Earthquake: Causes, Types, Seismic Waves, Measurement and Safety
An earthquake is a sudden shaking of the ground caused by the rapid release of energy in the Earth’s lithosphere. Some earthquakes are so weak that only instruments detect them. Others flatten cities in seconds. Because they strike without warning, understanding how an earthquake works is essential for science students, CSS and PMS aspirants, and anyone living in a seismically active region.
What Is an Earthquake?
An earthquake is the shaking of the Earth’s surface that occurs when stored strain energy is released along a fault, producing seismic waves. Most earthquakes result from the movement of tectonic plates, while others are caused by volcanic activity, collapsing underground cavities, explosions, or human activity. Earthquakes and volcanoes are both endogenic processes, meaning they originate from forces inside the Earth.
Definition (exam-ready): An earthquake is the sudden release of energy in the Earth’s lithosphere that creates seismic waves, resulting in ground shaking and, sometimes, surface rupture.
How Does an Earthquake Happen?
The Earth’s crust is broken into large plates that move slowly, only a few centimeters a year. At plate boundaries and along faults, the rocks lock together while the plates keep pushing. Stress builds up until the rock suddenly slips, and the stored energy is released as seismic waves. Scientists call this the elastic rebound process.
Key Terms Related to an Earthquake
| Term | Meaning |
|---|---|
| Focus (hypocenter) | The point inside the Earth where the earthquake begins |
| Epicenter | The point on the surface directly above the focus; shaking is usually strongest here |
| Fault | A fracture in rock along which movement occurs |
| Fault scarp | A step in the ground surface created by fault movement |
| Aftershock | Smaller earthquakes that follow the main shock |
| Foreshock | Smaller earthquakes that sometimes come before the main shock |
| Isoseismal line | A line joining places that experienced the same intensity |
| Seismograph | The instrument that records ground motion |
| Seismogram | The record produced by a seismograph |
Remember: a seismograph is the instrument, while a seismogram is the recorded output.
Seismic Waves: How an Earthquake Travels
Energy released at the focus travels outward as seismic waves, which fall into two groups: body waves and surface waves.
Body Waves
Body waves travel through the Earth’s interior and are faster than surface waves, so they are recorded first.
- P-waves (primary waves): The fastest seismic waves, about 1.7 times faster than S-waves, and the first to arrive. They are longitudinal, like sound waves, with particles moving in the same direction as the wave. They can travel through solids, liquids, and gases.
- S-waves (secondary waves): Slower than P-waves and able to travel only through solids. They are transverse, so particles move at right angles to the wave direction.
Surface Waves
Surface waves form when body waves reach the surface. They travel along the ground, are slower than body waves, and cause the most damage.
- Love waves: Produce purely horizontal, side-to-side motion. They are faster than Rayleigh waves.
- Rayleigh waves: Produce a rolling, elliptical motion, like ocean waves, moving the ground both up-and-down and back-and-forth. Much of the shaking people feel comes from them.
Shadow Zones: How Waves Reveal the Earth’s Interior
Seismologists noticed that S-waves are not recorded beyond about 105° from the epicenter, and P-waves are weak or absent between about 105° and 145°. Because S-waves cannot pass through liquids, this showed that the Earth’s outer core is liquid. These “shadow zones” are among the strongest pieces of evidence for the structure of the Earth’s interior.
Types of Earthquake
- Tectonic earthquake: The most common type, caused by the movement of tectonic plates along faults.
- Volcanic earthquake: Linked to volcanic activity and often occurring before or after an eruption.
- Collapse earthquake: Small tremors caused by the collapse of underground mines or caves.
- Explosion earthquake: Caused by chemical or nuclear explosions.
- Induced earthquake: Triggered by human activity, as described below.
Human-Induced Earthquakes
Some human activities can trigger an earthquake by changing stress in the crust, including:
- Deep mining
- Underground nuclear tests
- Filling large reservoirs behind dams (reservoir-induced seismicity)
- Extraction of oil, gas, and groundwater
- Injection of fluids deep underground, which can lubricate existing faults
How Is an Earthquake Measured?
Magnitude: The Richter and Moment Magnitude Scales
Magnitude measures the energy released at the source. The Richter scale, introduced in 1935, is logarithmic: each whole-number increase means ground motion is 10 times larger and roughly 32 times more energy is released. Today, scientists mostly use the moment magnitude scale (Mw), which is more reliable for large events. The scale has no fixed upper limit; the largest earthquake ever recorded was the 1960 Chile earthquake at magnitude 9.5.
Intensity: The Mercalli Scale
Intensity measures the effects of an earthquake on people, buildings, and the ground at a given place. The Modified Mercalli scale runs from I to XII.
| Feature | Magnitude | Intensity |
|---|---|---|
| Measures | Energy released at the source | Effects at a location |
| Scale | Richter / Moment magnitude | Mercalli (I–XII) |
| Value per earthquake | One number | Varies with distance |
Distribution of Earthquakes
- Circum-Pacific Belt (Ring of Fire): Surrounds the Pacific Ocean and accounts for roughly 81% of the world’s largest earthquakes. Japan, Indonesia, Chile, and California lie in this belt.
- Alpide (Mid-World Mountain) Belt: Stretches from the Mediterranean through Turkey, Iran, and the Himalayas to Indonesia, and accounts for roughly 17% of the largest earthquakes. Pakistan lies in this belt.
