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All earthquakes start beneath Earth’s surface. The focus of an earthquake is the point underground where rocks first begin to move. Seismic waves travel outward from the earthquake’s focus. The epicenter (EHP-ih-SEHN-tuhr) is the point on Earth’s surface directly above the focus. Scientists often name an earthquake after the city that is closest to its epicenter. The prefix epi- comes from a Greek word meaning “on top of.” An earthquake’s epicenter is directly over its focus. In general, if two earthquakes of equal strength have the same epicenter, the one with the shallower focus causes more damage. Seismic waves from a deep-focus earthquake lose more of their energy as they travel farther up to Earth’s surface. The depths of earthquakes along tectonic plate boundaries are related to the directions in which the plates move. For example, an earthquake along a mid-ocean spreading center has a shallow focus. There, the plates are pulling apart, and the new crust that forms is thin. Subduction zones have a wide range of earthquake depths, from shallow to very deep. Earthquakes can occur anywhere along the sinking plates. Focus and Epicenter Seismic waves spread out from the focus of an earthquake. Structures farther from the epicenter experience less shaking and less damage. fault The epicenter is the point on the surface directly above the focus. The earthquake begins at the focus. Structures near the epicenter experience more shaking and more damage. Seismic waves spread FPO out in all directions from the focus. What is the difference between the focus and the epicenter of an earthquake? 228 Unit 2: The Changing Earth