an earthquake that follows a large earthquake and has an epicenter at or near the initial earthquake. An aftershock does not have to be smaller than the initial earthquake. Aftershocks do decrease in frequency and magnitude over time.

a type of earthquake in which one block of rock earthquakes slides horizontally past another block of rock.

the point on the Earth's surface directly above the hypocenter.

a thin zone of crushed rock between two blocks of rock. A fault can be any length, from centimeters to thousands of kilometers.

the point where the earthquake rupture begins, usually deep within a fault. This is also known as the focus.

naturally occurring molten rock. Magma is commonly found in volcanoes.

an earthquake intensity scale. It is not based on mathematics but on observed occurrences.

a numerical, mathematical scale of earthquake magnitude.

a record of earthquake waves made by a seismograph.

an instrument that records earthquake waves.

a type of earthquake where one block of rock earthquake slides up or down relative to another block of rock.

large segments of the outer layer of the Earth that move relative to one another.