Just like wind and rain, earthquakes are a relatively common natural phenomenon. Therefore, people have long since started recording earthquakes and pondering their causes. In China, the earliest recorded earthquake was the "Taishan earthquake", which occurred in the Xia Dynasty nearly 4,000 years ago, and it may also be the earliest earthquake record in the world. However, ancient people didn't understand the causes of earthquakes very well and put forward various interesting theories. For example, in Japan, it was said that there was a large catfish underground, and as long as the catfish turned over, the earth would shake; in ancient Greece, people believed that the "gas" underground made the earth shake, and their myths even had gods who were in charge of earthquakes; while in ancient China, influenced by the Five Elements theory, it was believed that earthquakes were caused by "imbalance of Yin and Yang". There were also some more rational explanations. For example, some believed that earthquakes were caused by the collapse of underground caves; others, based on the phenomenon of volcanic eruptions, suggested that earthquakes were caused by the accumulation of too much fire underground. Due to the lack of instruments for observing earthquakes and sufficient evidence, many explanations were unscientific and inaccurate.
It wasn't until 1906 that people's understanding of earthquakes began to deepen. That year, an earthquake occurred in San Francisco, USA, and it was found that a row of continuous fences were broken into two segments by the earthquake, with obvious displacement between them. Through further research, scientists realized that earthquakes might be caused by the sudden rupture of underground rock layers. American engineer Reid proposed an earthquake model called the "elastic rebound" theory. According to this theory, underground rocks deform due to stress, and as time passes, the deformation becomes more and more severe. When the deformation reaches a certain level, the rocks will rupture, resulting in earthquakes. The "elastic rebound" is a highly simplified model, and the actual situation is much more complex. However, with this model, at least some earthquakes caused by geological structures can be explained. For example, the 2008 Wenchuan earthquake in China occurred in the Longmen Mountain fault zone, which had been explored by geologists for more than 400 kilometers. Today, the scientific community believes that the vast majority of natural shallow earthquakes are caused by geological forces that cause large-scale deformation of the earth's outer rock layers and sudden slippage along geological faults.
In the early 20th century, scientists began to establish a global earthquake network. After years of statistical analysis, they found that, on a global scale, earthquakes are mainly distributed in three major regions: one is the Pacific Rim, the second is the Mediterranean-Himalayan region, and the third is the mid-ocean ridge. That is to say, most earthquakes occur at the boundaries of tectonic plates, where tectonic plates either compress, stretch, or slide against each other, resulting in earthquakes. For example, the 2004 Sumatra earthquake, the 2010 Haiti earthquake and the 2011 Japan earthquake were typical interplate earthquakes. However, earthquakes can also occur within plates, such as many earthquakes in China that do not occur at plate boundaries. Scientists believe that this may be because mainland China is composed of multiple ancient small and medium-sized blocks, which can be regarded as lower-level "plates", and most intraplate earthquakes occur at the boundaries of these tectonic blocks.
Most earthquakes are tectonic earthquakes, but there are also other types of earthquakes, such as volcanic earthquakes, impact earthquakes (such as meteorite impacts), reservoir-induced earthquakes, and nuclear test earthquakes. These earthquakes are generally much weaker and less frequent than tectonic earthquakes.