Earthquake - all you want to know about earthquakes

Contents

1. What is an Earthquake? (Earthquake meaning)
2. What causes earthquakes?
3. Where do most earthquakes occur?
4. Fault types
5. Earthquake magnitude ratings
6. Earthquake damage

 Earthquakes 

What is an Earthquake? (Earthquake meaning)

An earthquake is a natural phenomenon characterized by the shaking of the earth's surface. It is caused by the sudden release of energy in the earth's crust, resulting in seismic waves propagating through the ground.

Earthquake: all you want to know about earthquakes

Earthquakes can occur anywhere on Earth, but more commonly they occur at the boundaries of tectonic plates. 

The energy released in an earthquake is generated by the movement of tectonic plates that form the outer envelope of the earth. These plates are in constant motion due to the convection currents in the underlying mantle.

The boundaries where the plates interact are areas of high stress, and when the stress exceeds the strength of the rocks, it causes them to fracture and slide past one another, releasing stored energy.

The size and magnitude of an earthquake are commonly measured using magnitude scales such as the Richter scale or the moment magnitude (Mw) scale. These scales quantify the amount of energy released by the earthquake. 

Earthquakes can range from minor, barely noticeable tremors to major events that can cause widespread destruction and loss of life. Earthquakes can cause a variety of impacts, including earthquakes, landslides, tsunamis (if the earthquake occurs under the ocean), and secondary hazards such as fires and gas leaks.

The severity of damage caused by an earthquake depends on factors such as the magnitude of the earthquake, proximity to populated areas, quality of infrastructure, and the preparedness of affected communities. 

Scientists and engineers study earthquakes to better understand their causes, predict their occurrence, and develop strategies to mitigate their effects. Measures to reduce the risks associated with earthquakes include earthquake-resistant structures, early warning systems, and emergency plans.

What causes earthquakes?

The main cause of earthquakes is the movement of the earth's tectonic plates. The Earth's crust is divided into several large plates that float on top of the underlying semi-liquid layer called the asthenosphere. These plates are constantly moving, albeit very slowly, due to the convection currents in the underlying mantle.

The boundaries where tectonic plates interact are the most common sites for earthquakes. Types of plate boundaries:

1. Convergent Boundaries: These are the points at which two plates clash. Depending on the type of plates involved, one can be pushed under the other in a process called subduction. Due to collision and subduction, strong pressures and stresses can build up in the crust, causing earthquakes.
2. Divergent Boundaries: These are areas where two plates diverge from each other. This movement creates a gap, and magma from the underlying mantle rises to fill it and form a new crust. The separation and stretching of the plates can cause earthquakes as the rocks fracture and adjust to the new configuration.
3. Transform Boundaries: The locations where two plates slide horizontally past each other are known as transformation boundaries. The plates lock together due to friction, but continued movement eventually overcomes the friction and causes the plates to suddenly slide. This sudden release of energy causes earthquakes to occur along the transform fault.

Besides plate tectonics, earthquakes can also be triggered by other factors, although they are comparatively rarer. This includes volcanic activity where magma rises and interacts with surrounding rocks, creating pressure and fracturing the crust. Human activities such as mining, reservoir-induced seismicity (due to filling large reservoirs), and hydraulic fracturing (fracking) for oil and gas production can also trigger seismic events. 

The energy released during an earthquake propagates in the form of seismic waves that shake the ground. The magnitude of an earthquake is measured using instruments called seismometers, and severity is often indicated using scales such as the Richter scale or the moment magnitude (Mw) scale, which quantify the amount of energy released.

Where do most earthquakes occur?

Most earthquakes occur along the boundaries of tectonic plates, which are large sections of the Earth's lithosphere that interact with each other. The majority of seismic activity is concentrated in specific regions around the world. Here are the primary areas where earthquakes occur:

• Pacific Ring of Fire: The Pacific Ring of Fire is the most seismically active region on Earth. It is a horseshoe-shaped belt that encircles the Pacific Ocean, running from the Americas' west coast, through Alaska's Aleutian Islands, through East Asia, and down to New Zealand. This region experiences frequent earthquakes and volcanic activity due to the collision and subduction of several tectonic plates.
• Circum-Pacific Belt: This belt extends beyond the Pacific Ring of Fire and includes areas such as western North America, Central America, and the western coast of South America. It is characterized by the subduction of the Pacific Plate beneath other plates, resulting in intense seismic activity.
• Mediterranean-Asian Belt: This belt encompasses the Mediterranean Sea region, including Southern Europe, northern Africa, and western Asia. It is an active seismic zone due to the convergence of the African Plate, Eurasian Plate, and Arabian Plate. It is known for significant earthquakes in countries like Greece, Turkey, Iran, and Italy.
• Mid-Atlantic Ridge: The Mid-Atlantic Ridge is a divergent boundary running through the Atlantic Ocean, where the North American Plate and the Eurasian Plate are moving apart. This region experiences frequent earthquakes as magma rises to form new oceanic crust.
• Alpine-Himalayan Belt: This belt spans from the Alps in Europe to the Himalayas in Asia. It is a seismically active area due to the collision between the African Plate and the Eurasian Plate, resulting in the formation of the Himalayan Mountain range.

