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Flashcards in Waves and the Earth Deck (14):

What is infrasound?

Sound with frequencies less that 20Hz


What are the uses of infrasound?

Communication between animals (elephants)
Detection of animal movement in remote locations
Detection of volcanic eruptions and meteors


What is ultrasound?

Sound with frequencies greater than 20kHz


What are the uses of ultrasound?

Communication between animals (Dolphins)
Foetal scanning


What is sonar?

Can be used to find depth of sea or detect fish
Loudspeaker on ship emits pulse of ultrasound, spreads through water, some reflected by sea bed
Microphone on ship detects echo, equipment measures time between sound sent out & echo returning
Depth = (speed x time) / 2


What are seismic waves generated by?

Earthquakes or explosions


Where can seismic waves be reflected and refracted?

At boundaries between the crust, mantle and core


What type of waves can seismic waves be?

Longitudinal (P) waves
Transverse (S) waves


How do P waves travel inside the earth?

Push & pull on rock particles as the wave passes
Travels in circles from focus like lines on a dartboard


How do S waves travel inside the earth?

Move rock particles from side to side as the wave passes from the focus


How are seismic waves refracted inside the earth?

Reflected & refracted when they pass into different rocks
Properties of rocks gradually change with depth causing speed of waves to gradually increase
If sudden change in rock properties, waves refracted sharply & some may be reflected


Why do scientist find it difficult to predict earthquakes and tsunamis even with available data?

Not currently possible to measure forces trying to move plates or friction between them.


How can seismometers be used to find the location of an earthquake?

P waves travel faster than S waves - detected by seismometer first
Time difference between arrival of P & S waves used to find how far away from seismometer the earthquake occurred
Data from 3 or more can be used to find epicentre - radius of distance of epicentre from each seismometer drawn on map - point where they intercept is epicentre


How are earthquakes caused?

Outermost layer of earth made of tectonic plates pushed by convection currents in mantle. Plates moving relative to each other. Friction between plates stops them moving until forces big enough to overcome friction and then they move with a sudden jerk. Each jerk causes an earthquake