Earthquake

From Wikipedia, the free encyclopedia

For other uses, see Earthquake (disambiguation).

"Seismic event" redirects here. For seismic migration, see Seismic migration.

Global earthquake epicenters, 1963–1998

Global plate tectonic movement

An earthquake (also known as a quake, tremor or temblor) is the perceptible shaking of the surface of the Earth, resulting from the sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes can be violent enough to toss people around and destroy whole cities. The seismicity or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time.
Earthquakes are measured using observations from seismometers. The moment magnitude is the most common scale on which earthquakes larger than approximately 5 are reported for the entire globe. The more numerous earthquakes smaller than magnitude 5 reported by national seismological observatories are measured mostly on the local magnitude scale, also referred to as the Richter magnitude scale. These two scales are numerically similar over their range of validity. Magnitude 3 or lower earthquakes are mostly almost imperceptible or weak and magnitude 7 and over potentially cause serious damage over larger areas, depending on their depth. The largest earthquakes in historic times have been of magnitude slightly over 9, although there is no limit to the possible magnitude. Intensity of shaking is measured on the modified Mercalli scale. The shallower an earthquake, the more damage to structures it causes, all else being equal.^[1]
At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacement of the ground. When the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a tsunami. Earthquakes can also trigger landslides, and occasionally volcanic activity.
In its most general sense, the word earthquake is used to describe any seismic event — whether natural or caused by humans — that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by other events such as volcanic activity, landslides, mine blasts, and nuclear tests. An earthquake's point of initial rupture is called its focus or hypocenter. The epicenter is the point at ground level directly above the hypocenter.


Do's during earthquake

• If you're indoors, stay there. Get under -- and hold onto --a desk or table, or stand against an interior wall. Stay clear of exterior walls, glass, heavy furniture, fireplaces and appliances. The kitchen is a particularly dangerous spot. If you’re in an office building, stay away from windows and outside walls and do not use the elevator.
   
• If you're outside, get into the open. Stay clear of buildings, power lines or anything else that could fall on you.
   
• If you're driving, move the car out of traffic and stop. Avoid parking under or on bridges or overpasses. Try to get clear of trees, light posts, signs and power lines. When you resume driving, watch out for road hazards.
   
• If you're in a mountainous area, beware of the potential for landslides. Likewise, if you're near the ocean, be aware that tsunamis are associated with large earthquakes. Get to high ground.
   
• If you’re in a crowded public place, avoid panicking and do not rush for the exit. Stay low and cover your head and neck with your hands and arms. 
  
  
  
  
  
  
                                                                                        
  
  
  Dont's during earthquake
   
  What rescuers and experts DO NOT recommend you do during an earthquake
  Based on years of research about how people are injured or killed during earthquakes, and the experiences of U.S. and international search and rescue teams, these three actions are not recommended to protect yourself during earthquakes:
  DO NOT run outside or to other rooms during shaking: The area near the exterior walls of a building is the most dangerous place to be. Windows, facades and architectural details are often the first parts of the building to collapse. To stay away from this danger zone, stay inside if you are inside and outside if you are outside. Also, shaking can be so strong that you will not be able to move far without falling down, and objects may fall or be thrown at you that you do not expect. Injuries can be avoided if you drop to the ground before the earthquake drops you.
  DO NOT stand in a doorway: An enduring earthquake image of California is a collapsed adobe home with the door frame as the only standing part. From this came our belief that a doorway is the safest place to be during an earthquake. True- if you live in an old, unreinforced adobe house or some older wood-frame houses. In modern houses, doorways are no stronger than any other part of the house, and the doorway does not protect you from the most likely source of injury- falling or flying objects. You also may not be able to brace yourself in the door during strong shaking. You are safer under a table.
  DO NOT get in the “triangle of life”: In recent years, an e-mail has been circulating which describes an alternative to the long-established “Drop, Cover, and Hold On” advice. The so-called “triangle of life” and some of the other actions recommended in the e-mail are potentially life threatening, and the credibility of the source of these recommendations has been broadly questioned.
  The “triangle of life” advice (always get next to a table rather than underneath it) is based on several wrong assumptions:
  
• buildings always collapse in earthquakes (wrong- especially in developed nations, and flat “pancake” collapse is rare anywhere);
• when buildings collapse they always crush all furniture inside (wrong- people DO survive under furniture or other shelters);
• people can always anticipate how their building might collapse and anticipate the location of survivable void spaces (wrong- the direction of shaking and unique structural aspects of the building make this nearly impossible) ; and
• during strong shaking people can move to a desired location (wrong- strong shaking can make moving very difficult and dangerous).

Some other recommendations in the “triangle of life” e-mail are also based on wrong assumptions and very hazardous. For example, the recommendation to get out of your car during an earthquake and lie down next to it assumes that there is always an elevated freeway above you that will fall and crush your car. Of course there are very few elevated freeways, and lying next to your car is very dangerous because the car can move and crush you, and other drivers may not see you on the ground!