Earthquakes Lesson: Definition, Causes & Key Concepts

Lesson Overview

• What Causes an Earthquake?
• Important Terms to Know
• Measuring Earthquakes
• Finding an Earthquake's Epicenter: Triangulation
• Where Do Earthquakes Usually Occur?
• Earthquake Safety: Before, During, and After
• Earthquake-Proof Structures: Seismic Safety
• Real-Life Earthquake Examples
• Effects of Earthquakes

Imagine standing still and suddenly feeling the ground beneath your feet shake uncontrollably. This shaking is called an earthquake, a powerful natural event. Earthquakes can be small, barely noticeable, or large enough to cause significant damage. Let's explore why earthquakes happen, how we measure them, and how you can stay safe.

What Causes an Earthquake?

Earthquakes happen because our planet is always moving. Earth's crust, the outer layer, is divided into large pieces called tectonic plates. These plates slowly move and sometimes get stuck at their edges, called fault lines. Pressure builds up, and when it becomes too strong, the plates suddenly move, releasing energy that causes the ground to shake.

Important Terms to Know

• Focus: The exact point beneath the Earth's surface where the earthquake starts.
• Epicenter: The location directly above the focus, on Earth's surface, often experiencing the strongest shaking.

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Measuring Earthquakes

Scientists use special instruments called seismographs to measure earthquakes. A seismograph records the shaking caused by seismic waves produced during an earthquake.

Richter Scale

The Richter scale is used to measure the energy released by earthquakes. It helps us understand how strong an earthquake was.

• Each whole number increase on the Richter scale means the earthquake released around 31 times more energy than the previous number.
• For example, an earthquake measured at 6 on the Richter scale releases about 31 times more energy than one measured at 5.

Intensity

Intensity describes the strength of shaking and damage caused by an earthquake at a particular location. Intensity can vary depending on how far you are from the earthquake's epicenter.

Finding an Earthquake's Epicenter: Triangulation

Scientists use a process called triangulation to find exactly where an earthquake started. They look at data from at least three seismograph stations:

• Each station measures the time it takes for seismic waves to reach it.
• Scientists draw circles representing these distances.
• Where the circles intersect is the earthquake's epicenter.

Where Do Earthquakes Usually Occur?

Earthquakes mostly happen in areas called fault zones, which are locations where tectonic plates meet. Examples include:

• The San Andreas Fault in California, USA
• The Pacific Ring of Fire, around the Pacific Ocean, is known for frequent earthquakes and volcanoes.

Earthquake Safety: Before, During, and After

Before an Earthquake

• Create an emergency escape plan with your family.
• Prepare an emergency kit containing water, food, medications, and important documents.
• Secure heavy furniture to walls to prevent tipping during shaking.

During an Earthquake

• Practice the Drop, Cover, and Hold On method:
  • Drop to the ground immediately.
  • Cover your head and neck, preferably by taking shelter under sturdy furniture like a desk.
  • Hold On to your shelter until the shaking stops.
• If outside, move away from buildings, trees, streetlights, and power lines.

After an Earthquake

• Check yourself and others carefully for injuries and give first aid if needed.
• Stay away from damaged buildings until authorities say it's safe.
• Be prepared for aftershocks, smaller earthquakes that follow the main one.

Earthquake-Proof Structures: Seismic Safety

Buildings designed to resist earthquake damage are called seismically safe structures. They have features such as:

• Base isolators: Allow buildings to move slightly during an earthquake without collapsing.
• Energy dampers: Absorb energy from the shaking to reduce damage.
• Reinforced materials: Strengthen walls and foundations to prevent them from breaking apart.

Real-Life Earthquake Examples

• Japan, 2011: A major earthquake caused a tsunami that resulted in significant damage. This event highlighted the importance of earthquake preparedness.
• San Francisco, 1906: An earthquake and fires destroyed much of the city, leading to better building practices.

Effects of Earthquakes

Earthquakes can affect people, buildings, and nature in several ways:

• Damage to buildings and homes: Shaking can cause buildings to collapse.
• Landslides: Shaking can loosen rocks and soil on slopes.
• Tsunamis: Large waves caused by underwater earthquakes can affect coastal areas.

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