Why does the Earth have an earthquake?

Earthquakes are sudden vibrations of the Earth's surface, often referred to as geodynamic events in ancient times. They are natural phenomena that occur frequently on Earth, similar to wind, rain, lightning, landslides, and volcanic eruptions.

There are various causes for ground shaking. Based on their origin, earthquakes can be classified into several types:

1. Collapse earthquakes – These occur when underground cavities or mine roofs collapse. They are usually small in scale and rare, typically found in limestone areas or large mining regions.

2. Induced earthquakes – These are triggered by human activities such as reservoir impoundment or water injection in oil fields. They are limited to specific locations like reservoirs or oil fields.

3. Artificial earthquakes – These result from human actions like underground nuclear testing or controlled blasting. They are not natural but caused by external forces.

4. Tectonic earthquakes – Caused by the movement and rupture of deep rock layers, these are the most common type, accounting for over 90% of all earthquakes and often causing significant damage.

5. Volcanic earthquakes – These occur due to volcanic activity, such as magma movement or gas explosions. They are limited to volcanic regions and make up about 7% of global earthquakes.

Throughout history, people have created myths and legends to explain earthquakes. In Japan, around the 12th century, there were stories about giant squids beneath the earth that would move and cause tremors. In one tale, it was believed that if a squid became angry, its tail would stir the ground, leading to an earthquake.

Ancient China also had similar myths, where a large creature was thought to lie under the earth, and when it turned over, it caused the ground to shake. In ancient Greece, philosophers like Epicurus suggested that trapped winds in the Earth's crust could cause earthquakes. Others, like Lukcelushi, proposed that sudden air pressure changes inside the Earth led to seismic events.

In the early 20th century, scientists began studying seismic waves more deeply, marking a new era in seismology. Three major theories emerged:

First, in 1911, Reid proposed the "Elastic Rebound Theory," suggesting that when strain energy builds up and exceeds rock strength, a fault forms, and the released energy causes an earthquake.

Second, in 1955, Matsuzawa introduced the "Magmatic Impact Theory," which claimed that uneven heat conduction in rocks could lead to sudden expansion and rupture.

Third, Bridgeman suggested that changes in mineral structure under high pressure and temperature could trigger earthquakes through sudden volume changes.

Although many mysteries remain unsolved, the interdisciplinary approach involving physics, chemistry, geology, and other sciences has significantly advanced our understanding of earthquakes. Today, we continue to explore and learn more about these powerful natural events.