With tremendous force a great mass of solid rock suddenly moves deep inside the earth．Shock waves travel upward, and the suce of the earth begins to tremble, ff the trembling is very strong, trees sway and fall, houses collapse, bridges twist and slide into rivers．In cities, fires start as gas lines break．With a loud rear, the ground splits open．Earthquake!
Experts cannot tell when earthquakes will occur．But they can tell us why they happen．In the center of the earth a huge, hot mass moves constantly, like rushing water．Vast areas of solid rock, called plates(板岩), rest on top of this mass．Sometimes two plates move and strain against each other．When they finally break, the earth's suce moves, and an earthquake begins．Some quakes have other causes.
Most of the world's earthquakes happen around or in the Pacific Ocean．Called the 'ring of fire' by scientists, this area suffers from about twenty powerful earthquakes each year．But there are others even too small to be felt．They occur a million times a year．However, they take place under the ocean or away from people．Therefore, there is no damage.
Scientists measure the strength of earthquakes with sensitive machines called seismographs(地震仪)．These instruments can record shock waves from earthquakes in any part of the world．Seismographs show us that some earthquakes can give off as much energy as 200 million tons of TNT．That is 10,000 times stronger than an atomic bomb!
Powerful earthquakes can sometimes take an enormous number of human lives．The worst earthquake on record took place in China in 1556．At that time 830,000 people died．In 1737, 300,000 were killed during an earthquake in India．In modern times, China was hit again with the world's second worst quake in 1976．It left 650,000 dead.
Many cities are taking steps to pr loss of life in earthquakes．Buildings are being made quakeproof．Rescue teams practice saving victims．Partly because of such steps, the San Francisco quake took only sixty lives.
We can infer from the passage that ______.