# seismic waves

## waves

Energy is transmitted in waves. Every wave has a high point called a crest and a low point called a trough. The height of a wave from the center line to its crest is its amplitude. The distance between waves from crest to crest (or trough to trough) is its wavelength. The parts of a wave are illustrated in Figure 1.1.

## earthquake waves

The energy from earthquakes travels in waves. The study of seismic waves is known as seismology. Seismologists use seismic waves to learn about earthquakes and also to learn about the Earths interior. One ingenious way scientists learn about Earths interior is by looking at earthquake waves. Seismic waves travel outward in all directions from where the ground breaks and are picked up by seismographs around the world. Two types of seismic waves are most useful for learning about Earths interior.

## earths interior

By tracking seismic waves, scientists have learned what makes up the planets interior (Figure 1.4). P-waves slow down at the mantle core boundary, so we know the outer core is less rigid than the mantle. S-waves disappear at the mantle core boundary, so we know the outer core is liquid.

## surface waves

Surface waves travel along the ground, outward from an earthquakes epicenter. Surface waves are the slowest of all seismic waves, traveling at 2.5 km (1.5 miles) per second. There are two types of surface waves. The rolling motions of surface waves do most of the damage in an earthquake.

## body waves

P-waves and S-waves are known as body waves because they move through the solid body of the Earth. P-waves travel through solids, liquids, and gases. S-waves only move through solids (Figure 1.2). Surface waves only travel along Earths surface. In an earthquake, body waves produce sharp jolts. They do not do as much damage as surface waves. P-waves (primary waves) are fastest, traveling at about 6 to 7 kilometers (about 4 miles) per second, so they arrive first at the seismometer. P-waves move in a compression/expansion type motion, squeezing and S-waves (secondary waves) are about half as fast as P-waves, traveling at about 3.5 km (2 miles) per second, and arrive second at seismographs. S-waves move in an up and down motion perpendicular to the direction of wave travel. This produces a change in shape for the Earth materials they move through. Only solids resist a change in shape, so S-waves are only able to propagate through solids. S-waves cannot travel through liquid.

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

the high point of a wave.

``````a) trough

b) amplitude

-->  c) crest

d) length
``````

the height of a wave from the center line to its high point.

``````a) trough

-->  b) amplitude

c) crest

d) length
``````

scientists can learn most everything about earths interior by studying the waves that come into one seismograph.

``````a) true

-->  b) false
``````

the distance between waves from trough to trough is its.

``````c) trough

d) amplitude

e) crest

-->  f) wavelength
``````

s-waves can move through

``````a) solids, liquids and gases.

b) solids and liquids, but not gases.

-->  c) solids, but not liquids and gases.

d) liquids and gases, but not solids.
``````

which of these statements is not true about s-waves?

``````a) s-waves are secondary waves

b) s-waves move up and down or side to side

c) s-waves cannot travel through liquids

-->  d) s-waves compress and expand
``````

which of these statements is not true about surface waves?

``````a) there are two types of surface waves.

-->  b) surface waves travel through solids, liquids and gases.

c) surface waves travel along the ground.

d) surface waves are the slowest of all seismic waves.
``````

we know that earth has a liquid outer core because

``````a) p-waves travel through the core at the same speed they travel through solids.

-->  b) s-waves disappear at the core-mantle boundary.

c) surface waves highlight the transition between the liquid outer core and the solid inner core.

d) all of these.
``````

p-waves speed up at the mantle core boundary.

``````a) true

-->  b) false
``````

scientists can learn about earth interior by using seismic waves because:

``````a) p-waves go faster in material that is more rigid.

b) s-waves do not make it to all seismic stations if they must travel through a liquid.

c) p-waves bend slightly when they travel between material types.

-->  d) all of these.
``````

## diagram questions

No diagram questions associated with this lesson