The Causes and Measurement of Earthquakes
1. Introduction
The Earth’s lithosphere is made up of several large tectonic plates that move around on the planet’s surface. The word ‘lithosphere’ comes from the Greek word ‘lithos’, meaning ‘stone’, and ‘sphaira’, meaning ‘sphere’. The lithosphere is the solid outer layer of the Earth that includes the crust and the uppermost mantle. The lithosphere is broken into several large tectonic plates that move around on the planet’s surface. Earthquakes happen when these plates move or collide with each other.
There are three main types of plate margins: divergent, convergent, and transform. Divergent plate margins happen when two plates move away from each other. The best example of this is the Mid-Atlantic Ridge, where the North American Plate and the Eurasian Plate are moving apart. This type of plate margin can also be found at the East African Rift Valley, where the African Plate is splitting into two plates – the Nubian Plate and the Somalian Plate.
Convergent plate margins happen when two plates move towards each other and collide. The best examples of this are subduction zones, where one plate moves underneath another plate. The Pacific Ring of Fire has many convergent plate margins, including the Japan Trench (where the Pacific Plate subducts beneath the Philippine Sea Plate) and the Peru-Chile Trench (where the Nazca Plate subducts beneath the South American Plate).
Transform plate margins happen when two plates slide past each other in different directions. An example of this is the San Andreas Fault in California, where the Pacific Plate is sliding northward past the North American Plate.
2. Types of plate margins and their movement
As stated earlier, there are three types of plate margins: divergent, convergent, and transform (Duffett, 2006). Each type of margin has a different way of movement, which will be elaborated below.
Divergent Margins
Divergent margins occur when plates move away from each other (Duffett, 2006). This can be caused by many factors such as mantle convection currents or variations in temperature (Duffett, 2006). When mantle convection currents rise, they push on the lithospheric plates and cause them to diverge (Duffett, 2006). Mantle convection currents are caused by heat escaping from the Earth’s core (Duffett, 2006). Variations in temperature can also cause divergent margins to form (Duffett, 2006). When there is a difference in temperature between two areas, it can cause one area to expand while the other contracts (Duffett, 2006). This can lead to cracking and eventually divergence of the lithospheric plates (Duffett, 2006).
The most well-known divergent margin is the Mid-Atlantic Ridge which runs through Iceland and down the middle of the Atlantic Ocean (Duffett, 2006). Another notable divergent margin is found in East Africa called The Great Rift Valley (Duffett, 2006).
While divergent margins don’t usually cause earthquakes since the plates are moving away from each other, they can sometimes cause volcanoes to form (Duffett, 2006). When magma rises to the surface it can form new landmass along divergent margins (Duffett, 2006).
Convergent Margins
Convergent margins occur when plates move towards each other and collide (Duffett, 2006). This can happen for a few reasons. One reason is that the Earth’s mantle has higher density than the lithosphere (Duffett, 2006). This difference in density causes the mantle to slowly sink while the lithosphere floats on top (Duffett, 2006). As the mantle sinks, it pushes the lithospheric plates together which causes them to converge (Duffett, 2006). Another reason convergent margins can form is because of variations in temperature (Duffett, 2006). When one area is warmer than another it causes convection currents to form which can push the lithospheric plates together (Duffett, 2006).
There are two main types of convergent margins: ocean-continent and ocean-ocean (Duffett, 2006). Ocean-continent convergent margins happen when an oceanic plate collides with a continental plate (Duffett, 2006). The continental plate is much thicker and more buoyant than the oceanic plate so it will ride overtop of the oceanic plate and force it to sink down into the Earth’s mantle (Duffett, 2006). An example of an ocean-continent convergent margin is the Peru-Chile Trench where the Nazca Plate subducts beneath the South American Plate (Duffett, 2006).
Ocean-ocean convergent margins happen when two oceanic plates collide with each other (Duffett, 2006). When this happens, both plates will start to sink down into the mantle since they are both dense (Duffett, 2006). The Pacific Ring of Fire has many examples of ocean-ocean convergent margins including the Japan Trench where the Pacific Plate subducts beneath the Philippine Sea Plate (Duffett, 2006).
While convergent margins don’t always cause earthquakes, they can sometimes cause volcanoes to form (Duffett, 2006). When one plate sinks beneath another it can cause magma to be pushed up to the surface which can result in a volcanic eruption (Duffett, 2006).
Transform Margins
Transform margins occur when two plates slide past each other in different directions (Duffett, 2006). This can happen for a few reasons. One reason is that there is a difference in density between the two plates (Duffett, 2006). When this happens, one plate will start to slide on top of the other since it is less dense (Duffett, 2006). Another reason transform margins can form is because of changes in temperature (Duffett, 2006). When one area is warmer than another it causes convection currents to form which can push the lithospheric plates together (Duffett, 2006).
One well-known transform margin is the San Andreas Fault in California where the Pacific Plate is sliding northward past the North American Plate (Duffett, 2006). This fault line runs for about 800 miles (1,300 kilometers) and is responsible for some of the largest earthquakes in California history including the 1906 San Francisco earthquake which had a magnitude of 7.8 on the Richter scale (Duffett, 2006)
Transform margins don’t usually cause earthquakes but when they do they are usually small since the plates are just sliding past each other (Duffett, 2006).
3. The causes of earthquakes
Earthquakes can be caused by many different things but the most common cause is plate tectonics. Plate tectonics is the scientific study of the movement and behavior of the Earth’s lithospheric plates (Duffett, 2006). Earthquakes happen when these plates move or collide with each other.
The three main types of plate margins are divergent, convergent, and transform (Duffett, 2006). Divergent margins happen when two plates move away from each other. Convergent margins happen when two plates move towards each other and collide. Transform margins happen when two plates slide past each other in different directions.
Divergent margins usually don’t cause earthquakes since the plates are moving away from each other. Convergent margins can sometimes cause earthquakes when one plate subducts beneath another. Transform margins can sometimes cause earthquakes but they are usually small since the plates are just sliding past each other.
4. Measuring earthquakes
Earthquakes are measured using a seismometer which is a device that detects and measures vibrations in the ground (Duffett, 2006). Seismometers were first invented in the early 1800s by a man named John Michell (Duffett, 2006). Seismometers work by detecting the shaking of the ground and measuring the strength of the shaking (Duffett, 2006). The strength of the shaking is called the earthquake’s magnitude (Duffett, 2006). The magnitude is typically measured on the Richter scale which ranges from 1 to 10 (Duffett, 2006).
Earthquakes can also be measured by their intensity which is a measure of how strong the shaking was at a particular location (Duffett, 2006). The intensity is typically measured on the Modified Mercalli scale which ranges from I to XII (Duffett, 2006).
Seismometers are usually located in seismograph stations which are buildings that house seismometers and other equipment used to measure earthquakes (Duffett, 2006). Seismograph stations are usually run by government agencies or universities (Duffett, 2006).
5. Conclusion
Earthquakes are a natural phenomenon that occurs when the Earth’s lithospheric plates move or collide with each other. Earthquakes can happen at any time and any place on Earth. They are typically caused by plate tectonics but can also be caused by other things such as volcanic eruptions or meteorite impacts. Earthquakes are measured using a seismometer which detects and measures vibrations in the ground. Seismometers were first invented in the early 1800s by a man named John Michell. Seismometers typically measure the magnitude of an earthquake on the Richter scale and the intensity of an earthquake on the Modified Mercalli scale.