earthquake, trembling or shaking movement of the earth's surface. Most earthquakes are minor tremors. Larger earthquakes usually begin with slight tremors but rapidly take the form of one or more violent shocks, and end in vibrations of gradually diminishing force called aftershocks. The subterranean point of origin of an earthquake is called its focus; the point on the surface directly above the focus is the epicenter. The magnitude and intensity of an earthquake is determined …showed first 75 words of 2715 total…

…showed last 75 words of 2715 total…ons. Measurement of sudden sea level changes from satellites are also used to warn of potential tsunami. Probably the most destructive tsunami occurred following the explosive eruption of the volcano Krakatoa in the East Indies on Aug. 27, 1883, when over 36,000 people were killed as a result of the wave. Waves were up to 100 ft (30 m) high with speeds between 350 and 450 mi per hr (560-720 km per hr). Its passage was traced as far away as Panama.