Which of the following sounds is produced by an earthquake before the main shock wave begins:

Waves over the frequency of 20 hertz are not audible to humans, and infrasound is a sound wave below the lower limit of human hearing, while ultrasound waves are above the upper limit of human hearing.

Earthquake is the sudden movement of the earth's tectonic plates, which causes tremors in the earth's crust and produces infrasound before the main shock wave begins. During this process, there is a sudden release of seismic waves that causes the ground to shake, and these seismic waves are classified as infrasound waves.

The main source of earthquakes is when tectonic plates contact, resulting in mountain formation and earthquakes and volcanoes.

This infrared sound, also known as low frequency sound, refers to sound waves with frequencies lower than the lower limit of audible frequency, which is generally considered to be twenty hertz, and hearing of these infrared frequencies is thus not possible or less perceivable because the frequency is very low, but at higher intensities, these infrared vibrations can be felt in various parts of the body.

The study of these infrared sound waves is commonly referred to as infrasonic, which includes sound below twenty hertz down to one by tenth hertz and rarely to one by hundredth hertz. These infrasonic disturbances in the atmosphere can extend to approximately fifty kilometres above the earth's surface, with the ability to travel considerable distances around.

Hence, the correct option is (2)