Langfang 4.2 magnitude earthquake, why others were woken up by the shaking, but I was sleeping soundly? !

According to the China Earthquake Networks Center, a magnitude 4.2 earthquake occurred at 1:21 a.m. today in Yongqing County, Langfang City, Hebei Province (39.42 degrees north latitude, 116.6 degrees east longitude), with a focal depth of 20 kilometers. A magnitude 2.9 earthquake occurred at 1:25 a.m. today in Yongqing County, Langfang City, Hebei Province (39.42 degrees north latitude, 116.57 degrees east longitude), with a focal depth of 15 kilometers.

Image from CCTV News

Regarding the Langfang earthquake, many netizens in the Beijing-Tianjin-Hebei region said that they were awakened by the shaking in their sleep, or awakened by the earthquake warnings issued by their mobile phones. However, some netizens said that they did not feel strong shaking. What's going on?

Why do people feel earthquakes differently?

Why can some people in the same area feel the vibrations strongly while others feel nothing at all?

Earthquake refers to the feeling of ground shaking or tremors that people feel when an earthquake occurs. The strength of the earthquake varies depending on factors such as the magnitude of the earthquake, the depth of the epicenter, the distance from the epicenter, and the geological environment. It is also affected by personal physique and location.

According to the national standards of the People's Republic of China, there are three types of earthquakes: mild, strong, and very strong:

01 Slight vibration

The main characteristics are: people in the room can feel it; doors and windows make slight noises, hanging objects swing, and utensils make noises. It is equivalent to intensity III, IIVV.

02Strong tremor

The main characteristics are: feeling violent shaking, unstable standing, waking up from a dream; doors, windows, roofs, and roof frames shaking and making noises; tables vibrating and moving, and objects on the tables moving or falling. Equivalent to intensity V, VI, and VII.

03Extremely strong vibration

The main characteristics are: feeling swaying and bumping, difficulty walking, people who are moving may fall, people in an unstable state may fall off the ground, and have a feeling of being thrown up; sometimes unimaginable phenomena may be observed, such as strong ground sounds, strange ground lights, unpleasant ground air, etc. Equivalent to intensity VIII or above.

The "different" shock feeling is related to these factors

Earthquake parameters: the “black hand” behind the earthquake

Some parameters of the earthquake itself (such as magnitude, focal depth and distance from the epicenter) play a major role in the feeling of the earthquake. Generally speaking, at the same location, if the magnitude is the same, the shallower the focal point, the stronger the feeling; the deeper the focal point, the weaker the feeling. If the focal depth is the same, the larger the magnitude, the stronger the feeling; the smaller the magnitude, the weaker the feeling.

Geological environment: "seismic changes" in different landforms

When an earthquake occurs, two main types of seismic waves are generated: longitudinal waves (P waves) and transverse waves (S waves). Longitudinal waves travel faster and reach the ground first. The vibration direction of its particles is consistent with the propagation direction of the wave, giving people the feeling of ups and downs. Although people can feel the vibration, its destructive power on buildings is relatively weak.

The propagation speed of transverse waves is slower than that of longitudinal waves, and the direction of particle vibration is perpendicular to the direction of wave propagation, which will cause horizontal shaking of the ground and is extremely destructive to buildings.

As seismic waves propagate through the Earth's interior and surface, they are affected by the regional geological and geomorphic environment and local geological structure.

If the earthquake encounters valleys, steep cliffs and other terrains near the surface, reflection will occur. According to relevant research, after the reflected wave is superimposed on the incident wave, positive phase interference between peaks and peaks and troughs will be formed in some areas, thereby enhancing the earthquake sensation; while in other areas, the superposition of peaks and troughs may cancel each other out, resulting in a weakened earthquake sensation. This also explains why people who are not far away from each other can have obvious differences in the sensation of the same earthquake.

In addition, seismic waves have different propagation characteristics in different geological structures. When an earthquake occurs, people will feel different vibrations depending on the hardness of the soil in their area.

Different geological structures

Specifically, seismic waves travel faster in deep rocks, and when they "run" into shallow soft strata, the wave speed slows down and the amplitude increases. So if people are on different types of surfaces, even if they are at the same distance from the earthquake site, residents living near soft strata will still feel stronger vibrations than those in hard rock areas.

Personal constitution: How different bodies feel

Similar to motion sickness and seasickness, some people also feel dizziness, nausea and other discomfort symptoms during an earthquake. This is related to the body's sensory organ, the vestibular organ in the inner ear. That is, due to the change in the accelerated motion state of the external environment, the vestibular organ in the inner ear is stimulated, causing the central nervous system to react, thereby causing symptoms such as dizziness and vertigo.

Specifically, the vibration and shaking of the ground during an earthquake stimulates the vestibular system of the inner ear. When this stimulation exceeds the individual's tolerance range, it causes discomfort.

In addition, the infrasound waves generated by earthquakes may also have a resonance effect on the human inner ear organs, further aggravating the symptoms of dizziness and nausea.

However, each individual's physical condition and tolerance are different, so the reaction to the accelerated motion state of the outside world is also different. Some people will feel a stronger vibration, while others may feel nothing at all.

Location: "Contrast of shock sensations" in different spaces

Where were you during an earthquake? Were you stationary or moving? Were you awake or asleep? Generally, for earthquakes of magnitude less than 3, it is difficult for people walking on flat ground to feel them, but people in high-rise buildings may feel them. For earthquakes of magnitude 3 to 4, some people outdoors can feel them, and people in high-rise buildings will feel them more clearly.

This is because low-rise buildings are shorter and have a smaller amplifying effect on seismic waves, so the tremors are weaker; while high-rise buildings are more likely to amplify seismic waves due to the resonance effect, causing residents on high floors to feel stronger vibrations.

In addition, the seismic resistance of a building will also affect the feeling of earthquake. For example, buildings with reinforced concrete structures can absorb and transmit earthquake energy more easily than brick and wood structures, thereby enhancing the feeling of earthquake.

Personal status impact

The state of an individual (such as whether they are still or awake) can also affect the perception of an earthquake. For example, a person who is still is more sensitive than one who is moving, and a person who is sleeping may not be able to sense an earthquake in time. In addition, people with sensitive physical conditions (such as the elderly or those with certain diseases) are more likely to feel the vibrations brought by an earthquake.

Author: Tadpole Jun

Reviewer: Liu Ying, Li Peiyuan