by two major discontinuities. This is not too surprising given
that our earth has been an active planet for the past nearly five billion
years and that these materials have had ample time to separate according
to density, under the influence of gravity. Moreover, this internal density
zonation is just an extension of the pattern we can observe at the surface,
where the less dense materials of the atmosphere overlie the oceans, and
these in turn cover the denser upper part of the solid earth. |
