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Earthquakes produce seismic waves, which propagate on the earth and can be detected and recorded by seismographs. Seismic waves can not only provide the focal location and depth of seismic events, but also tell researchers valuable information about the internal structure and composition of the earth.

However, not all earthquake events occur naturally, and some are caused by human activities, such as underground nuclear tests. These man-made events may pose a threat to global security, the environment and human health. Therefore, it is important for seismologists to be able to distinguish between natural and man-made earthquakes, especially those involving nuclear explosions.

In this article, we will introduce some methods that seismologists use seismic waves to distinguish between earthquakes and underground nuclear explosions.

One of the main differences between P-wave / S-wave ratio earthquake and underground nuclear explosion is the ratio of P-wave (P-wave) and S-wave (S-wave) produced by each event. P waves are faster and can pass through solids and liquids, while S waves are slower and can only pass through solids. The explosion produces more P waves than S waves, while the earthquake produces more S waves than P waves.

This is because the explosion releases energy equally in all directions, creating a spherical wavefront that expands outward from the source. The wavefront is mainly composed of P waves and rarely contains S waves. On the other hand, the earthquake releases energy along the fault plane, and the fault plane slips or breaks due to tectonic stress. This produces a complex wavefront, which is mainly composed of S waves, and the content of P waves varies according to the direction and geometry of the fault.

The P-wave / S-wave ratio can be measured by comparing the amplitude or energy of each wave at different distances from the source. High P-wave / S-wave ratio indicates the source of similar explosion, while low P-wave / S-wave ratio indicates the source of similar earthquake.

Another difference between an earthquake and an underground nuclear explosion is the frequency of their seismic waves. Frequency is a measure of the number of oscillations of a wave in a unit time. In unit time, high-frequency waves have more oscillations than low-frequency waves. Nuclear explosions produce more high-frequency waves than earthquakes, and earthquakes produce more low-frequency waves. This is because the duration of the explosion is shorter and the focal range is smaller than that of earthquakes.

An explosion usually lasts less than a second, while an earthquake can last a few seconds or minutes. The explosion also has a dotted source, while the source of the earthquake extends along the fault plane. These factors affect the energy released by each event at different frequencies. The frequency content of seismic waves can be measured by analyzing its frequency spectrum, which shows the energy at different frequencies. The high-frequency spectrum represents the source of an explosion, while the low-frequency spectrum represents the source of a similar earthquake.

Aftershock sequence the third difference between an earthquake and an underground nuclear explosion is their aftershock sequence. Aftershocks are smaller earthquake events that follow larger earthquakes in space and time. Aftershocks occur because the main event changes the rock around the stress field, causing new cracks or sliding along existing cracks.

Explosions may cause earthquakes and some aftershocks, but they are usually smaller and fewer aftershocks than natural earthquakes. This is because the explosion will not create new cracks in the rock, but earthquakes will. The aftershock sequence can be measured by calculating the number and magnitude of aftershocks. The sparse aftershock sequence represents the source of an explosion, while the dense aftershock sequence represents the source of a similar earthquake.

Challenges and limitations while these methods are useful in distinguishing earthquakes from underground nuclear explosions, they are not foolproof. Factors such as depth, distance, geology and interference will affect the seismic signal, making it difficult to interpret correctly.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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