Earthquakes
There are two main categories of earthquake waves (seismic waves); body waves and surface
waves.
Body Waves
The body waves travel through the Earth. The faster of these body waves is called the Primary (P)
wave. Its motion is the same as that of a sound wave; as this wave propagates is alternately pushes
(compresses) and pulls (dilates) the rock. Dilation is the term used in solids and liquids rarefaction is
used in gasses. These P waves are able to travel through both solid rock (e.g. granite mountains) and
liquids (e.g. volcanic magma and water).
The slower wave through the body of rock is called the Secondary (S) wave. As an S wave
propagates, it shears the rock sideways at right angles to the direction of travel. If a liquid is sheared
sideways, it will not move back to its original position. Due to this, S waves cannot propagate in the
liquid parts of the Earth (oceans, lakes or the molten core).
The actual speed of P and S waves depends on the density and elastic properties of the rocks and
soil that they pass through. In most earthquakes, the P waves are felt first. The effect is similar to a
sonic boom that bumps and rattles windows. Some seconds later, the S waves arrive with their upand-down and side-to-side motion, shaking the ground surface vertically and horizontally. This is the
wave motion that is so damaging to structures.
Surface Waves
The surface waves come in two types: Love waves and Rayleigh waves. Both are transverse, but in
one the ground moves from side-to-side and in the other they move up-and-down. These waves are
like ripples of water that travel across a lake.
The Love waves motion is like an S waves and has no vertical displacement. It moves the ground
from side-to-side in a horizontal plane, but at right angles to the direction of the propagation. The
horizontal shaking of Love waves is particularly damaging to the foundations of structures.
The second type of surface wave is known as a Rayleigh wave. Like rolling ocean waves, Rayleigh
waves move both vertically and horizontally in a vertical plane pointed in the direction in which the
waves are travelling.
Surface waves travel more slowly than body waves (P and S waves). Of the two surface waves, Love
waves generally travel faster than Rayleigh waves. Love waves do not propagate through water, but
they do affect water along coastlines by pushing water sideways (like water near the sides of a
vibrating tank of water). As Rayleigh waves have a vertical component to their motion they can affect
bodies of water.
P and S waves have a characteristic that affects shaking; when they move through layers of rock in
the crust, they are reflected or refracted at the interfaces between rock types. Whenever either wave
is refracted or reflected, some of the energy of one type is converted to waves of the other type.
Example
A P wave travels upwards and strikes the bottom of a layer of alluvium. Part of its energy will pass
through the alluvium as a P wave and part will pass through as a converted S wave. Some of the
energy will also be reflected back down as P and S waves
The velocity of the body wave increases as it travels deeper into the Earth. This means that the
average speed between nearby locations is less than the average speed between distant locations,
because the wave is traveling though deeper rock as it cuts through the Earth.
Example
The straight line through the Earth between Vancouver and Paris goes through deeper rock than the
straight line between Vancouver and Toronto. This means that the average speed of a wave traveling
from Vancouver to Paris is greater than the average speed of a wave traveling from Vancouver to
Toronto.
Assignment
1. Refer to Table 1. On the same graph paper, plot a distance (from epicenter) against time for
wave to travel for a P and S wave.
2. Examine Table 2. This shows the time of day an Earthquake arrived at the sensing equipment
in various cities around the world. The P wave always arrives ahead of the S wave. The
difference in arrival times can be used to find the distance to the epicenter by using your
graph.
3. Complete Table 2 and predict the location of the epicenter of the earthquake. Using a globe
may help you with this.
Table 1: Data for time of travel of P and S waves
Distance
(km)
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
Time for P wave
(minutes)
2.2
4.0
5.7
7.2
8.4
9.7
10.8
11.8
12.6
13.2
Time for S wave
(minutes)
4.4
7.5
10.4
12.8
15.0
17.0
18.9
20.6
22.1
23.2
Table 2: Time of arrival of seismic surface waves at different locations
Location
Arrival Time of P
wave
Arrival of S wave
New York
Seattle
Mexico
Paris
2:24:05 pm
2:24:40 pm
2:29:00 pm
2:30:15 pm
2:30:59 pm
2:28:34 pm
2:36:00 pm
2:38:45 pm
Predicted location of Epicenter: ___________________________
Difference in
Arrival Times
(minutes)
Distance to
Epicenter
(km)