Model Answer
0 min readIntroduction
An earthquake, also known as a seismic event, is a sudden and violent shaking of the ground caused by the passage of seismic waves through Earth's rocks. These waves are generated by the abrupt release of energy in the Earth’s lithosphere, creating seismic waves that radiate outward from the source. Earthquakes are among the most destructive natural hazards, causing significant loss of life and property damage globally. Understanding their causes and how their intensity is measured is crucial for mitigation and preparedness efforts. Recent devastating earthquakes in Turkey and Syria (February 2023) underscore the importance of this knowledge.
Understanding Earthquakes
Earthquakes are primarily caused by the movement of tectonic plates. The Earth’s lithosphere is divided into several large and small plates that are constantly moving, interacting at their boundaries. This interaction can result in stress buildup, which, when exceeding the rock’s strength, is released in the form of seismic waves.
Main Causes of Earthquakes
1. Tectonic Earthquakes
These are the most common type of earthquakes and are associated with the movement of tectonic plates. There are three main types of plate boundaries:
- Convergent Boundaries: Where plates collide (e.g., Himalayas, Andes). Subduction zones, where one plate slides beneath another, are particularly prone to large, deep-focus earthquakes.
- Divergent Boundaries: Where plates move apart (e.g., Mid-Atlantic Ridge). These typically produce smaller, shallow-focus earthquakes.
- Transform Boundaries: Where plates slide past each other horizontally (e.g., San Andreas Fault). These can generate large, shallow-focus earthquakes.
The Pacific Ring of Fire, encompassing areas around the Pacific Ocean, is a prime example of a region with intense tectonic activity and frequent earthquakes.
2. Volcanic Earthquakes
These earthquakes are associated with volcanic activity. The movement of magma beneath the surface can cause stress and fracturing of surrounding rocks, leading to earthquakes. These are generally less powerful than tectonic earthquakes and often precede volcanic eruptions. Examples include earthquakes associated with Mount St. Helens (1980) and Mount Pinatubo (1991).
3. Induced Earthquakes
These earthquakes are triggered by human activities. Common causes include:
- Reservoir-Induced Seismicity: The weight of water in large reservoirs can increase stress on underlying faults.
- Wastewater Disposal: Injecting wastewater from oil and gas operations into deep wells can lubricate faults and trigger earthquakes.
- Hydraulic Fracturing (Fracking): The process of fracking itself can sometimes induce small earthquakes.
- Underground Nuclear Explosions: Testing of nuclear weapons underground can cause seismic activity.
Oklahoma, USA, has experienced a significant increase in induced earthquakes due to wastewater disposal from oil and gas production.
Measuring Earthquake Intensity
Earthquake intensity refers to the effects of an earthquake at a particular location, based on observed damage and human perception. Several scales are used to measure earthquake intensity:
1. Richter Scale
Developed by Charles F. Richter in 1935, the Richter scale measures the magnitude of an earthquake – the energy released at its source. It is a logarithmic scale, meaning that each whole number increase represents a tenfold increase in amplitude and approximately a 32-fold increase in energy. While historically significant, the Richter scale is less accurate for large earthquakes.
2. Mercalli Intensity Scale
The Mercalli scale, developed in the early 20th century, measures earthquake intensity based on observed effects on people, structures, and the natural environment. It uses Roman numerals (I to XII) to describe the intensity, with I representing imperceptible shaking and XII representing total destruction. The Modified Mercalli Intensity Scale (MMI) is commonly used today.
| Mercalli Intensity | Description |
|---|---|
| I | Not felt except by a very few under especially favorable conditions. |
| V | Felt by all, many frightened. Some objects fall from shelves. |
| VIII | Damage slight in specially designed structures; considerable damage in poorly designed structures. |
| XII | Damage total. Waves seen on ground surfaces. Objects thrown upwards into the air. |
Moment Magnitude Scale (Mw): This is the most commonly used scale today for measuring earthquake magnitude, especially for large earthquakes. It is based on the seismic moment, which is related to the physical size of the fault rupture and the amount of slip.
Conclusion
Earthquakes are a complex geological phenomenon with diverse causes, primarily linked to tectonic plate interactions. Understanding these causes and employing accurate measurement scales like the Richter, Mercalli, and Moment Magnitude scales are vital for assessing seismic risk and implementing effective mitigation strategies. Continued research and monitoring are crucial for improving our ability to predict and prepare for these devastating natural disasters, especially in vulnerable regions worldwide.
Answer Length
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