An earthquake is the shaking of the ground caused by huge slabs of rock suddenly slipping past each other deep below Earth’s surface. About 55 earthquakes happen somewhere on the planet every single day, though most are too small for anyone to feel. Some are powerful enough to shake buildings and crack roads, while others are so tiny that only special instruments can detect them.
Why the Ground Shakes
Earth’s outer layer isn’t one solid shell. It’s broken into giant pieces called tectonic plates that fit together like a jigsaw puzzle. These plates are always moving, but very slowly. The Pacific Plate, for example, slides past the North American Plate at about two inches per year, roughly the speed your fingernails grow.
Even though the plates move slowly, they don’t slide smoothly. Their edges are rough, so they get stuck against each other. Pressure builds up over time, the same way tension builds when you bend a stick. Eventually the stress becomes too much, the rocks snap past each other, and all that stored energy bursts out as waves that ripple through the ground. That burst of energy is what we feel as an earthquake.
The cracks in the Earth where plates push, pull, or slide against each other are called faults. Some faults have blocks of rock sliding sideways. Others have one plate diving underneath another, which can also create volcanoes. Still others pull apart, causing the ground between them to drop down like a sinking bridge.
Epicenter, Focus, and Fault Lines
Every earthquake starts at a specific point underground where the rock first breaks. Scientists call this the focus (sometimes called the hypocenter). The spot on the surface directly above the focus is the epicenter. When a news report says “the epicenter was 10 miles south of a city,” that’s the point on the map where the shaking was strongest.
A fault line is simply a crack in Earth’s crust where the rocks on either side have shifted. Some fault lines stretch for hundreds of miles. California’s San Andreas Fault is one of the most famous, running nearly the entire length of the state.
Where Earthquakes Happen Most
Earthquakes don’t strike randomly. Most happen along the edges of tectonic plates. The most active zone on Earth is the Ring of Fire, a horseshoe-shaped belt that circles the Pacific Ocean. It runs along the coasts of South America, North America, Japan, and down through New Zealand. This is where the Pacific Plate bumps up against many surrounding plates, making it the most earthquake-prone and volcanic region in the world. About 500 of Earth’s roughly 1,350 active volcanoes sit along this ring.
That doesn’t mean earthquakes only happen there. They can occur anywhere a fault exists, including places far from plate edges. Parts of the central United States, for instance, have experienced powerful earthquakes in the past.
How Scientists Measure Earthquakes
Scientists use instruments called seismographs to record ground shaking. A seismograph works in a clever way: the whole device is bolted firmly to the ground, but inside it there’s a heavy weight hanging on a spring. When the ground shakes, the frame moves with it, but the weight stays still because of its own heaviness. The difference between the moving frame and the still weight gets recorded as a squiggly line called a seismogram. Bigger squiggles mean stronger shaking.
You might have heard of the Richter scale. Charles Richter invented it in the 1930s to measure earthquakes in southern California, but it turned out to work well only for certain sizes and distances. Today, scientists use a newer system called the moment magnitude scale, which works reliably for earthquakes of all sizes, from tiny rumbles to the most massive quakes on record. Each whole number up the scale represents about 32 times more energy. So a magnitude 6 earthquake releases roughly 32 times more energy than a magnitude 5.
How Many Earthquakes Happen Each Year
Scientists locate around 20,000 earthquakes worldwide every year. The vast majority are small. Based on records going back to about 1900, a typical year brings about 15 earthquakes in the magnitude 7 range and one magnitude 8 or greater. Those magnitude 8+ events are sometimes called “great” earthquakes, and they can cause serious damage over large areas.
How Long the Shaking Lasts
Most earthquakes are over fast. A moderate earthquake, around magnitude 4 or 5, releases its energy in about one second of shaking. A larger quake might shake for a few minutes. The 1964 Great Alaska Earthquake, one of the most powerful ever recorded at magnitude 9.2, ruptured a fault 500 miles long and shook the ground for nearly five minutes. After very large earthquakes, sensitive instruments can detect energy waves traveling around the globe for several days.
Tsunamis: Earthquakes Under the Ocean
When a strong earthquake happens beneath the ocean floor, it can push the seafloor suddenly upward or pull it downward. That movement shoves a huge column of water out of place. As the water tries to settle back to its normal level, it creates long, fast-moving waves called tsunamis. Out in the deep ocean, a tsunami wave may be only a foot or two tall, but as it reaches shallow water near shore, it piles up and can become enormous and destructive.
Can Animals Sense Earthquakes?
An earthquake actually sends out two main types of waves. The first type, called P-waves, travels fastest and arrives first, but the shaking it causes is small and hard for people to notice. The second type, S-waves, arrives next and causes the strong shaking everyone feels. Many animals have sharper senses than humans and can feel those early P-waves seconds before the bigger S-waves hit. That’s why dogs, cats, or birds sometimes act nervous right before people feel anything. It’s not a mysterious sixth sense. They’re simply detecting vibrations that human bodies aren’t sensitive enough to pick up.
How to Stay Safe: Drop, Cover, and Hold On
If you ever feel the ground start to shake, there are three steps to remember:
- Drop to your hands and knees so you don’t get knocked down.
- Cover your head and neck with your arms. If a sturdy desk or table is nearby, crawl under it.
- Hold on to that furniture until the shaking stops completely.
If you’re in bed when an earthquake strikes, stay there and cover your head and neck with a pillow. The goal is always to protect yourself from things that might fall, like books, light fixtures, or ceiling tiles. Practicing these steps with your family ahead of time makes it much easier to remember them when the ground is actually moving.