Earthquakes

When the ground shakes

Earth's outer shell is made of huge rock pieces — plates. They drift very slowly, about as fast as your fingernails grow. Sometimes two plates get stuck, then suddenly slip — that is an earthquake! The orange lines on the map are plate boundaries; most earthquakes happen there. Tashkent had a strong earthquake in 1966, after which the city was rebuilt.

The lithosphere is split into about 15 major plates, driven 2–10 cm a year by convection in the mantle. Three kinds of motion happen at boundaries: spreading (the Mid-Atlantic Ridge, where new crust forms), collision (the Himalayas, where the Indian plate rams into Eurasia; around the Pacific an ocean plate dives under a continent — subduction) and sliding past (San Andreas). Strength is measured by magnitude: each unit is about 32 times more energy. The strongest recorded was Chile, 1960, M9.5. The rings mark famous historic earthquakes.

Fun facts

• About 500,000 earthquakes happen every year, but only around 100 cause damage.
• The 1966 Tashkent earthquake left 300,000 people homeless, yet the city was rebuilt in 3.5 years.
• The Moon has "moonquakes" too — Apollo seismometers recorded them.
• Seismic waves "X-ray" Earth from inside — that is how we learned it has a core.