Geology Reference
In-Depth Information
Earthquake waves are recorded by instruments buried underground called
seismometers
all over the
planet. When an earthquake occurs, the seismometer sends a signal to a machine in a lab (a
seis-
mograph
) that records the earthquake wave movements on a printout called a
seismogram.
Scientists
watch the seismographs as they print the seismograms to see when the P waves and S waves arrive.
Keep in mind that S waves can travel only through solid material. Scientists studying earthquakes in the
early part of the twentieth century noticed that S waves didn't arrive at seismographs across the globe
from their originating epicenters. The scientists determined that there was a region through which S
waves wouldn't travel, and they dubbed the region the
shadowzone.
These observations have led sci-
entists to conclude that the outer core of the earth is liquid and have allowed them to estimate at what
depth this liquid layer begins.
Remember: While S waves can travel through solid material, they do not travel through the solid inner
core of the earth because they are blocked (refracted) by the liquid outer core, through which they
cannot travel.
P waves can travel through liquids, but they create a shadow zone of their own due to slight changes
in their direction at the liquid-solid boundaries at the mantle-outer core and outer core-inner core re-
gions of the earth. At these boundaries P waves are refracted at an angle to their original direction.
They also slow down as they pass through the liquid outer core so that they arrive later than expected
on the other side of the globe. Figure 4-3 in Chapter 4 shows you how S waves and P waves travel
through the interior of the earth and create shadow zones.
Two types of waves travel out from the
focus,
or origination point, of an earthquake. One
type is
surface waves,
which travel across the surface of the earth just like ripples travel
across the surface of a pond when you throw a pebble into it. These waves travel in all
directions from the
epicenter,
which is directly above the earthquake's focus. Surface
waves can move up and down or side to side and are responsible for most of the dam-
age we see from an earthquake.
Another type of energy wave, a
body wave,
travels through the earth's interi-
or. Body waves are either
primary waves
(P waves) or
secondary waves
(S waves).
Here's how they differ:
P waves
are also called
compressional waves.
They move quickly by compressing
rocks, which means the rocks contract and expand as the wave moves through
them. One way to visualize this is to imagine a Slinky coil stretched out. If you
compress and release one end, the energy will move along the coil by contracting
