Meaning of Earthquake

What is an Earthquake?

An earthquake is the shaking or trembling of the earth’s surface. It happens when energy stored in the earth’s crust is suddenly released. This energy travels through the ground as waves, making buildings shake and the ground crack.

Think of it like bending a stick until it breaks. The stick stores energy as you bend it. When it snaps, that energy releases suddenly. The same thing happens in the earth’s crust. Rocks bend under pressure until they can’t take anymore. When they break or slip, an earthquake occurs.

The point inside the earth where the earthquake starts is called the focus or hypocenter. The point on the earth’s surface directly above the focus is called the epicenter. The epicenter is usually where the most damage happens.

Causes of Earthquakes

1. Tectonic Plate Movements

This is the main cause of earthquakes worldwide. The earth’s crust is broken into large pieces called tectonic plates. These plates float on the hot, semi-liquid mantle below. They move slowly, about 2-10 centimeters per year.

When plates push together, pull apart, or slide past each other, pressure builds up. Eventually, the rocks break or slip, releasing energy as an earthquake. The San Andreas Fault in California is a famous example where two plates slide past each other.

2. Volcanic Activity

Volcanoes can cause earthquakes in two ways. First, as magma moves underground, it pushes rocks aside, creating tremors. Second, when a volcano erupts, the sudden release of pressure can shake the ground. Mount Cameroon in West Africa has caused small earthquakes during eruptions.

3. Human Activities

People can trigger earthquakes through activities like:

• Mining: Removing coal, oil, or minerals creates empty spaces that can collapse
• Dam construction: Large dams like Kainji Dam add weight that can trigger movement in faults
• Underground nuclear testing: Explosions can shake the ground violently
• Geothermal energy extraction: Pumping water underground can lubricate faults

4. Isostatic Adjustment

After ice sheets melt or large amounts of sediment erode away, the land slowly rises to balance itself. This adjustment can cause earthquakes, though they are usually minor.

Measuring Earthquakes

Scientists use two main scales to measure earthquakes:

The Richter Scale

Developed in 1935, this scale measures the magnitude (size) of an earthquake from 1 to 10. Each number is ten times stronger than the one before. A magnitude 5 earthquake is 10 times stronger than magnitude 4, and 100 times stronger than magnitude 3.

The Mercalli Scale

This scale measures the intensity (damage) caused by an earthquake using Roman numerals from I to XII. It describes what people feel and the damage to buildings. The same earthquake can have different Mercalli ratings in different places.

A seismograph is the instrument that detects and records earthquakes. It has a weight suspended on a spring that stays still while the ground shakes. A pen attached to the weight draws lines on a rotating drum, creating a seismogram that shows the earthquake’s waves.

Types of Earthquake Waves

Earthquakes produce three types of seismic waves:

1. Primary Waves (P-waves)

These are the fastest waves, traveling at 6-7 km per second. They move through solids, liquids, and gases. P-waves push and pull rocks in the same direction they travel, like a slinky. They arrive first at seismograph stations.

2. Secondary Waves (S-waves)

These travel slower than P-waves, at about 3-4 km per second. They only move through solids, not liquids. S-waves shake rocks side to side and up and down. They cause more damage than P-waves.

3. Surface Waves (L-waves)

These are the slowest but most destructive waves. They travel along the earth’s surface like ripples on water. Surface waves make the ground roll and twist, causing most of the damage during earthquakes.

Effects of Earthquakes

Positive Effects

• Formation of rift valleys and lakes: Earthquakes create depressions that fill with water, like Lake Tanganyika in the East African Rift Valley
• Exposure of mineral resources: Cracks in rocks can reveal valuable minerals and gemstones
• Creation of hot springs: Groundwater heated by volcanic activity emerges as hot springs with medicinal properties
• Scientific research: Earthquakes help scientists understand the earth’s internal structure

Negative Effects

• Loss of life: The 2010 Haiti earthquake killed over 200,000 people
• Building collapse: Poorly constructed buildings fall, trapping people inside
• Landslides: Shaking loosens soil on slopes, causing deadly mudslides
• Tsunamis: Underwater earthquakes create giant waves that flood coastal areas
• Fire outbreaks: Broken gas pipes and electrical lines start fires
• Economic losses: Destroyed infrastructure costs billions of Naira to rebuild
• Displacement: People lose their homes and become refugees
• Psychological trauma: Survivors experience fear and mental health problems

