Earthquakes are caused by the movement of 50-mile-thick plates that comprise the earth's crust. More than two thousand earthquakes occur daily somewhere on our planet, but about 95 percent of them are too weak to be felt ＿＿（11）＿＿ sensitive seismometers. ＿＿（12）＿＿ are felt by humans, most cause relatively little damage, particularly if they occur in sparely ＿＿（13）＿＿ areas. Scientists have observed that certain phenomena seem to occur before an earthquake as the pressure within the plates intensifies. Myriads of tiny cracks appear in the rock, causing it to expand and uplift the ground above it. The pressure squeezes water out of the rock, thereby ＿＿（14）＿＿ nearby water levels. Also the rock becomes more resistant to electric current, and radon gas is released. Scientists hope that these ＿＿（15）＿＿ phenomena will provide keys to predict when and where an earthquake will occur, thereby enabling them to warn people to evacuate the area. While the earthquake itself cannot be prevented, at least lives may be saved.