Question

Earthquakes produce which kind of sound before the main shock wave begins.
A. Ultrasound
B. Infrasound
C. Audible sound
D. None of the above

Answer 1

Hint: Waves above the frequency of twenty hertz are not audible to human beings and infrasound is the sound wave that is below the lower limit of human hearing and ultrasound waves are considered above the upper limit of human hearing.

Complete step by step answer:
Earthquake is basically the sudden movement of tectonic plates of earth which causes tremor of earth crust and it produces infrasound before the main shock wave begins during this process there is a sudden release of seismic waves that makes the ground shake and these seismic waves are classed as infrasound waves.
Main cause of earthquakes is generally when the tectonic plates collide over one another causing mountain building resulting in earthquakes and volcanoes.
This infrared sound also referred to as low frequency sound means the sound waves having the frequency below the lower limit of audible frequency which is generally considered as twenty hertz and hearing of these infrared frequency is therefore not possible or we can say less perceivable because frequency is very low but at higher intensities it is possible to feel these infrared vibrations in various parts of body.
The study of these infrared sound waves is generally referred to as infrasonic which includes sound beneath twenty hertz down to one by tenth hertz and rarely to one by hundredth hertz, this infrasonic disturbances in the atmosphere can extends to generally fifty km above earth's surface actually having the tendency to travel considerable distances around the globe.
Hence the correct option is B i.e. Infrasound.

Note:These infrared sounds are generally used for investigation of internal structure of the planet and in nature these infrasound waves generally occur in earthquakes , waterfalls, volcanoes and also in various atmospheric phenomena such as thunder, wind and weather patterns.