Earthquakes in Nigeria

Nigeria is not in a major earthquake zone, but we still experience minor tremors. Recorded earthquakes in Nigeria include:

• Kwoi, Kaduna State (1984, 1990): Magnitude 4.5, caused panic and minor damage
• Ijebu-Ode, Ogun State (2000, 2009): Magnitude 4.0, cracked buildings
• Warri, Delta State (2006): Linked to oil and gas extraction activities
• Abuja (2016, 2018): Minor tremors felt in the Federal Capital Territory

These earthquakes are usually caused by local faults or human activities like oil drilling and dam construction. The Nigerian Geological Survey Agency monitors seismic activity across the country.

Earthquake Safety Measures

Before an Earthquake

• Construct buildings with earthquake-resistant designs using flexible materials
• Secure heavy furniture and appliances to walls
• Keep emergency supplies (water, food, torch, first aid kit) ready
• Know safe spots in each room (under strong tables, in doorways)
• Have a family emergency plan and meeting point

During an Earthquake

• If indoors: Drop, cover, and hold on. Get under a strong table or desk
• If outdoors: Move away from buildings, trees, and power lines to open ground
• If in a car: Stop safely and stay inside until the shaking stops
• Do not use elevators or run outside during shaking
• Protect your head and neck with your arms

After an Earthquake

• Check yourself and others for injuries
• Expect aftershocks (smaller earthquakes that follow the main one)
• Stay away from damaged buildings
• Turn off gas, water, and electricity if you suspect leaks or damage
• Listen to radio for emergency information
• Only use phone for emergencies to keep lines open

Common Exam Mistakes

WAEC Chief Examiners report these common errors:

• Confusing focus and epicenter: Remember, focus is inside the earth, epicenter is on the surface directly above
• Mixing up P-waves and S-waves: P (Primary) waves are faster; S (Secondary) waves are slower
• Not explaining how seismographs work: Don’t just name it, describe the suspended weight and rotating drum
• Listing effects without examples: Always give real examples like the Haiti earthquake or Nigerian locations
• Poor English when describing safety measures: Use clear imperative sentences: “Drop under a table” not “You should be dropping”
• Confusing earthquake causes with effects: Tectonic movement is a cause; tsunami is an effect

Practice Questions

Multiple Choice Questions

1. The point on the earth’s surface directly above where an earthquake starts is called the:
a) Focus
b) Epicenter ✓
c) Fault line
d) Seismograph

2. Which type of seismic wave travels fastest?
a) Surface waves
b) Secondary waves
c) Primary waves ✓
d) Love waves

3. The instrument used to detect and record earthquakes is called:
a) Barometer
b) Thermometer
c) Seismograph ✓
d) Anemometer

4. Which of these human activities can trigger earthquakes?
a) Farming
b) Dam construction ✓
c) Road building
d) Tree planting

Essay/Theory Questions

1. (a) What is an earthquake? (2 marks)
(b) Explain four causes of earthquakes. (8 marks)

Tip: For part (b), structure each cause with explanation and example. Don’t just list them.

2. Describe five negative effects of earthquakes on human activities. (10 marks)

Tip: Each effect needs proper explanation (not just one word) and an example if possible.

3. (a) Distinguish between the focus and epicenter of an earthquake. (4 marks)
(b) State three safety measures to take during an earthquake. (6 marks)

Tip: “Distinguish” means show the differences clearly. Use a comparison format.

Memory Aids

Remember earthquake wave speeds:
“Primary Pupils Run Faster, Secondary Students Stroll, Lazy Learners Last”
P-waves fastest → S-waves slower → L-waves (surface) slowest

Remember what to do during an earthquake:
“DROP, COVER, HOLD”
Drop to the ground → Cover under furniture → Hold on tight

Remember the main causes:
“TV Has Volume”
Tectonic movement → Volcanic activity → Human activities

Related Topics

• Volcanoes and volcanic eruptions
• Plate tectonics and continental drift
• Landforms created by earth movements
• Weathering and mass wasting
• Tsunamis and coastal hazards