What is the epicenter and the focus of the earthquake
Earthquakes are among the most impressive natural phenomena on earth and have a very high destructive potential. Because of this, an earthquake quickly turns into a natural disaster. Earthquakes can cause landslides and avalanches. The most feared side effects of earthquakes are tsunamis, which often claim far more lives than the triggering earthquake.
What is an earthquake?
By definition, earthquakes are measurable vibrations of the earth's body. The tremors are mostly caused by spontaneous mass shifts along fracture zones in the earth's crust. In this case one speaks of tectonic earthquakes. These displacements can be so great that they affect the Earth's axis of rotation. A tectonic earthquake can shift entire regions.
If fluids such as magma penetrate the earth's crust, the penetrating material can cause fractures in the rock, which also cause earthquakes. These tremors are called volcanic earthquakes.
How do earthquakes occur?
Tectonic earthquakes occur at fault zones or plate boundaries. In the course of the continental drift, the earth's crustal plates shift relative to one another. The plates move between 2 and 4 cm on an annual average. The plates can collide and cause mountains to grow, or they can drift apart and create gorges and valleys. Particularly along fault zones, plate parts slide past each other or form upward and downward movements. But it can also happen that the plates get stuck. Then great tensions build up in the rock. If the shear strength of the rock is exceeded, it breaks suddenly: an earthquake occurs.
During an earthquake, energy is released in the form of seismic waves. These spread from the earthquake focus in all directions. There are different types of seismic waves. They differ in the direction of vibration, frequency, speed and energy. The fastest are the P waves, followed by the S waves. Different types of seismic waves have different destruction potentials.
Magnitudes and seismograms
The strength of an earthquake is expressed in terms of magnitude. The magnitude is mainly determined from the amplitude of a seismogram, which is recorded with the aid of a seismometer.
A seismometer is a device that registers the vibrations of the earthquake waves. The magnitude can be mathematically related to the energy of an earthquake and is thus mathematically defined. The strength of an earthquake can also be described by its intensity. The visible effects of an earthquake are primarily observed here.
There are different scales for classifying the magnitude of the earthquake. The best-known scale is the Richter scale. The underlying amplitude measurements from seismograms are only accurate for relatively short distances from the epicenter. Theoretically, the Richter scale is open at the top. In practice, earthquakes with magnitudes greater than 9.5 cannot be recorded in a differentiated manner. The most common scale today is the moment magnitude scale Mw. It is based on the seismic moment. This is the scalar product of the size of the fracture surface in the subsurface, the mean displacement of the rock blocks and the shear modulus of the rock. The moment magnitude scale is limited to Mw 10.6, as it is assumed that at this value the solid earth's crust breaks completely. When increasing by one magnitude level, 32 times the amount of energy is released.
When earthquake strengths are stated on vulkane.net, I refer to this magnitude.
Epicenter, hypocenter and the earthquake focus
In connection with earthquakes, the terms epicenter and hypocenter appear again and again. The epicenter is the point on the earth's surface above the focus of the earthquake. The latter is the place where the earthquake takes place underground. The hypocenter is identical to the focus of the earthquake and describes the spatial position of the earthquake in the earth's crust. So the depth of the earthquake is also taken into account.
Foreshock, main tremor, aftershock
Strong earthquakes are often heralded by a series of weak foreshocks. Seismologists try to use these foreshocks to make predictions about upcoming major tremors. However, a series of weak earthquakes need not be followed by major tremors. A strong earthquake can just as easily occur spontaneously. After a major quake, several weaker earthquakes usually occur, which are known as aftershocks.
Swarms of earthquakes and swarms of earthquakes
One speaks of a swarmquake when several earthquakes of comparable magnitude occur in a given location. So there is no much stronger earthquake, so that one cannot differentiate between main and aftershocks. Swarm quakes of small magnitude often occur in connection with magma ascent. Swarm quakes with magnitudes greater than 5 are mostly of tectonic origin.
Earthquakes manifest themselves predominantly along the plate boundaries and other fault zones, or at volcanoes. In Germany it is the Rheingraben along which the greatest (albeit very low) earthquake risk exists. In Italy, the earth shakes particularly frequently in the Apennines and Sicily. In the west, Portugal is the focus of earthquakes. Towards the eastern Mediterranean, Greece and Turkey are particularly at risk. The earthquake belt then continues through Iran and along the coasts of Southeast Asia. Along the Pacific coast there are not only a particularly large number of volcanoes, but also earthquakes: Indonesia, Japan and Kamschtka are earthquake hot spots. The earthquake regions continue across the Aleutian Islands, across Alaska and Canada to the west coast of the USA. While earthquakes occur relatively rarely on the east coast of the USA, things are very different in the Caribbean. Of course there are also frequent earthquakes in western Central America, just like along the west coast of South America. The Andes are a mountain range with frequent earthquakes and volcanic eruptions. In Africa, the earth shakes frequently in the East African Rift Valley. It is less dangerous in the middle of the continents: Central Asia, Canada, the middle of the USA, Central Greenland and Australia are very rarely shaken.
The most catastrophic earthquakes in recent years
In the last two decades there has been an accumulation of extremely catastrophic earthquakes, although I don't know which one is more memorable: the Sumatra earthquake of 2004 or the Japan earthquake of 2011. The latter led to the disaster of the Fukushima nuclear power plant and led the energy transition in Germany. Both earthquakes triggered tsunamis, which dramatically increased the disaster. The Sumatra disaster killed more than 230,000 people and brought it to a magnitude of 9.1. In Japan there were officially over 11,000 dead. Unofficially, it was spoken of 28,000 victims. The main Japanese island was offset by 240 cm, the axis of the earth by 10 cm. The earthquake had a magnitude of 9.0.
The events in Haiti must have been no less dramatic when an earthquake with a magnitude of 7.0 triggered a catastrophe in 2010: 316,000 people officially died, many of the victims were buried under the dilapidated buildings of the capital Port au Prince. Almost 2,000,000 people were left homeless.
The strongest earthquake ever recorded manifested itself in 1960 under the Chilean Valdivia. It had a magnitude of 9.5 and claimed 1655 lives.
Behavior in the event of an earthquake
How do you behave in an earthquake? There is also a leaflet from GFZ-Potsdam.
As a volcano filmmaker, I have already experienced and reported on several earthquakes. However, I hardly had time to react. Generally one should try not to panic. The greatest danger comes from falling parts. Even medium-strength earthquakes, with magnitudes around 5, can severely damage simple buildings. Roof tiles, facade parts, scaffolding and bursting window panes become deadly projectiles. If you are in a building, furniture can fall over, parts of the ceiling come down, or the building can even collapse completely. If possible, one should seek out open spaces outside. If you are in a house, you should lie down next to sturdy pieces of furniture. Anyone near an exit can try to leave the building. But be careful: you can be hit by falling rubble, especially in the entrance area. Never use elevators. There is also a risk of fires and explosions if gas pipes break. Basements can fill up with water.
If you are planning a trip to an earthquake-prone region, it can make sense to choose accommodation based on the aspect of earthquake safety. Cheap or old hotels with ailing buildings are to be avoided. In addition, if you are close to the coast you should consider that you are exposing yourself to the risk of a tsunami. This can roll on even if the earthquake was thousands of kilometers away.
In valleys and at the foot of steep slopes, avalanches, rockfalls or mud flows can represent an additional hazard. As a precaution, it can't hurt to have an earthquake warning app installed on your smartphone or to be notified by SMS. When you find yourself in a disaster area, the telephone network often no longer works. A small battery operated radio can serve as a reliable source of information.
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