Earthquakes

How can we define and explain earthquakes?

An earthquake is the sudden and violent shaking of tectonic plates and is caused by the sudden release of energy due to a buildup of stress between tectonic plates. This buildup of stress is a consequence of rocks from the Earth's Tectonic Plates getting caught on each other and generating friction (this happens because tectonic plates are constantly moving over or horizontally past one another). The strain eventually overrides the elasticity of the rocks, which results in the release of stress, leading to a shaking motion on the surface.

Two important definitions that you need to know are the ‘focus’ and ‘epicentre’ of an earthquake.

• The focus of an earthquake is the point between the tectonic plates where the rocks break off and the earthquake starts. The waves of energy spread from this point, with the greatest damage being close to the focus.
• The epicentre is the point on the Earth’s surface, above the earthquake’s focus.

How are earthquakes measured?

Earthquakes are measured based on the moment magnitude scale (MMS), which quantifies the total seismic moment released by an earthquake. It is calculated in reference to the distance that the ground moves along the slip and the force required to move it. It is often recorded using a seismograph.

The moment magnitude scale is logarithmic. This means that from one integer to the next, the amplitude of ground motion is ten times more and the energy released is 30 times greater. The scale ranges from 1Mw to 10Mw, where Mw stands for moment magnitude.

How do the physical processes of tectonic plates impact the magnitude of earthquakes?

The different physical processes, which are based on the type of plate margin, impact the magnitude of tectonic hazards, including earthquakes.

Earthquakes at divergent plate margins

Earthquakes at divergent plate margins often have low magnitudes (under 5.0) and a shallow focus (less than 60km deep).

Earthquakes at convergent plate margins

The frequency of earthquakes at convergent plate margins depends on the types of tectonic plates that meet.

1. Margins with two oceanic plates colliding and margins with oceanic and continental tectonic plates experience frequent large earthquakes up to magnitude 9.0. They have a focus between 10km to 400km. The focus follows the line of the subducting plate, also known as the Benioff Zone.
2. The margins of two continental plates cause earthquakes of high magnitude and shallow focus. These margins have huge faults and generate infrequent but major earthquakes, which are distributed over a large area.

Earthquakes at conservative plate margins

Earthquakes at conservative plate margins often have a shallow focus and reach magnitudes up to 8. They can be very destructive and often have aftershocks due to additional stress along the fault.

What are intraplate earthquakes and how are they caused?

Approximately 5% of earthquakes occur within the plates instead of at the plate margins. These earthquakes that take place away from the plate boundaries are called intraplate earthquakes. Plates travel over a spherical surface, and this creates areas of weakness. An earthquake occurs at these zones of weakness.

Examples of intraplate earthquakes are those caused by the New Madrid Seismic Zone on the Mississippi River. Here, earthquakes reach magnitudes of up to 7.5, even though the zone is thousands of kilometres from any plate margins.

What are the different types of earthquake waves?

As mentioned in the definition, an earthquake is caused by the sudden release of energy due to a buildup of stress between tectonic plates. This energy exists in the form of seismic waves. There are different types of earthquake waves, which include body waves (P waves and S waves) and surface waves (L waves and Rayleigh waves). Waves of energy travel through the ground as a result of the sudden release of stress from the rocks.

Earthquake waves: what are body waves?

Body waves are of higher frequency and travel through the interior of the ground. They travel faster than surface waves.

• P waves (also known as primary waves) travel the fastest and move through solid rock and fluids (up to 5000 metres per second in granite). They are similar to the motion of sound waves. P waves compress and dilate rocks and can also travel through the air.
• S waves (or secondary waves) are the slower waves that can only travel through solid rock.

Earthquake waves: what are surface waves?

There are different types of surface waves, but the two we focus on here are L waves and Rayleigh waves.

• L waves (or Love waves) travel through the Earth’s crust and have a lower frequency. Despite travelling slower than body waves, L waves cause most of the damage made by earthquakes. L waves move the ground from side to side.
• Rayleigh waves roll similarly to waves on water (up and down and side to side). Although they are slower than the L waves, most of the ground shaking that is felt during an earthquake is due to Rayleigh waves.

What are the consequences and effects of earthquakes?

The consequences of earthquake waves include ground shaking and crustal fracturing. Crustal fracturing occurs within the earth but can impact the surface through buckling and fractures. The secondary hazards caused by earthquakes include tsunamis, landslides, liquefaction, subsidence, and fires. The 2004 Indian Ocean earthquake mentioned at the beginning of the article caused a devasting tsunami.