# Seismic Waves and Earthquakes

Longitudinal waves ? the vibration goes forwards and backwards along the direction of travel. Think of sound. Sound is a series of collisions of particles. In air a drum skin vibrates (for example). As this skin goes up it pushes all the air particles up and they compress together. These will then push into the air particles above them and cause them to do the same. The sound travels through the air as a series of collisions. A good way to see this is to put some marbles between two long rulers. If you make one marble move along it will go as far as the next marble, hit it and stop. The one it hit will move along to the next one and hit that ? and so on. Another way to see longitudinal waves would be to use a slinky (a giant spring). If you stretch it out a bit the take one end and push it forwards and back you will see the spring?s coils bunch together. The ?bunching? of the coils will pass along the spring. The individual coils only go forwards and back a little way. Sound waves are examples of longitudinal waves. What are Transverse waves? Transverse waves vibrate sideways. Think of waves over water. These are transverse waves. The wave travels across the surface of the water but the individual molecules of water on the surface go UP and down, not sideways. Watch a piece of wood on the water. It bobs up and down. The vibrations are at 90 degrees to the direction of travel. Other examples of transverse waves are waves along a rope (think how you make the wave ? by moving the rope sideways) and all the electromagnetic spectrum waves ? radio waves, microwaves, infra red, visible light, ultra violet, X-rays and gamma rays. What is a seismic wave? These are waves from vibrations in the Earth (core, mantle, oceans), which also occur on other planets, for example the moon. They can be natural ? due to earthquakes, volcanic eruptions or meteor strikes - or man-made ? due to explosions or anything that thumps the earth hard
They are used in prospecting/research with the same principle as echo-sounding. P-waves (P for pressure) ? longitudinal waves which can travel through solid and liquid, including molten rock. S-waves (S for shake) ? transverse waves which travel through solid matter only. Patterns of waves picked up by seismometers can be used:
To locate distant earthquakes and major explosions.
To research the internal structure of the Earth: S-wave ?shadows?
provide evidence that the core of the Earth is molten.
What are P-waves and what are S-waves? P-waves are primary waves, and S waves are secondary waves. They can travel through fluids, such as water and air, as well as though solids. When an earthquake strikes, the first set of waves are compression waves, much like sound waves. Secondary waves ?wave? in a stereotypical fashion ? much like the waves visible on the surface of water. These cause the ground to sway. The ground moves up and down when an S wave moves past. These waves only move through rock. [image]What is the inside of the earth like? [image] What happens to S-waves as they pass through the earth? The ground moves up and down when an S wave moves past. What happens to P-waves? These cause the ground to sway. What are the effects of Earthquakes and what causes them? The effect: [image]On August 1976, a devastating earthquake of approximate magnitude 8 hit the island of Mindanao in the Southern Philippine Islands. The epicentre of this earthquake was in the Gulf of Moro in the Celebes Sea. A large tsunami generated by the earthquake resulted in the death of thousands of people (more than 10,000) in coastal communities in the Sulu Islands, North and South Zamboanga, North and South Lanao, North Cotabato, Maguindanao, and Sultan Kudarat, on the island of Mindanao. What caused it? Phivolcs said that the earthquake, which was located near Tagum, Davao, was tectonic in origin and caused by the movement of the Philippine Fault Zone.

Seismic Waves and Earthquakes 9.3 of 10 on the basis of 1950 Review.