Earthquakes

Between 2000 and 2011 over 90 million people were affected by earthquakes and more than 700,000 people lost their lives, the majority of which (549, 092), were due to four single events:

  1. the 2004 Indian Ocean earthquake, which killed 165, 708 people and affected 5 million.
  2. the 2005 Kashmir earthquake, which killed 73,338 and affected 5 million.
  3. the 2008 Sichuan, China earthquake, which killed 87,476 people and affected 4 million. 
  4. the 2011 Haiti earthquake, which killed 222,570 people and affected 3 million.

The impacts of earthquakes vary spatially and temporally: the area hit, the time of day, and the levels of development, awareness and mitigation can all affect the destructive power of an earthquake. An earthquake with a magnitude of 6 can occur in California and affect very few people, but an earthquake of the same magnitude could occur in Iran and kill many thousands.

What are earthquakes..?

Earthquakes are the shaking or trembling of the Earth induced by the abrupt rupture and release of energy that is tectonic or volcanic in origin.

Where do earthquakes occur..?

Most earthquakes occur along the edge of the oceanic and continental plates.

The earth’s crust – the outer layer of the planet – is made up of several pieces, called tectonic plates. The plates under the oceans are called oceanic plates and the rest are continental plates. These plates move around by the motion – convection currents – of the molten mantle that lies underneath the crust.

Tectonic plates are always bumping into each other, pulling away from each other, or moving past each other. Earthquakes usually occur where two plates are running into each other or sliding past each other.

Earthquakes also occur far from the edges of plates, along faults. Faults are cracks in the earth where sections of a plate (or two plates) are moving in different directions. Faults are caused by all the bumping and sliding the plates do and are more common near the edges of the plates.

History shows that earthquakes occur in the same general patterns year after year, namely in three main areas:

  1. the circum-Pacific seismic belt, or the “Ring of Fire,” which loops around the Pacific ocean taking in the South American coast, Central America, the West coast of America and on to Japan, the Philippines the South Pacific Islands and New Zealand. 81% of the world’s earthquakes occur here due to the number of converging plates and young, growing mountains and deep ocean trenches.
  2. the Alpide, which extends from Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic. This area accounts for 17% of the world’s earthquakes and also some of the most destructive in history.
  3. the mid-Atlantic Ridge, which is a fracture zone along the bottom of the Atlantic Ocean where molten mantle comes to the surface and creates new crust.

Earthquakes do occur outside these areas though, in the USA or central Europe for example.

Due to global seismic monitoring via national and international initiatives – for example, the United States Geological Survey’s, “Global Seismographic Network” – the occurrence, intensity and location of earthquakes are now well documented, allowing for better tracking of the impacts of earthquake events.

When do earthquakes occur..?

The temporal occurrence of earthquakes remains essentially random, unexplained and unpredictable. However, some earthquakes are believed to have been linked to mining, the building of a dam, volcanic activity and bomb testing.

Why do earthquakes occur..?

Earthquakes are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves that make the ground shake.

When two blocks of rock – or two tectonic plates – are rubbing against each other, they stick a little: they can’t just slide smoothly against each other because the rocks catch on each other. When this happens, the rocks – or tectonic plates – are still pushing against each other, but they can’t move. After a while, the rocks break because of all the pressure that has built up and the sudden release of pressure results in an earthquake.

During the earthquake, and afterward, the plates are still moving and they continue to move until they get stuck again.

The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus at ground level is called the epicentre and this is what is often mentioned in the press.

Earthquake Myths

Can you predict earthquakes?

No. No scientist has ever predicted a major earthquake. They do not know how, and they do not expect to know how any time in the foreseeable future. However, based on scientific data, probabilities can be calculated for potential future earthquakes. For example, scientists estimate that over the next 30 years the probability of a major earthquake occurring in the San Francisco Bay area is 67%.

Can animals predict earthquakes?

The earliest reference to unusual animal behaviour prior to a significant earthquake is from Greece in 373 BC. Rats, weasels, snakes, and centipedes reportedly fled several days before a destructive earthquake. Anecdotal evidence abounds of animals, fish, birds, reptiles, and insects exhibiting strange behaviour anywhere from weeks to seconds before an earthquake. Elephants apparently moved to higher ground just before the 2004 Boxing Day earthquake and subsequent tsunami. However, consistent and reliable behaviour prior to seismic events, and a mechanism explaining how it could work, still eludes us.

Can the ground open up during an earthquake?

No. Hollywood lies. Movement occurs along the plane of a fault, not perpendicular to it. If faults opened up, no earthquake would occur because there would be no friction to lock them together.

Shallow crevasses can form during an earthquake-induced landslide, but faults, however, do not open up during an earthquake.

Will California eventually fall off into the ocean?

No. Again, Hollywood lies. The strike-slip earthquakes on the San Andreas Fault are a result of the plates moving past one another. The plates of the San Andreas Fault are moving horizontally past one another, so California is not going to fall into the ocean. However, Los Angeles and San Francisco will one day be adjacent to one another!

Do earthquakes cause volcanoes?

No. Earthquakes may occur in an area before, during, and after a volcanic eruption, but they are the result of the active forces connected with the eruption, and not the cause of volcanic activity.

Do solar flares or magnetic storms cause earthquakes?

No. Damn you Hollywood.

Earthquake FAQ’s

What is the biggest earthquake ever?

The biggest earthquake recorded since 1900 was the May 22, 1960 Chile earthquake that measured 9.5 on the Richter Scale.

Which Country has the most earthquakes?

The answer to this question is not as straightforward as it may seem. In order to most accurately answer it I need to rephrase the question:

  1. For which country do we locate the most earthquakes? Japan. The whole country is in a very active seismic area, and they have the densest seismic network in the world, so they are able to record many earthquakes, including lots of small ones.
  2. Which country actually has the most earthquakes? Indonesia is in a very active seismic zone, also, but by virtue of its larger size than Japan, it has more total earthquakes.
  3. Which country has the most catastrophic earthquakes, or which has had the most damage and fatalities? Both China and Iran are in seismically active areas, have very long historical records, and have had many catastrophic earthquakes. Turkey is also worth mentioning in this category.

Foreshocks, aftershocks – what’s the difference?

Foreshock” and “aftershock” are relative terms. Foreshocks are earthquakes which precede larger earthquakes in the same location. Aftershocks are smaller earthquakes which occur in the same general area during the days to years following a larger event or “mainshock.”

As a general rule, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the main shock. The frequency of these aftershocks decreases with time.

Do earthquakes occur in Antarctica?

Earthquakes do occur occasionally in Antarctica, but not very often. There have been some big earthquakes–including one magnitude 8–in the Balleny Islands. Smaller quakes are much more likely to go undetected in Antarctica because there are very few seismograph stations.  The interior of Antarctica does have icequakes however, which, although they are much smaller, are perhaps more frequent than earthquakes, but occur within the ice sheet itself instead of the land underneath the ice.

Where can I buy a Richter scale?

The Richter scale is not a physical device, it is a mathematical formula: the magnitude of an earthquake is determined from the logarithm of the amplitude of waves. We use seismographs to measure earthquakes.

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