1 Earthquake prediction and earthquake warning are not distinguishable
Earthquake prediction is the process of predicting the location, size, and timing of future earthquakes before they occur. However, with the current level of scientific and technological development of mankind, it is not possible, that is, earthquakes cannot be predicted at present.
Earthquake early warning is the basic premise that after an earthquake occurs, the speed of electromagnetic waves (300,000 km/s) is much faster than the speed of seismic waves (about 3-6 km/s), and the basic premise is to inform in advance through electromagnetic wave communication before the arrival of seismic waves, especially before the arrival of highly destructive surface waves. Therefore, earthquake early warning is essentially a time difference, and the farther away from the epicenter, the more time there will be to rush out. Conversely, if you are too close to the epicenter, it may cause the warning to be invalid (blind spot), which is roughly within 50 kilometers of the epicenter.
2 Earthquakes do not occur at one point
After an earthquake occurs, the news usually reports information such as the location of the epicenter, the depth of the hypocenter, etc. It's easy to misunderstand that an earthquake occurs at a point. Note: The location of the hypocenter represents only the place where the earthquake rupture began; An earthquake consists of three main phases: the beginning, the expansion and the cessation of fault rupture. Therefore, seismic occurrence is a process in which a three-dimensional fault rapidly ruptures and propagates (typically at speeds of up to 1-3 km/s) and excites seismic waves, not just a point.
3 The epicenter and the epicenter are two different things
Hypocenter: The location where the earthquake rupture began, which is a three-dimensional quantity (longitude, latitude, depth). Epicenter: The projection of the hypocenter on the earth's surface, which is a two-dimensional quantity (longitude, latitude). Note that the initial rupture location is not necessarily the location where the final slippage is greatest, nor is it necessarily the location where the damage is most severe.
4 The depth of the hypocenter is difficult to determine
Recall news reports that we often see cases where the depth of the hypocenter is 10 km. Strictly speaking, the current state of the art has poor constraints on the depth of the earthquake source; In other words, it's not accurate. In most cases, the uncertainty of the source depth is very large, and in the vernacular, the error is very large. Therefore, an approximate depth of occurrence is given, such as 10km.
In order to further determine, sufficient observation data (such as the constraints of special seismic phases, such as deep seismic phases), a reasonable subsurface medium velocity model, and manual intervention and verification are required. Essentially, this is because the determination of the depth of the hypocenter is a nonlinear inversion problem, which itself is difficult and in most cases lacks good observation data.
5 Why are there sometimes inconsistencies between the magnitude reported in China and abroad?
For example, the Wenchuan earthquake was given a magnitude of 8.2 in China, while the United States USGS gave a magnitude of 7.9. This is all true because magnitude is measured differently. In China, the surface wave magnitude (Ms) is usually used, while the United States USGS gives the moment magnitude (Mw). The surface wave magnitude is measured according to the maximum amplitude of the far-range field wave with a wavelength of 20 seconds, while the moment magnitude is measured according to the area and amount of sliding, so there will be some discrepancies in the results. In general, for large earthquakes, the numerical value of the surface wave magnitude will be a little larger.
6 Magnitude and intensity are two different things
Magnitude measures the magnitude of an earthquake; Severity measures the degree of destruction.
For example, if a magnitude 5 earthquake occurs, for the epicenter location, (1) if the depth of the epicenter is 200 km, the damage caused by it is negligible and the intensity is very small; However, if the depth of the epicenter is only 200 m, it may cause very strong surface shaking, which is very damaging and intense.
Therefore, for a certain earthquake, there is only one magnitude; And the intensity, different positions are different. The greater the intensity, the stronger and more destructive the surface shaking at that location when the earthquake occurs. In general, the farther away from the epicenter, the smaller the shaking and the less intense the earthquake will be.
7 magnitude can be negative
Magnitude is the logarithm of energy based on 10, and then add or subtract some coefficients, so when the earthquake energy is relatively small, the magnitude may be negative. For example, laboratory earthquakes are usually -2, -3, etc.
Because the coefficient before the magnitude (Ms) of the surface wave is 1.5, it is said that the magnitude difference is 1, and the energy difference between the seismic waves is 32 times (10^1.5).
8 If you stagger the earth in half, the magnitude of the earthquake will not exceed the magnitude of 12
From simple estimates, it can be seen that the moment magnitude of an earthquake that is 100 m staggered in half of the earth is less than 12.
Realistically, an earthquake with a magnitude of more than 10 is unlikely to occur on Earth. Wake up to the magnitude 30 earthquake that some people imagine, it's impossible.
9 Earthquake prediction is currently difficult to achieve
Earthquake prediction here refers to the size, time and location (time, space, intensity) of earthquakes that may occur in a certain area in the future. There is currently an estimate of the location and magnitude limit of the likely future occurrence of major earthquakes. However, it is not possible to estimate the timing of this event. Therefore, earthquake prevention and disaster reduction is still a passive defense, and it is most reliable to build a stronger building.
10 Earthquakes occur all the time in the world, and most of them are not felt by us
The number of magnitude 6 earthquakes in the world each year is: hundreds; The difference in magnitude is about 10 times greater, such as magnitude 5 or higher: thousands of earthquakes per year worldwide. Number of earthquakes per year worldwide: millions.
11 Most earthquake precursors are unreliable
Correlation does not represent cause and effect, and most of the time the so-called precursors are just chances. But we also need to make a lot of observations and accumulate valuable data. After all, the time scale for a person's life is usually up to 100 years, and the so-called recurrence cycle of a major earthquake is hundreds to thousands of years. In order to overcome the problem of earthquakes, it is necessary to make a large number of reliable observations over a long period of time as the basis for research.
In short, the most important thing to avoid known faults and build buildings in strict accordance with local fortification standards is to avoid known faults and reduce earthquake disasters.
We don't have to worry about it, most people don't have the opportunity to experience a major earthquake.
Source: Seismological Science