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

Series of 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 6 mi beneath the oceans to 35 mi 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 430 mi.

fault

Fracture in the Earth’s crust separating two blocks that slide against one another during an earthquake.