Earthquakes Lesson: Causes, Effects, and Safety Measures

Lesson Overview

• What Are the Causes of Earthquakes?
• What Are Seismic Waves? 
• How to Measure Earthquakes
• What Are the Effects of Earthquakes?
• Earthquake Preparedness and Mitigation Strategies
• Case Study: The 2025 Myanmar Earthquake

Earthquakes are natural phenomena that result from the sudden release of energy within the Earth's crust, leading to seismic waves that cause the ground to shake. These events can profoundly impact human societies, infrastructure, and the environment. This comprehensive guide aims to provide Grade 12 students with an in-depth understanding of earthquakes and their causes, effects, measurement, and preparedness strategies. By delving into these topics, students will be well-prepared to answer related questions and develop critical thinking skills regarding seismic events.

What Are the Causes of Earthquakes?

Here are the causes of earthquakes: 

Tectonic Plate Movements

The Earth's lithosphere is divided into several large and small tectonic plates that float atop the semi-fluid asthenosphere. The interactions of these plates are the primary drivers of seismic activity. The main types of plate boundaries include:

1. Convergent Boundaries: At these boundaries, plates move toward each other, and one plate is forced beneath another in a process known as subduction. This interaction often leads to powerful earthquakes and volcanic activity. For example, the subduction of the Indian Plate beneath the Eurasian Plate has formed the Himalayas, a region prone to significant seismic events.
   
2. Divergent Boundaries: Here, plates move away from each other, allowing magma to rise and create a new crust. While earthquakes at divergent boundaries are generally less intense, they are common along mid-ocean ridges.
   
3. Transform Boundaries: Plates slide horizontally past one another at transform boundaries, causing friction that can result in earthquakes. The San Andreas Fault in California is a well-known example of a transform boundary.
   

Elastic Rebound Theory

The elastic rebound theory explains how energy is stored and released during an earthquake. As tectonic plates move, they can become locked due to friction, causing stress to build up over time. When the accumulated stress exceeds the frictional forces holding the rocks together, it is released suddenly, causing the rocks to snap back to their original shape and releasing energy in the form of seismic waves.

What Are Seismic Waves? 

Seismic waves are the energy waves generated by the sudden movement of the Earth's crust during an earthquake. They are categorized into two main types:

1. Body Waves: These waves travel through the Earth's interior and are divided into:
   
   • Primary (P) Waves: P-waves are compressional waves that move in a push-pull motion, traveling fastest and arriving first at seismic stations. They can move through solids, liquids, and gases.
     
   • Secondary (S) Waves: S-waves are shear waves that move perpendicular to the direction of propagation. They are slower than P-waves and can only travel through solids.
     
2. Surface Waves: These waves travel along the Earth's surface and typically cause the most damage during an earthquake. They include:
   
   • Love Waves: Love waves cause horizontal shearing of the ground.
     
   • Rayleigh Waves: Rayleigh waves produce a rolling motion, combining vertical and horizontal movements.
     

How to Measure Earthquakes

Accurately measuring earthquakes is crucial for assessing their potential impact and implementing appropriate response strategies. The two primary scales used are:

1. Richter Scale: Developed in 1935 by Charles F. Richter, this logarithmic scale measures the magnitude of an earthquake based on the amplitude of seismic waves recorded by seismographs. Each whole number increase on the Richter scale represents a tenfold increase in measured amplitude and approximately 31.6 times more energy release.
   
2. Modified Mercalli Intensity (MMI) Scale: This scale assesses the intensity of an earthquake based on observed effects on people, buildings, and the Earth's surface. It ranges from I (not felt) to XII (total destruction). Unlike the Richter scale, the MMI scale is subjective and can vary based on location and structural conditions.
   

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What Are the Effects of Earthquakes?

Earthquakes can have a wide range of effects, including:

1. Ground Shaking: The primary effect of an earthquake, ground shaking can lead to structural damage or collapse of buildings and infrastructure.
   
2. Surface Rupture: The displacement of the Earth's surface along a fault line can damage roads, pipelines, and buildings.
   
3. Ground Failure: Earthquakes can induce landslides, liquefaction (where saturated soil temporarily loses its strength), and subsidence.
   
4. Tsunamis: Underwater earthquakes can displace large volumes of water, generating tsunamis that can cause widespread coastal damage.
   

Earthquake Preparedness and Mitigation Strategies

Effective preparedness and mitigation can significantly reduce the risks associated with earthquakes. Key strategies include:

1. Building Codes and Engineering: Implementing and enforcing seismic building codes ensures that structures can withstand ground shaking. Retrofitting older buildings can also enhance their resilience.
   
2. Emergency Planning: Developing comprehensive emergency response plans, including evacuation routes and communication strategies, is vital for community safety.
   
3. Public Education: Educating the public about earthquake risks and safety measures, such as "Drop, Cover, and Hold On," can improve individual and collective preparedness.
   
4. Early Warning Systems: Investing in seismic monitoring and early warning systems can provide critical seconds to minutes of advance notice, allowing people to take protective actions.
   

Case Study: The 2025 Myanmar Earthquake

On March 28, 2025, a magnitude 7.7 earthquake struck central Myanmar, causing extensive damage and loss of life. The earthquake occurred along the Sagaing Fault, a major strike-slip fault marking the boundary between the Indian and Eurasian tectonic plates. The shallow depth of the earthquake, approximately 10 km, contributed to its destructiveness, as seismic energy was released close to the surface.

The event highlighted several critical aspects:

• Infrastructure Vulnerability: Many buildings in the affected area were not constructed to withstand significant seismic activity, leading to widespread structural failures.
  
• Emergency Response Challenges: Ongoing civil conflict and limited resources hindered effective rescue and relief operations.
  
• Seismic Awareness: The rarity of large earthquakes in the region has led to a lack of preparedness among the population.

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