Waves & the Earth

Waves & the Earth

1. Infrasound

   • Sound and light both travel as waves. Sound waves are longitudinal vibrations which travel through mediums, which can be solid liquid or gas. The frequency of a sound waves determines it pitch.
     • Humans can her hear sounds with frequencies between 20 Hz and 20 000 Hz. Sound waves with low frequencies produce low pitches, and those with frequencies below 20 Hz are called infrasound which can be detected using microscopes.
       • Some animals produce and hear infrasound, such as elephants and giraffes. Infrasound travels further in air than the sound waves of higher frequencies before they become too faint to detect. Whales also use infrasound, which can travel for hundred of kilometers through the ocean. Animals' use of infrasound means that biologists can use microphones to detect where animals are and how much they move in a year.
         • Volcanic eruptions release infrasound, meaning that eruptions can be detected and monitored. As well as this, meteors produce infrasound, so scientists can detect the paths of meteorites to ensure people's safety.

2. Ultrasound

   • Soundwaves with frequencies over 20 kHz are called ultrasound. Some animals, such as dolphins use ultrasound to communicate with each other. This is called Sonar.
     • Bats use sonar to detect obstacles and to communicate. The ultrasound waves made by animals are reflected by objects, meaning that they can listen to the echoes to work out where an object is. Humans can use sonar equipment to detect the depth of the sea or the location of fish. A loudspeaker on the ship emits a pulse of ultrasound, which spreads out across the sea, and some of it is reflected by the seabed. A microphone measures the time it takes for the echo to return and the distance the ultrasound travels can be calculated using the speed equation.
       • Ultrasound can also be used to make images of things inside the body such as unborn babies. When an ultrasound scan is made, some of the waves are reflected each time they interact with the foetus creating an image of the baby.

3. Seismic Waves

   • Earthquakes transmit waves through the earth called seismic waves. When they reach the surface, the earth shakes. Seisometers measure seismic waves.
     • The place inside the earth where rock suddenly moves or fractures is called the focus of the earthquake. The point on the surface of earth directly above the focus is called the epicentre. Ab earthquake causes two types of waves within the earth: P waves (longitudinal) and S Waves (Transverse), P waves travel faster than S waves. P waves push and pull the rock as they move through earth and S waves move the rock from side to side.
       • Scientists investigate the layers of rock near the earth by setting off small explosions or even dropping a large mass from a truck. They place seisometers to record the seismic waves at different distances from the source. Seismic waves are reflected and refracted as they pass into different rocks, therefore the speed of the waves gives scientists information about the rocks beneath the surface. Oil companies use this information to find oil and gas beneath the earth's surface.
         • The properties of rock gradually change with depth, causing the speed of seismic waves to increase gradually. This makes the waves gradually refract upwards and travel on a curved path. When there is a sudden change in rock properties, such as a layer of different rock, the waves are refracted sharply at this boundary and some are reflected.
           • Scientists use information from seisometers to all around the world to analyse seismic waves produced by earthquakes. Analysis of data can help to model the structure of the earth.

4. Detecting Earthquakes

   • Scientists use a network of seisometers around the world to detect earthquakes and work out where they occurred. If it happens in a remote location, scientists can often alert emergency services to get people out of the disaster area.
     • P waves travel faster than S waves, so arrive at the epicenter first. This tie difference allows scientists to work out how far away the earthquake is from the seisometer. Data from three or more seisometers can be used to work out the location of the earthquake, this is called triangulation.
   • The crust of the earth is made out of tectonic plates, pushed by very slow convection currents in the mantle. These plates are all moving relative to each other. Friction between plates stops them moving, before the forces become big enough to overcome the friction, causing them to jerk away from each other. These jerks are the causes of earthquakes.
     • This means that scientists can use the plate boundaries to predict where an earthquake might occur. However, it is not currently possible to measure the movement of the plates, making predictions very difficult.
       • If an earthquake happens under the sea, the movement causes a tsunami. However scientists cannot tell whether or not a tsunami will happen from seisometer tracings. Tsunami warning systems include pressure sensors that detect the tsunami waves in water. They travel at an average of 220m/s in deep water, so people on the far side of an ocean from a tsunami ay get a few hours warning.