EARTHQUAKE

• An earthquake is the shaking or trembling of the Earth's surface caused by seismic waves resulting from the sudden release of energy within the Earth's crust or upper mantle. 
• Seismographs or seismometers are the instruments used to detect, measure, and record the seismic waves produced by earthquakes. These devices are vital in monitoring and understanding seismic activity and the Earth's movements.

FOCUS AND EPICENTRE

• The focus, also known as the hypocenter, is the precise location within the Earth's crust where an earthquake originates.
• the epicentre refers to the point on the Earth's surface directly above the focus. This is usually where the seismic waves are most intense, and as the waves travel outward from 

this point, their intensity decreases as they propagate away from the epicentre.

SEISMIC WAVE

Seismic waves are energy waves that travel through the Earth following an earthquake or other natural events involving the release of energy. These waves spread out in all directions from the point of origin (the focus or hypocenter), causing the ground to shake. 

Primarily there are two type of  seismic wave;

1. Body Waves: These waves travel through the Earth's interior.  There are two primary types: P-waves (Primary waves) and S-waves (Secondary waves).

• Primary Waves (P-waves): These are compressional waves that move by compressing and expanding the material they pass through. They can travel through solids, liquids, and gases. P-waves are the fastest seismic waves.

• Secondary Waves (S-waves): These waves travel more slowly than P-waves and can only move through solid rock. They're shear waves that cause the rock to move perpendicular to the direction of the wave's travel.

These waves are important because they provide crucial information about the Earth's interior structure and properties.

2. Surface wave

• Love waves - surface waves associated with seismic activity. They are named after A.E.H. Love. Love waves are relatively faster than Rayleigh waves and primarily cause horizontal shearing of the ground in a side-to-side, or horizontal, motion. They travel along the Earth's surface and are responsible for the horizontal shaking felt during an earthquake.

• Rayleigh waves are another type of surface wave associated with seismic activity. Named after Lord Rayleigh, these waves cause the ground to move in an elliptical rolling motion. They produce a combination of up-and-down and side-to-side movement in the direction of their travel.

The Rayleigh wave is responsible for the majority of the shaking that people feel during an earthquake

EARTHQUAKE TYPES

On the basis of causative factors

• Tectonic Earthquakes: These are the most common type of earthquakes and are caused by the movement of tectonic plates along fault lines in the Earth's crust.
• Volcanic Earthquakes: These earthquakes are associated with volcanic activity and can occur before or after a volcanic eruption. They are caused by the movement of magma and rocks within and around the volcano.
• Collapse Earthquakes: These earthquakes occur in underground mines and are caused by the pressure generated within rocks due to mining activities.
• Explosion Earthquakes: These are artificially induced earthquakes and are typically associated with high-energy explosions, such as nuclear explosions or large-scale construction activities.

Each type of earthquake has its unique characteristics and causes, but tectonic earthquakes are by far the most common and significant in terms of their impact on the Earth's surface and human activities.

RICHTER SCALE  is a logarithmic scale used to measure the magnitude of an earthquake. It was devised by Charles F. Richter in 1935. 

• The scale quantifies the energy released by an earthquake and assigns it a single number, referred to as the magnitude.
• The Richter scale typically ranges from 0 to 9, although it is not technically limited to this range. The numbers on the scale are continuous, meaning that they can have decimal values (e.g., 5.5).
• The magnitude of an earthquake is determined based on the amplitude of seismic waves recorded on seismographs. The larger the amplitude of the waves, the higher the magnitude of the earthquake.
• The Richter scale provides a quantitative way to compare the sizes of different earthquakes. It is important to note that while the Richter scale measures the amplitude of seismic waves, 
• The moment magnitude scale (Mw) has largely replaced the Richter scale in modern seismology. The moment magnitude scale is more accurate and applicable to a wider range of earthquake sizes.

MERCALLI  SCALE -Unlike the Richter or moment magnitude scales that provide a single number to characterise an earthquake's overall magnitude, the Mercalli Intensity Scale focuses on the earthquake's impact and observed damage in various areas

•  it assigns an intensity value to different locations affected by the earthquake. These intensity values range from I (not felt) to XII (total destruction).

CAUSES OF EARTHQUAKE

FAULT ZONE - The immediate case of most shallow earthquakes is the sudden release of stress along a fault rupture (crack) in the earth’s crust.

Sudden slipping of rock formations along fault rupture in the earth’s crust happens due to the constant change in volume and density of rocks due to intense temperature and pressure in the earth’s interior.

PLATE TECTONICS - The movements along these fault lines are attributed to different tectonic boundaries, leading to distinct seismic activities:

At convergent boundaries, like subduction zones, reverse faults cause powerful earthquakes. Megathrust earthquakes, associated with subduction zones, are extremely impactful, like the 2004 Indian Ocean earthquake.

Strike-slip faults, which are common in transform boundaries, like the San Andreas Fault, result in significant earthquakes, usually up to magnitude 8.

Normal faults, found in divergent boundaries, also induce earthquakes, but they tend to be less severe, generally below magnitude

VOLCANIC ACTIVITY - Volcanic earthquakes tend to be less severe and typically originate from the movement of magma within the volcano or tectonic processes related to volcanic activity.

These earthquakes can act as precursors or early indicators of volcanic eruptions, providing an alert before an eruption, as observed during the 1980 eruption of Mount St. Helens.

The geographic alignment of major earthquakes often corresponds to the distribution of volcanoes, especially along the Circum-Pacific Belt and oceanic ridges. However, many earthquake epicenters are unrelated to active volcanoes, even if they occur within the same regions.

