Will there often be small earthquakes before big earthquakes?
Will there be frequent small earthquakes before big earthquakes?
Small earthquakes don't always happen. The reason of the earthquake is that the plates on the earth collide with each other, resulting in dislocation and rupture at the edge and inside of the plates. However, earthquakes also need to have enough energy. If small earthquakes occur frequently in a place, the energy in the crust will be released frequently without accumulating energy, so the remaining energy is insufficient, so generally speaking, it will not cause a big earthquake.
Seismic waves propagating inside the earth are called body waves, which are divided into longitudinal waves and shear waves. The waves with the same vibration direction and propagation direction are longitudinal waves, and the longitudinal waves from underground make the ground bump up and down. Waves whose vibration direction is perpendicular to the propagation direction are shear waves. Shear waves from underground will cause horizontal ground vibration. Because the propagation speed of longitudinal waves in the earth is faster than that of shear waves, in earthquakes, longitudinal waves always reach the surface first, while shear waves always lag behind. In this way, when a recent earthquake occurs, most people will feel up and down at first, and it will take a few seconds to ten seconds to feel a strong horizontal shaking. Shear waves are the main cause of damage.
Earthquake accumulation and disasters
After the earthquake, the subsequent aftershocks have obvious characteristics of temporal and spatial aggregation. Theoretically, whether it is the Lushan earthquake on June 1 2022 or the Marcand earthquake on June 10, 2022, there are bound to be subsequent aftershocks, but generally speaking, the total energy released by aftershocks may be less than 5% of the main earthquake. However, previous studies also show that the maximum magnitude in the aftershock sequence can reach the magnitude of the main shock minus 1, indicating that the potential danger of aftershocks of some large earthquakes cannot be ignored. Judging from the trigger mechanism of main shock-aftershock, it is entirely possible that the magnitude of an aftershock approaches or even exceeds that of the main shock. This possibility can be described by the classical Gutenberg-Richards relation, that is, in the aftershock sequence, the number n of aftershocks with magnitude greater than m should also satisfy log 10N(M)=a-bM, where a and b are constants.
The 6. 1 earthquake that occurred in Lushan, Sichuan on June12022 may have sounded the alarm for us, revealing that the aftershocks may have enough potential dangers. For the Marcand earthquake on June 10, 2022, it is very important to monitor the activity law in the later period of the earthquake sequence. In fact, some strong earthquakes with similar magnitudes occurred in pairs, which may have almost hinted at the importance of aftershock disasters, such as the earthquakes with M = 7.3 and M = 7.4 in Longling, China on May 29th, 1976. In addition, controlled by the unique mechanism of stress change after some main earthquakes, the spatial distribution of aftershocks will increase with time. All these bring great challenges for seismologists to study the time, space and scale of earthquakes.
How did the earthquake happen?
Influenced by plate movement, volcanic eruption, human activities and many other factors, there are forces with complex temporal and spatial characteristics at different depths of the earth where we live. Under the action of a specific force, the material in the earth will be deformed, and when the deformation occurs in the shallow part of the lithosphere, the material deformation caused by this force usually conforms to the elastic relationship; That is to say, just like a compressed spring, its compression or extension is linearly proportional to the force acting on its two ends. Generally speaking, in order to quantify the force acting on an object, the force can be converted into the force acting on a unit area, that is, stress. With the increasing stress on the elastic lithosphere, brittle fracture will occur in the elastically deformed medium in a short time. This process is like folding a chopstick with our hands. With the increase of force, chopsticks first bend like elastic deformation, and then break, resulting in brittle fracture. In the elastic (crustal) lithosphere, when the fault is accompanied by instantaneous relative motion between the two sides, an earthquake is formed. At this time, the magnitude of the earthquake can be calibrated by moment magnitude, that is, MW = (lgm0)/1.5-10.7, where M0=μ? W? S is the seismic moment (M0, N.m), which is equal to the product of the area (w, m2) of the seismic fracture surface, the displacements (s, m) on both sides of the seismic fracture surface and the elastic modulus (μ, N/m2) of the elastic crust. Of course, there are also some methods to calibrate earthquake magnitude according to the amplitude characteristics of seismic waves. There is a certain empirical conversion formula between them and earthquake moment magnitude, so I won't repeat it here.
It should be noted that after billions of years of long geological evolution, the interior of the earth under our feet has been filled with a large number of faults with different scales, different modes of movement and different activity habits within a certain depth. Under the action of stress, these existing faults are more prone to (re) fracture and sliding than the elastic medium around them. This explains the main reason why most earthquakes with large magnitude occur on existing faults.
When the load reaches the stress extreme value required by fault sliding (rock fracture), the fault moves relatively fast to form an earthquake, which excites different types of seismic waves.
However, although we know the basic principles of earthquakes, seismologists still know little about the three elements of earthquakes (time, place and magnitude). The main reasons are as follows: firstly, we have limited knowledge of the detailed characteristics of faults tens of kilometers deep in the earth's crust; Secondly, even if we can know the exact shape of faults, we still don't know how strong the seismic (rupture-resistant) ability of each fault is; Third, seismologists don't know how strong the absolute stress is on each fault. We believe that with the continuous development of science and technology, just like today's daily weather forecast, the era when human beings can predict earthquakes will surely come.