It's important to note that earthquakes can occur in other regions as well, although they are less frequent. These include intraplate earthquakes that happen within tectonic plates, such as the New Madrid seismic zone in the central United States.

Fault types

There are three main types of faults that occur along tectonic plate boundaries, each associated with different types of movement. These faults are:

• Transform Faults: Transform faults occur where two tectonic plates slide horizontally past each other. The movement is mainly horizontal, with little to no vertical displacement. Transform faults are commonly found along transform boundaries, such as the San Andreas Fault in California, USA. These faults are associated with strike-slip earthquakes, where the rocks on either side of the fault move horizontally in opposite directions.
• Divergent Boundaries: Divergent boundaries are characterized by the separation and pulling apart of two tectonic plates. This process forms a gap or rift where new crust is created as magma rises from the underlying mantle. The primary fault type associated with divergent boundaries is a normal fault. In a normal fault, the hanging wall (the block of rock above the fault) moves downward relative to the footwall (the block of rock below the fault). Normal faults are often observed in regions with spreading centers, such as the Mid-Atlantic Ridge.
• Convergent Boundaries: Convergent boundaries occur when two tectonic plates collide or come together. The type of fault that forms at a convergent boundary depends on the type of crust involved. There are two main types of convergent boundaries:

       a. Subduction Zones: In subduction zones, one tectonic plate is forced beneath another plate in a process called subduction. The fault type associated with subduction zones is a thrust fault or reverse fault. In a thrust fault, the hanging wall moves upward and over the footwall, resulting in a shortening or compression of the crust. Subduction zones are often associated with powerful earthquakes and the formation of mountain ranges. The subduction zone along the west coast of South America, where the Nazca Plate is subducting beneath the South American Plate, is an example of a region with thrust faults.

       b. Continental Collision Zones: In regions where two continents collide, neither plate can be subducted due to their similar densities. Instead, the crust buckles and folds, forming complex mountain ranges. The fault type associated with continental collision zones is a thrust fault, similar to subduction zones. The collision between the Indian Plate and the Eurasian Plate, resulting in the formation of the Himalayas, is an example of a region with thrust faults due to continental collision.

Earthquake magnitude ratings

Earthquakes are measured using two commonly used magnitude scales: the Richter scale (R) and the moment magnitude scale (Mw). Charles F. Richter created the Richter scale in 1935, measures the amplitude of seismic waves recorded by seismographs and is logarithmic, meaning each whole number increase represents a tenfold increase in the amplitude and approximately 31.6 times more energy release. 

It is suitable for measuring the magnitude of smaller earthquakes (typically up to magnitude 6 or 7) but becomes less accurate for larger and more powerful earthquakes.

The Moment Magnitude Scale (Mw) is a more modern and widely used scale for measuring earthquake magnitude. It estimates the total energy released by an earthquake based on the seismic moment, considering the area of the fault, average slip, and rock rigidity. The Mw scale is logarithmic like the Richter scale but provides a more accurate measurement of larger earthquakes. Both Richter and Mw scales provide a quantitative measure of earthquake magnitude, allowing scientists to compare the size and energy release of different earthquakes. It is crucial to note that each whole number increase on the magnitude scale represents a significant increase in energy and potential damage caused by an earthquake.

Earthquake damage

Here are some broad consequences and sorts of harm that earthquakes may cause:

• Ground Shaking: The fundamental consequence of an earthquake is ground shaking. The intensity and duration of shaking depend on the magnitude of the earthquake and the distance from the epicenter. Strong shaking can cause buildings, bridges, and other structures to collapse or suffer significant damage. It can also trigger landslides, avalanches, and liquefaction (when saturated soil temporarily loses strength and behaves like a liquid).
• Structural Damage: Buildings, bridges, and infrastructure are particularly vulnerable to earthquake shaking. Poorly constructed or older structures are at a higher risk of damage or collapse. Common types of structural damage include the cracking or collapse of walls, floors, and foundations, as well as damage to columns, beams, and roofs.
• Tsunamis: Underwater earthquakes, particularly those that occur in subduction zones, can generate tsunamis. Large ocean waves that may travel great distances and wreak catastrophic damage when they hit coastal locations. Tsunamis can result in flooding, destruction of buildings near the shore, and loss of life.
• Aftershocks: Aftershocks are minor earthquakes that happen in the same location as the primary shock. They can continue for hours, days, or even weeks following a significant earthquake. Aftershocks can cause further damage to already weaken structures and infrastructure, hampering rescue and recovery efforts.
• Human Casualties: Earthquakes can cause injuries and fatalities. The severity of human casualties depends on factors such as the population density, preparedness measures, and the ability to respond effectively to the disaster. Collapsed buildings, landslides, and tsunamis pose significant risks to human safety.
• Economic Impact: Earthquakes can have substantial economic consequences. The costs associated with rebuilding damaged infrastructure, repairing or replacing buildings, and providing assistance to affected communities can be significant. Additionally, the disruption of economic activities, such as loss of business, interruption of transportation, and damage to utilities, can have long-term financial implications.

Efforts to mitigate earthquake damage involve implementing building codes and regulations, conducting seismic assessments of critical infrastructure, educating the public about earthquake preparedness, and developing early warning systems to provide advance notice of impending earthquakes.