The interaction between volcanic activity and seismicity contributes to our understanding of the Earth's complex geological processes

RESERVOIR INDUCED SEISMICITY - When a large artificial lake forms due to dam construction, it can cause significant changes in the stress conditions within the Earth's crust, potentially leading to earthquakes.

The primary mechanisms involved in this process are:

• Changes in Stress: The enormous weight of the water in the reservoir applies pressure on the underlying rock strata, which can alter the stress along existing faults or fractures. This change in stress can eventually lead to fault movement, causing earthquakes.
• Weakening of Soil Structure: The percolation of water into the soil can affect the soil structure and also lubricate the faults, making them more susceptible to movement.
• The Koyna Dam reservoir in Maharashtra, India, is a classic example where seismic activity was believed to be triggered by the filling of the reservoir. The seismic event in that region has been linked to the reservoir's water storage.
• Similarly, the 2008 Sichuan earthquake in China is suggested to be related to the Zipingpu Dam. The earthquake occurred in close proximity to the dam, raising the possibility that the water impoundment might have induced stress changes that contributed to the earthquake.
• Reservoir-induced seismicity is an important aspect to consider when building large dams, as it can significantly impact the geological stability of the surrounding region.

Wadati–Benioff zone 

• Wadati Benioff zone is a zone of subduction along which earthquakes are common. The most powerful earthquakes occur along this zone (most powerful earthquakes occur along the convergent boundary).
• Differential motion along the zone produces numerous earthquakes, the foci of which may be as deep as about 700 kilometres.
• Wadati–Benioff zones can be produced by slip along the subduction thrust fault (Himalayan Region – C-C convergent boundary) or slip on faults within the downgoing plate (O-O and C-O convergent boundary).

EARTHQUAKE PREDICTION

Earthquake prediction remains a challenging and uncertain field in seismology. While seismologists use various methods to understand seismic activity and potential patterns, accurately predicting the exact time, location, and magnitude of earthquakes in advance is not currently possible due to the complex nature of the Earth's crust and seismic activities. Scientists continuously study seismic events and the associated data to enhance their understanding of earthquakes, but making precise, short-term predictions remains elusive.

EFFECTS OF EARTHQUAKE

seismic events such as earthquakes can lead to various kinds of secondary and often catastrophic events, as noted below:

• Damage to Human Life and Property: Earthquakes can cause destruction to buildings and infrastructure. In urban areas, poor construction and building design can exacerbate the impact, resulting in high casualties and economic losses.
• Landslides and Avalanches: Tremors can destabilise slopes, leading to landslides. In mountainous areas, avalanches may occur due to seismic activity. The debris from these events can cause significant damage.
• Floods: Earthquakes can disrupt dams or block river courses due to landslides. This obstruction can lead to flash floods, causing substantial damage to the surrounding areas.
• Tsunamis: Seismic activity under the ocean floor can displace water, generating massive sea waves or tsunamis. These tsunamis can cause widespread devastation when they hit coastal areas.
• Nuclear Accidents: In certain situations, earthquakes can cause significant damage to nuclear facilities. The 2011 Tohoku earthquake in Japan caused a massive tsunami, leading to a nuclear meltdown at the Fukushima Daiichi nuclear power plant. This resulted in radioactive fallout and worldwide concern about the spread of radiation.

All these secondary events, resulting from seismic activity, contribute to the overall devastation and challenges faced by affected regions.

FAQs regarding earthquake

1. What causes earthquakes?

Ans. Earthquakes are primarily caused by the release of stress along a fault rupture in the Earth's crust. This rupture results from the constant changes in volume and density of rocks due to intense temperature and pressure in the Earth's interior. Movements along tectonic plate boundaries, volcanic activity, and reservoir-induced seismicity can also trigger earthquakes.

2. What are the types of earthquakes based on causative factors?

Ans. There are various types of earthquakes:

• Tectonic Earthquakes: Caused by the movement of tectonic plates along fault lines in the Earth's crust.
• Volcanic Earthquakes: Associated with volcanic activity, occurring before or after a volcanic eruption.
• Collapse Earthquakes: Occur in underground mines due to pressure generated within rocks from mining activities.
• Explosion Earthquakes: Induced by high-energy explosions such as nuclear tests or construction activities.

3. How are earthquakes measured?

Ans. The magnitude of an earthquake is measured using scales such as the Richter Scale and the Moment Magnitude Scale. The Richter Scale quantifies the energy released by an earthquake, assigning a single number to its magnitude. Moment magnitude scales are more accurate and versatile, widely used in modern seismology.

4. Can earthquakes be predicted?

Ans. Predicting the exact time, location, and magnitude of earthquakes remains challenging due to the complex nature of the Earth's crust. Seismologists use various methods to understand seismic activity, but precise short-term predictions are not currently possible.

5. What are the effects of earthquakes?

Ans. Earthquakes can lead to various secondary events causing catastrophic damage:

• Damage to life and property: Destruction to buildings and infrastructure can result in high casualties and economic losses.
• Landslides and floods: Tremors can cause landslides and block river courses, leading to floods and damage.
• Tsunamis: Seismic activity under the ocean can displace water, causing massive sea waves that hit coastal areas.
• Nuclear accidents: Seismic events can damage nuclear facilities, potentially leading to accidents and radioactive fallout.

6. What are seismic waves and their types?

Ans. Seismic waves are energy waves that travel through the Earth. They're categorized into two primary types:

• Body Waves: Comprising Primary (P-waves) and Secondary (S-waves) waves that travel through the Earth's interior.
• Surface Waves: Including Love waves and Rayleigh waves, which travel along the Earth's surface, causing horizontal and rolling ground movements.