Earthquakes occurring in various regions of our planet surprise and frighten many people with the loud noise that is heard during the initial tremors. The rumble at the beginning of an earthquake is likened to an explosion or a loud collision. Most of the acoustic signals accompanying earthquakes are in the infrasonic range, which is not perceived by most people. But some of them, according to eyewitnesses, are quite audible and resemble the noise of a waterfall, distant pops, cannon shots or muffled roar.
According to the observations of seismologists, there is nothing anomalous about these sounds. It is the primary P-waves, caused by the movement of layers and tectonic plates, that are responsible for the loud noise. These waves have the highest propagation speed and arrive first. When they reach the Earth’s surface, they penetrate into the atmosphere, and at that moment, some of their energy in the air is converted into a sound similar to noise or an explosion. Surface vibrations occur later and silently, as they are caused by slower waves. Therefore, the rumble in the ground can be considered one of the precursor signals of an impending earthquake.
Factors influencing the intensity and frequency of sounds during an earthquake
The duration and nature of sound before tectonic tremors depend on several factors, including:
- nature of tectonic plate deformations causing earthquakes – such as shifting, rupturing, or compression;
- intensity of tremors;
- depth of the earthquake epicenter;
- distance on the surface to the epicenter;
- type of rock in the fault zone.
Thus, the strongest sound was recorded by seismologists during the 2011 earthquake in Japan with a magnitude of 9.1, when the Eurasian tectonic plate subducted beneath the Okhotsk plate. According to researchers, earthquakes that occur in ancient rock layers typically sound louder and more powerful. They have stronger energy and transmit it more intensely than rocks in younger crystalline shields. As distance from the epicenter increases, the rumble preceding the earthquake changes characteristics – instead of an explosion, a dull rumble is heard.
Acoustic noises after earthquake
Since major earthquakes are often followed by aftershocks, the strength of which is sometimes comparable to the intensity of the primary tremor, residents in some regions hear a frightening rumble even after the seismic activity. Greek seismologists were concerned about a series of eerie underground sounds on the island of Lesbos. This happened after a strong earthquake with a magnitude of 4.9 on January 7, 2023. Scientists carefully analyze such events because:
- precursor noise may indicate a stronger repeat earthquake;
- it’s impossible to reliably determine whether there is a new fault in the plate, which could trigger tremors and cause serious damage;
- due to seismic history of Lesbos, the noise persisting after the earthquake could be another precursor factor for subsequent tremors.
Concerns of seismologists were confirmed when a subsequent earthquake with a magnitude of 4.8 on the Richter scale occurred three days later. Therefore, researchers believe that the loud sounds from the depths of the Earth before an earthquake should be perceived as precursors, positive signs that should not cause panic, but instead prepare for the impending event. A person hearing the Earth’s rumble has 3-5 minutes to prepare for the earthquake, considering the speed of P-waves (6 km/s). Additionally, the duration of the rumble may indicate the intensity of the approaching earthquake and its proximity to the Earth’s surface.