Re: What are the differences in the waves pertaining to earthquakes?

Message ID: 860441651.Es

Subject: What are the differences in the waves pertaining to earthquakes?

Michele,

The three types of earthquake waves: P, primary or compressional waves, S, secondary or shear waves, and L, surface waves are distinguished based on their different behaviors, that is, by their different modes of travel.

P- & S-waves are body waves, that is, they can travel through the interior of the earth. L-waves are called surface waves because they only travel along the surface of the earth. L-waves are generated when P- or S-waves intersect the ground surface.

P-waves are the fastest of the three, which is why they are called primary waves. As they pass through the earth, they travel through earth materials (solid rock, molten rock, or loose sediment) the same way that sound travels through the air -- by compressing the molecules within them. This means that, as the wave passes through the material, the molecules are first pushed together (compressed) then pulled apart (expanded) in a series of movements. The compression and expansion occurs in the direction of wave travel. If you are familiar with the toy known as a "slinky", they work in a similar way, as do coiled springs. Compressional waves can travel through matter in any of its three states: solid, liquid, or gas. The denser the material, the faster they travel.

S-waves travel faster than L-waves but slower than P-waves, which is why S-waves are called secondary waves. S-waves travel through materials the same way a wave passes through a piece of rope when you shake it -- in a series of sideways, or shearing, movements. So S-waves move perpendicular to the direction of wave travel. You can observe this by holding one end of a six- or seven-foot long rope while a friend holds the other end. When you shake the rope, you can see the shear waves pass along it, but the wave crests move at right angles to the direction of wave travel. As S-waves pass through the earth's interior, they can travel only through solids -- so they cannot pass through the liquid outer core of the earth. This is one of the ways scientists know that the outer core is composed of liquid.

L-waves are the slowest earthquake waves and exhibit complex motion. As they travel up and down along the earth's surface like a rolling ocean wave, or side-to-side like a snake, L-waves deform the ground surface along with anything that rests on it. They are very dangerous since they can lift up the foundation of a building at one end, while at the other end the building is dropping downwards, or can move one end of the house forward while the other end is forced back. Unless specially designed, few buildings can withstand this kind of motion. L-waves are the most destructive type of earthquake wave.

With the help of a seismograph, P-waves (or S-waves or both) are used to determine the magnitude of earthquakes. The difference in travel time between P- and S-waves is used to determine the location of earthquakes. The shape of P- and S-waves provides information concerning the type of movement along the fault.

A good source for further information is the college textbook "Geology and the Environment" by B.W. Pipkin, West Publishing Company, 1994. Nearly any introductory environmental geology book will contain a fairly detailed discussion of earthquake wave mechanics at a level that is suitable for most high-school and college students.