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About Earthquakes

What happens when an earthquakes occurs?

An earthquake occurs when two blocks within the earth move suddenly past each other. The surface that the slip occured on is know as a fault. Where the earthquake started is called a hypocentre and the location on the surface directly about it is called the epicentre. When the earthquake occurs it releases energy in the form of vibrations that travel through the earth, known as seismic waves. There are several types of waves that travel at various speeds.

There are two categories of seismic waves:

 

Within the body wave category there are P-waves and S-waves, also known as compressional and shear waves. These waves are often used to find out where an earthquake occurred, know as the hypocentre.

There are also two main types of surface waves, called Rayleigh waves and Love waves. These can have quite damaging effects at large distances away from the earthquake. They are commonly described by people as a swaying motion like being on a boat due to their long wavelengths.

For more details about seismic waves see Braile's Seismic Wave Demonstrations and Animations

What causes earthquakes and where do they happen?

The driving forces that causes blocks in the earth to move in different directions from each other is Plate Tectonics. The outermost layer of the earth, know as the crust, is broken up into plates and these move around due to the convection of the layer underneath know as the mantle. 

(source: U.S. Geological Survery)

The edges of these plates, known as plate boundaries, are where the majority of earthquakes occur. Earthquakes can also occur in the interior of plates, such as in Australia, and associated with volcanoes.

 (source: NASA http://denali.gsfc.nasa.gov/dtam/seismic/)

 

How are earthquakes recorded?

Very sensitive instruments called seismometers record earthquakes and other small ground motions. Scientists look at the recordings from these instruments to determine an earthquake’s size and location. Below is a simplified figure by the USGS showing how a seismometer measures ground vibrations. 

 

Here is an example of an earthquake recorded by seismometer located at Mt Stromlo, Canberra, Australia. It is so sensitive it picked up waves that no one in Canberra could feel that originated from an earthquake in Indonesia. The plot is called a seismogram.

 

How can scientists tell where the earthquake happened?

Seismograms are used to determine the location of an earthquake. P-waves and S-waves travel at different speeds and this difference tells us how far the earthquake was from the seismometer recording it. It is a bit like the difference between the speed of light and sound when lightening strikes. The P-wave travels faster so is like the lighting flash and the S-wave is like the thunder. The closer in time the P-wave and S-wave arrive the closer you are to where the earthquake occured. Unfortuately, we can't tell from just a single seismogram what direction it came from, we need many. 

To learn more about how to locate earthquakes you can work through this Virtual Earthquake activity. 

How do scientists measure the size of earthquakes?

The size of an earthquake is commonly described in two ways.

Earthquake Magnitude

The earthquake magnitude scale (sometimes referred to as the Richter Scale) is a measure of the amount of energy released during an earthquake. The scale is logarithmic, meaning that every time you go up one whole number the approximate amount of ground shaking from the earthquake goes up 10 times. For example, a magnitude 6 earthquake would have ~10 times more ground shaking and 30 times more energy than a magnitude 5. The most accurate measure of magnitude is seismic moment or the moment magnitude. This estimate of the energy is directly related to the size of the area that ruptured and how far it slipped.

To put the magnitude scale into context this figure by the U.S. Geological Survey shows events in human history that have a similar energy release to certain earthquake magnitudes. For example, the atomic bomb that destroyed Hiroshima was equivalent to a magnitude 6.2 earthquake. 

Earthquake Intensity

Earthquake intensity commonly known, as The Modified Mercalli Intensity Scale, is a description of the severity or amount of damage caused by an earthquake. Intensity decreases with distance from the earthquake and is often determined based on people’s accounts of what happened during the earthquake (known as felt reports) and surveys of damage.

The intensity for the same magnitude earthquake in two different cities is likely to be different. The intensity will be influenced by:

This table provides a brief description of the scale:

I Instrumental Not Felt
II Weak Felt by persons at rest, on upper floors or very sensitive
III Slight Felt indoors. Vibration is like the passing of a light truck.
IV Moderate Felt by many. Vibration is like the passing of heavy trucks
V Rather Strong Felt by most, including outdoors. Small unstable objects displaced
VI Strong

Felt by all. Furniture moved. Weak plaster/masonry cracks. People frightened and run outdoors.

VII Very Strong Difficult to stand. Damage to masonry and chimneys.
VIII Destructive

Partial collapse of masonry. Frame houses moved. Heavy furniture moved.

IX Violent General panic. Masonry seriously damaged or destroyed.
X Intense Many buildings and bridges destroyed.
XI Extreme

Few structures remaining. Rails bent greatly. Pipelines severely damaged.

XII Catastrophic Nearly total destruction.

How do you prepare for an earthquake?

Before:

PLAN:

Earthquakes can occur at anytime so discuss with family members what you would do if an earthquake struck in the following situations:

Make sure you identify safe places to take cover at home and at your office/school. Make a plan to reconnect with family. Do not rely on phone communication as networks get overloaded during emergencies.

PREPARE:

Create an emergency survival kit. This is essential for all natural disasters not just earthquakes. If you live in an area prone to storms and floods or live in a remote community you might require more than 72 hours of supplies. Some essential items are:

During:

If you are inside…

If you are outside…

If in a vehicle, stop the car and stay inside the vehicle. Stay clear of overpasses and bridges.

After:

After a large earthquake there are often aftershocks during which you should follow Drop, Cover and Hold. Aftershocks may continue for several days to months but should decrease in severity over time.

Once the shaking has stopped it is useful to do the following:

Do not go sightseeing or enter damaged buildings. Stay calm and help others if possible.

 

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