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earthquake 
Sudden tremor in a region of the Earth’s crust caused by one rock mass sliding against another.
focus
Point in the Earth’s crust where an earthquake is triggered. Also called the hypocenter.
seismic wave
Vibrations generated at the focus that disperse in all directions, causing shaking of the Earth’s surface.
Earth’s crust
Solid layer at the Earth’s surface whose thickness varies from 10 km beneath the oceans to 60 km beneath the mountains.
isoseismal line
Curved line connecting the points on the Earth’s surface that have been subject to tremors of the same intensity.
epicenter
Point on the Earth’s surface located directly over the focus, where the most violent tremors are felt.
depth of focus
Distance between the focus and the epicenter; it can reach 700 km.
fault
Fracture in the Earth’s crust separating two blocks that slide against one another during an earthquake.
