An earthquake is a sudden shaking of the ground. Earth's outer shell is broken into giant pieces called tectonic plates that are always slowly moving — about as fast as your fingernails grow! Sometimes the edges of two plates get stuck. Pressure builds up for years, and then — suddenly — the plates slip and all that stored-up energy zooms out in waves called seismic waves.
The spot deep underground where the quake starts is called the focus, and the point on the surface right above it is the epicentre. Scientists measure earthquakes with an instrument called a seismograph, and they describe their strength using magnitude numbers. Each whole number up means about 32 times more energy!
Damage in the city of Kesennuma, Japan, after the huge magnitude 9.0 earthquake in March 2011 — the most powerful earthquake ever recorded in Japan. See source ↓
📏 How strong was that quake?
Magnitude 2 or 3 quakes happen all the time and most people can't even feel them. A magnitude 6 can damage buildings, and a magnitude 8 or 9 is a huge disaster. The biggest earthquake ever measured was magnitude 9.5!
📜 A bit of history
Almost 2,000 years ago, in the year 132 AD, a Chinese scientist named Zhang Heng invented the world's first earthquake detector — a bronze pot with dragon heads holding balls in their mouths. When the ground shook, a ball dropped into a frog's mouth below, showing which direction the quake came from!
📚 Picture Sources
- Photo 1 — Earthquake damage in Kesennuma, Japan, 2011. From Wikimedia Commons (CC BY 2.0): https://commons.wikimedia.org/wiki/File:Kesennuma_earthquake_damage,_2011.JPG