Effects of an Earthquake
- Collapse of buildings, bridges, and roads
- Landslides, ground cracks, and fissures
- Liquefaction, where saturated soil behaves like a liquid
- Fires caused by broken gas lines and electrical faults
- Tsunamis when the sea floor suddenly shifts
- Changes in landforms, such as the uplift or sinking of coastlines (for example, the 1819 Kutch earthquake)
- Damming of rivers and formation of new lakes
- Economic loss, displacement, and long-term psychological stress
Major Earthquake Disasters Around the World
| Year | Location | Magnitude | Notes |
|---|---|---|---|
| 1906 | San Francisco, USA | 7.9 | Fires destroyed much of the city; over 3,000 deaths |
| 1931 | Dogger Bank, UK | 6.1 | Strongest recorded in the UK; minor damage |
| 1949 | Queen Charlotte, Canada | 8.1 | Largest in Canadian history; little damage due to offshore location |
| 1976 | Tangshan, China | 7.5 | Around 242,000 official deaths |
| 1989 | Newcastle, Australia | 5.6 | 13 deaths; Australia’s deadliest in modern times |
| 2004 | Indian Ocean (Sumatra) | About 9.1 | Tsunami killed around 230,000 across the region |
| 2010 | Haiti | 7.0 | Over 200,000 deaths estimated; poor infrastructure worsened the toll |
| 2011 | Tōhoku, Japan | 9.1 | Tsunami killed nearly 20,000 and triggered the Fukushima nuclear accident |
| 2015 | Gorkha, Nepal | 7.8 | Over 9,000 deaths; historic monuments destroyed |
| 2023 | Turkey and Syria | 7.8 | Over 50,000 deaths |
| 2025 | Myanmar | 7.7 | See below |
Case Study: The 2025 Myanmar Earthquake
On 28 March 2025, a 7.7-magnitude earthquake struck the Mandalay region of Myanmar, followed minutes later by a 6.4-magnitude aftershock. The shaking reached Thailand, where a high-rise under construction in Bangkok collapsed. The earthquake struck at a shallow depth near large cities, which made it especially destructive. The official toll exceeded 1,600 deaths within a day, and later counts rose further. It shows why shallow earthquakes near populated areas are so dangerous, and why building quality matters.
Earthquakes in Pakistan
Pakistan sits where the Indian Plate collides with the Eurasian Plate, and it is crossed by major fault systems such as the Chaman Fault and the Makran subduction zone. This makes it one of the more seismically active countries in South Asia.
- 1935 Quetta earthquake: Destroyed the city of Quetta and killed tens of thousands of people.
- 2005 Kashmir earthquake (8 October): Magnitude 7.6, with over 87,000 deaths, more than 100,000 injured, and around 3.5 million people left homeless in Azad Kashmir and Khyber Pakhtunkhwa. It remains the deadliest disaster in Pakistan’s history.
- 2013 Awaran earthquake (24 September): Magnitude 7.7 in Balochistan, with over 800 deaths. A small island emerged off the coast near Gwadar.
- 2015 Hindu Kush earthquake (26 October): Magnitude 7.5, with its epicenter in Afghanistan at a depth of around 200 km. Strong shaking was felt across Khyber Pakhtunkhwa, Gilgit-Baltistan, and Islamabad, and about 400 people died across Pakistan and Afghanistan. The great depth reduced the damage, but it was still very destructive. It was also felt in Iran, China, and India.
- 2019 Mirpur earthquake (24 September): Magnitude 5.8, with at least 40 deaths and damage to roads, bridges, and homes.
What to Do During an Earthquake
Drop, Cover, and Hold On is the internationally recommended rule:
- Drop to your hands and knees so you do not fall.
- Cover your head and neck under a sturdy table, or against an interior wall away from windows.
- Hold on until the shaking stops.
Additional safety tips:
- Do not run outside during shaking, because falling debris is a major danger.
- If outdoors, move to an open area away from buildings, trees, and power lines.
- If driving, stop in a safe place and stay inside the vehicle.
- Avoid elevators, and expect aftershocks.
- Keep an emergency kit with water, food, a flashlight, a first-aid kit, and important documents.
Can an Earthquake Be Predicted?
Scientists can identify where an earthquake is likely, but they cannot yet predict the exact time, place, and magnitude. What they can do is build early warning systems, which detect fast P-waves and send alerts seconds before damaging S-waves and surface waves arrive. Even a few seconds allow trains to slow down and people to take cover.
How to Reduce Earthquake Damage
- Follow earthquake-resistant building codes, with reinforced concrete frames, shear walls, and flexible designs
- Retrofit old buildings
- Avoid construction on soft soil or steep, unstable slopes
- Hold regular drills in schools and offices
- Strengthen emergency response and public awareness
Frequently Asked Questions
What is an earthquake?
It is the sudden shaking of the ground caused by the release of energy in the Earth’s lithosphere, usually along a fault.
What is the difference between the focus and the epicenter?
The focus is the underground point where the earthquake begins, while the epicenter is the point on the surface directly above it.
Which waves cause the most damage?
Surface waves, especially Rayleigh and Love waves.
What is the difference between magnitude and intensity?
Magnitude measures the energy released at the source, while intensity measures the effects felt at a particular place.
Why is Pakistan prone to earthquakes?
It lies along the boundary of the Indian and Eurasian plates, which are still colliding.
Conclusion
An earthquake is one of nature’s most powerful and unpredictable events, but understanding its causes, waves, and effects makes communities safer. We cannot stop an earthquake, but earthquake-resistant buildings, early warning systems, regular drills, and public awareness can greatly reduce loss of life. For students preparing for CSS and PMS General Science and Ability, this topic is high-yield: master the terms, wave types, scales, distribution, and Pakistan’s earthquake history.
