Glossary of Important Terms
Familiarize yourself with these terms to help identify an earthquake hazard:
Aftershock – An earthquake of similar or lesser intensity that follows the main earthquake.
Earthquake – A sudden slipping or movement of a portion of the earth’s crust, accompanied and followed by a series of vibrations.
Epicenter – The place on the earth’s surface directly above the point on the fault where the earthquake rupture began. Once fault slippage begins, it expands along the fault during the earthquake and can extend hundreds of miles before stopping.
Fault – The fracture across which displacement has occurred during an earthquake. The slippage may range from less than an inch to more than 10 yards in a severe earthquake.
Liquefaction – Fluidization of water-saturated silty or sandy sediment in which the individual particles lose cohesive contact with each other, causing the sediment to lose strength and flow similar to wet concrete or a slurry. It is usually caused by shaking during an earthquake, but can also occur within a moving landslide. It can cause buildings, overpasses, and other structures to settle, break apart, tilt, or slide. Due to the way URM foundations are constructed, liquefaction impacts can be much more severe than for a non-URM. Parts of the Wasatch Front and other areas of Utah have high potential for liquefaction during an earthquake.
Magnitude – The amount of energy released during an earthquake, which is computed from the amplitude of the seismic waves. A magnitude of 7.0 on the Richter Scale indicates an extremely strong earthquake. Each whole number on the scale represents an increase of about 30 times more energy released than the previous whole number represents. Therefore, an earthquake measuring 6.0 is about 30 times more powerful than one measuring 5.0.
Masonry – Building material derived primarily from earthy materials such as clay, sand, gravel, cinders, lime (both for cement and as cut stone), stone, and other materials. While masonry materials tend to be strong under static load, without proper reinforcement, they tend to be nonelastic and frequently fail (crack, crumble, topple, buckle, etc.) during the violent side-to-side shaking of an earthquake. When constructed properly using wood, steel, fiberglass, or other materials to add strength and support, masonry materials can still be good building materials in earthquake country.
Seismic Waves – Vibrations that travel outward from the earthquake fault at speeds of several miles per second. Although fault slippage directly under a structure can cause considerable damage, the vibrations of seismic waves cause most of the destruction during earthquakes.
Unreinforced Masonry Building (URM) – A URM is a building in which the primary supporting walls are brick, blocks, stone, adobe, or concrete (masonry) that does not have adequate steel or wood structural support to properly withstand shaking associated with a severe earthquake.