Earthquakes produce which kind of sound before the main shock wave begins. 

Infrasound is a sound wave that is below the lower limit of human hearing, whereas ultrasound is a sound wave that is above the higher limit of human hearing. Humans cannot hear waves above the frequency of 20 hertz.
Before the main shock wave starts, there is a sudden release of seismic waves that cause the ground to tremble. These seismic waves are classified as infrasound waves. An earthquake is essentially the sudden movement of the earth's tectonic plates, which causes tremors in the crust and produces infrasound.
When tectonic plates collide over one another, mountain development results in earthquakes and volcanoes, which is usually the primary source of earthquakes.

This infrared sound, also known as low frequency sound, refers to sound waves with frequencies below the lower limit of audibility, which is typically considered to be twenty hertz. Hearing of these infrared frequencies is therefore not possible or, alternatively, less perceptible because frequency is very low, but at higher intensities it is possible to feel these infrared vibrations in various body parts.
The term "infrasonic" refers to the study of these infrared sound waves, which includes sound below twenty hertz down to one by tenth hertz and rarely to one by hundredth hertz. Infrasonic disturbances in the atmosphere can extend up to about fifty kilometres above the earth's surface and have the ability to travel great distances.

Hence, the correct answer is option B.