Subjects and lessons to explore.

Law

Earth & Space Science · Foundations

Earthquakes

10 min read 5 practice questions
8 key termsSourcesReport an error
Want it in plain words first? Jump to Eli explains — the same idea, no jargon.
On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Quick check
  8. Study tools
  9. Sources & references

In 30 seconds

An begins when rock suddenly slips along a after accumulated stress overcomes friction. The slip releases energy that travels as seismic waves and shakes the ground. describes the event’s size at its source; describes shaking at a particular place. One earthquake has one magnitude on a chosen scale but can have many intensities because distance and local ground conditions differ.

Why this matters

Earthquake language appears in maps, alerts, news, and science courses, yet magnitude and intensity are often confused. Distinguishing the source event from local shaking makes reports more accurate and shows why two places can experience one earthquake differently. This foundation supports later work on seismic waves and hazards while avoiding a false promise of date-specific prediction or individualized risk advice.

The college version

From stored stress to shaking

An earthquake is ground shaking caused by sudden slip on a fault. Faults are fractures or zones of fracture in rock across which movement can occur. Tectonic motion can continue while rough fault surfaces remain stuck by friction. Stress then accumulates in the surrounding rock. When the forces driving motion overcome the resisting friction, part of the fault slips and stored energy is released. That energy travels outward through Earth as seismic waves, producing the vibrations people may feel.

The rupture begins below the surface at the , also called the focus. The is the point on Earth’s surface directly above that starting point. These terms identify locations, not measures of the earthquake’s size or damage. A fault can extend far beyond the initial rupture point, and a large event can involve slip over a broad area. Later lessons describe seismic-wave types and their travel paths; the central idea here is that slip and energy release connect a process in rock to shaking at the surface.

Not every earthquake is at a plate boundary, but many occur on faults associated with plate motion. The boundary model gives a broad geological context; the fault-slip model describes the immediate rupture process. Keeping these scales separate prevents the mistaken claim that every ground tremor reveals one simple plate interaction.

Magnitude and intensity answer different questions

Magnitude measures the size of an earthquake at its source. On a given magnitude scale, an event has one magnitude, estimated from instrumental observations and physical properties of the rupture. A records ground vibration, and networks of instruments help scientists determine an event’s time, location, and magnitude. Magnitude is therefore not a report of what one person felt at one address.

Intensity measures the shaking or effects at a particular location. One event can produce a map of many intensity values. Shaking generally changes with distance from the rupture, but local surface geology, rupture direction, depth, and other factors also matter. Two stations at similar distance can record different motion when one is on soft sediment and another is on bedrock. The Modified Mercalli Intensity scale commonly used in the United States describes effects with Roman numerals, but an intensity report should always be understood as site-specific.

The distinction clarifies news language. Saying an event had magnitude 6.0 describes its source size on that scale. Saying a neighborhood experienced strong intensity describes conditions at that location. Neither statement by itself completely specifies damage, because buildings and infrastructure also differ. Magnitude and intensity are related measurements, but they are not interchangeable rankings of “how bad” an earthquake was everywhere.

Use measurements with limits

Earthquake science uses instruments, maps, and physical models to locate events and describe shaking. These tools support monitoring and hazard understanding, but they do not make exact short-term prediction routine. A scientific explanation should distinguish an observation—such as a seismogram or a mapped intensity—from an inference about the event. It should also identify what the measurement does not establish.

Consider two communities affected by one earthquake. Community A is closer to the rupture and sits on soft sediment; Community B is farther away on firmer rock. A simplified prediction is that A may experience stronger shaking, but a real evaluation needs actual measurements and local conditions. It would be incorrect to say that A’s intensity changes the event’s magnitude, or that a magnitude alone fixes the shaking at every site.

This limit is especially important for public communication. General science lessons can explain terms and encourage students to use official sources for current information. They should not offer a personal safety plan, declare a building safe, or predict the next earthquake. The accurate takeaway is evidence-based and bounded: fault slip releases waves, instruments describe the event, and local conditions shape the shaking experienced at different places.

Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine two rough pieces of rock pressed together. They may stay stuck while forces keep pushing. When they finally slip, the stored push is released and makes waves through the ground. That is an earthquake. Scientists use instruments to measure the event, but people in different places can feel different shaking from the same earthquake.

Picture it like this

A snapped ruler held against a desk can make vibrations that spread through the desk. The snap is like a sudden release, while different spots on the desk may shake differently depending on how far they are from it and what they rest on.

Where the picture stops working

A ruler is not a fault, and desk vibrations do not reproduce Earth’s rock properties, rupture area, or seismic-wave behavior. The analogy only illustrates sudden release and travel of vibrations, not earthquake forecasting or local damage. It also does not represent the complex way different surface materials can change recorded shaking.

Worked example

Two instruments record the same earthquake. The event is assigned one magnitude because magnitude describes the source. A station on firm bedrock farther from the rupture records modest shaking, while a nearer station on softer sediment records stronger shaking. Those different local intensities do not mean two different earthquakes occurred or that the event acquired two magnitudes. They show why source size and site effects must be reported separately.

Key takeaway

Earthquakes result from sudden fault slip and released seismic energy. Magnitude characterizes the source event, while intensity characterizes shaking at particular locations; keeping them separate makes earthquake reports scientifically useful.

Quick check

3 questions here, of 5 in this lesson’s practice set. Answers stay hidden until you check.

Question 1 of 3foundational

What immediately causes an earthquake?

Choose an answer, then check it.
Question 2 of 3intermediate

Which statement correctly compares magnitude and intensity?

Choose an answer, then check it.
Question 3 of 3intermediate

Two stations are equally distant from one earthquake, but one is on soft sediment and one on bedrock. What is the best inference?

Choose an answer, then check it.
Practice all 5

Keep learning

Ready to build on this? Continue to the next lesson.

Practice this lesson
Study tools & related lessonsYou’ll learn to · Common mistakes · Easily confused · Key vocabulary · Related

You’ll learn to

  • Explain fault slip as the immediate cause of an earthquake.
  • Distinguish a fault, hypocenter, and epicenter.
  • Compare magnitude with intensity.
  • Explain why local shaking can vary for one event.
  • State a justified limit on earthquake prediction.

Common mistakes

  • Saying an earthquake has many magnitudes because people felt it differently.

    On a given scale, an event has one magnitude; intensity varies by location.

  • Using epicenter as the place where rupture begins.

    The hypocenter is the subsurface start; the epicenter is directly above it.

  • Treating intensity as only distance from the source.

    Distance matters, but local ground conditions and rupture characteristics also affect shaking.

  • Assuming basic science can predict an exact local event.

    Monitoring and hazard science have limits; use qualified official sources for current information.

Easily confused

Magnitude vs. Intensity

Magnitude describes source size; intensity describes site-specific shaking or effects.

Hypocenter vs. Epicenter

Hypocenter is the rupture start within Earth; epicenter is the surface point directly above it.

Fault slip vs. Seismic wave

Slip is the rupture process; seismic waves carry released energy away from it.

Key vocabulary

Earthquake
Ground shaking caused by sudden slip on a fault and the radiated seismic energy.
Fault
A fracture or fracture zone in rock across which movement can occur.
Hypocenter
The point within Earth where an earthquake rupture starts.
Epicenter
The point on Earth’s surface directly above an earthquake’s hypocenter.
Seismic wave
A wave of energy that travels through or along Earth after seismic energy is released.
Magnitude
A measure of an earthquake’s size at its source on a specified magnitude scale.
Intensity
A measure of shaking or effects at a particular location.
Seismometer
An instrument that records ground vibration.

Sources & references

  1. What is an earthquake and what causes them to happen? — U.S. Geological Survey
  2. The Science of Earthquakes — U.S. Geological Survey
  3. What is the difference between earthquake magnitude and earthquake intensity? — U.S. Geological Survey
  4. Earthquake Magnitude, Energy Release, and Shaking Intensity — U.S. Geological Survey

EliExplains lessons are original prose written from the open, credible references above. See Copyright & Licensing.

Researched 2026-08-20

Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.