Earthquakes (or temblors) happen millions of times each year, but most are so small that you can’t feel them. Strong ones can make buildings collapse and cause fires or water leaks. They also can knock down power lines and block roads. Learn more about these unpredictable Earth tremors and what you can do to stay safe during one.
An earthquake occurs when rocks suddenly slip past each other along a crack in the Earth’s surface. These slips generate vibrations that spread outward from the place where the quake begins, which is called its focus. The spot on the Earth’s surface directly above the focus is called the epicenter.
When the earthquake’s rolling waves reach the ground, they produce extreme horizontal forces that can damage a building’s columns, walls and floors. The shaking caused by Love and Rayleigh waves creates lateral accelerations that can exceed 1 G, or about 40 inches (102 centimeters) per second. Buildings are designed to withstand vertical forces associated with gravity, but they’re not engineered for these horizontal forces, which can overstress them and cause them to break apart.
An earthquake’s size depends on how much rock is moved and the distance between the rock’s point of origin and its center of rupture. Scientists measure an earthquake’s magnitude by using data from the instruments that record its motion — seismographs and seismic waves. They arithmetize the difference between the speed of the P and S waves on the seismograph to get the earthquake’s moment magnitude. They can also use the information from at least three seismograph stations to draw a circle on a map that interacts in one spot, which indicates the location of the earthquake’s epicenter.
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