The terms used to describe the movements of the earth are earthquake or quake, and they consist in the vibrations generated in the internal areas of the Earth being propagated in waves through the rocks. As a result of these vibrations, the tectonic plates will move, a situation that is often caused by volcanic activity.
The term earthquake is generally used to describe those movements of the tectonic plates which cause major human or material damages, and the term quake or seismic movement is reserved for those aftershocks that go unnoticed. The term non-seismic is used to describe movements resulting from human action. To assess the magnitude of a seismic wave, we use the Richter scale.
Through studies in connection with the seismic activity held at seismic observatories spread across the globe, it was noticed that the movements of the earth’s crust have a circular center of propagation of the seismic waves. The surface point is where the earthquake’s intensity is measured and it’s called the epicenter. Because of this, the intensity of the earthquake is defined both by hypocenter’s depth (measured up to 700 km) and by the intensity and direction of wave propagation.
The intensity which is measured in the epicenter will be so much higher, depending on how close the hypocenter is to the surface. In case of an earthquake, we can distinguish between several types of movements: horizontal, vertical and torsional. Depending on the causes of the earthquake, we can distinguish between earthquakes: tectonic and volcanic earthquakes.
The distribution of earthquakes
There is a map that tracks seismic territories in which intense and frequent earthquakes manifest: the
Mediterranean-Himalayan belt, which includes the Carpathian Mountains and the Vrancea seismogenic zone, the Pacific ring of fire, which produces 80% of all strong earthquakes around the globe and where 90% of the annual seismic energy is concentrated. The rest of seismic areas such as the East African Rift, the Atlantic and Pacific inner side have a comparatively low seismic activity.
Most earthquakes occur frequently in Japan. In Romania, the most powerful earthquakes are located in the Vrancea Mountains and are produced by movements of the earth’s crust. There are, however, areas where there are no earthquakes. These are called aseismic area and are found in Africa, Brazil, Canada, Australia, the Russian platform and Greenland.
Production of earthquakes
The moment when the earthquake begins, the first shock waves will start propagating from the epicenter. The first waves that appear are called primary waves, or P waves. These waves are longitudinal waves propagating like sound and produce bakc and forth movements in the direction of propagation. These waves are followed by some secondary ones, or S waves. Thanks to them, the rocks will move perpendicularly to the direction of propagation. The last type of wave is the surface wave and causes waving, emphasizing the devastating effects of secondary waves.
1. P wave:
- determines the movement of the earth parallel to the direction of propagation
- It is a compression-type longitudinal wave
- moves in the direction of compression-expansion
- its amplitude is proportional to the magnitude of the earthquake
- its not dangerous for buildings because it contains only 20% of the total energy of the earthquake
- people perceive the surface wave just like a bounce, a tiny vertical shock
2. S Wave:
- determins the movements of the soil particles in a direction transverse to that of propagation
- it is a shear-like wave
- it is felt on the surface like a shearing motion
- its movements are similar to a snake advancing, with undulating movements of left and right
- it is dangerous because it carries 80% of the total energy of the earthquake, causing damage proportional to the duration and magnitude earthquake oscillation
1. Divergent edges
If two plates whose edges are made up of oceanic crust that are moving away from each other meet, in the empty space, a hot molten rock will emerge, forming volcanoes. The rock will cool in ocean water, hardening and leading to the formation of a new oceanic crust. This crust will push the two oceanic plates, moving them away from one another, causing earthquakes in the area. The place where this phenomenon occurs is called a divergent area.
As a result of the clash between two plates, their edges will be destroyed. The result of destruction depends on the type of crusts that the edges are made out of. So if an oceanic and a continental plate collide, the oceanic one will be forced to go under the continental one, because it is thinner. This is called subduction. The crack where the plate slides underneath the other one is called fossa.
When two oceanic plates collide, the same thing can happen, where one of the plates will go underneath the other. When two continental plates collide, it creates areas of mountains, because the edges colliding will pucker. Through this method, mountains such as the Himalayans were formed. The area where two plates collide is called the convergence zone.
3. When tectonic plates pass by each other, they will slip and unite, rubbing against each other and causing each other to vibrate, creating earthquakes.