How do we know?
↻ One of the pages with a moving picture
A heavy weight stays still while the ground shakes.
- Earth’s outer skin is cracked into giant puzzle pieces called plates.
- Rock edges get stuck along a crack called a fault. Energy builds up.
- The rocks suddenly slip! The quake starts at the hypocenter.
- Seismic waves spread like ripples on a pond. The epicenter is the spot right above.
In a seismograph, a heavy weight stays still while the base shakes, so it records the shaking. Records from 3 or more stations show where a quake began.
Plates slide by about 2 inches a year along the San Andreas Fault. A map of recent big quakes shows them lining up along the plate edges.
Quakes are not only on Earth. Apollo instruments recorded quakes on the Moon, and some lasted over 10 minutes.
Wow number
20,000 earthquakes are found around the world each year. That is about 55 a day!
Myth or fact?
In a quake, the ground opens wide and swallows towns. Myth. Faults do not open up.
The Moon has quakes. Some lasted over 10 minutes! Fact. Apollo instruments recorded them.
Think about it
No one can predict quakes yet. How could your school get ready?
Where in the world?
San Andreas Fault, California, USA where two plates slide by each other about 2 inches a year. Recent big quakes around the world line up along plate edges — 35.8997° N, 120.4357° W.
Where these facts come from
Every number on page 20, and who measured it
This page explains how earthquakes happen and how seismographs work.
The Science of Earthquakes — The U.S. Geological Survey, the government's Earth scientists
usgs.gov/programs/earthquake-hazards/science-earthquakes
We checked it on 31 October 2026
This page tells us how many earthquakes are found each year.
Why are we having so many (or so few) earthquakes? Has naturally occurring earthquake activity been increasing? — The U.S. Geological Survey, the government's Earth scientists
usgs.gov/faqs/why-are-we-having-so-many-earthquakes-has-naturally-occurring-earthquake-activity-been
We checked it on 31 October 2026
This page tells us how fast the plates slide along the San Andreas Fault.
How fast do tectonic plates move? — The U.S. Geological Survey, the government's Earth scientists
usgs.gov/faqs/how-fast-do-tectonic-plates-move
We checked it on 31 October 2026
This page tells us the ground doesn't open up and swallow towns.
Can the ground open up during an earthquake? — The U.S. Geological Survey, the government's Earth scientists
usgs.gov/faqs/can-ground-open-during-earthquake
We checked it on 31 October 2026
This page sorts real earthquake facts from made-up ones.
Earthquake Facts & Earthquake Fantasy — The U.S. Geological Survey, the government's Earth scientists
usgs.gov/programs/earthquake-hazards/earthquake-facts-earthquake-fantasy
We checked it on 31 October 2026
This page tells us the Moon has quakes too.
Moonquakes — NASA, the U.S. space agency
science.nasa.gov/moon/moonquakes
We checked it on 31 October 2026
Has this changed?
How earthquakes happen is not going to change.
What does change is our knowledge: no one can predict quakes yet, and scientists keep working on it.
Words from this page
- plates
- The giant pieces that Earth’s outer skin is cracked into.
- fault
- A crack in the rock where two pieces can slip past each other.
- epicenter
- The spot on the ground right above where a quake began.
- seismograph
- A machine that records the shaking of the ground.
Try it!
Feel how a fault sticks, then slips.
1. Loop a rubber band around a heavy book on a table. 2. Pull the band very slowly. Does the book glide, or stick, then jump? Keep your face back from the stretched band.
What happens: the band stretches while the book sticks, then it jerks ahead. Faults store energy and slip like that.
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