This glossary of earthquake and landslide terms was prepared to assist our readers understand earthquakes, landslides and mudslides.

 

Aftershock
is an earthquake that occurs from hours to months after the larger earthquake or mainshock has occurred. Aftershocks occur in the same general area as the mainshock and are believed to be the result of minor readjustments of stress within the fault zone. Aftershocks decrease in frequency and strength over time. Generally, the larger the mainshock, the larger the aftershocks.

Angle of Rest
also known as the angle of repose. This is the angle at which material (such as soil) will remain stable.

Avalanche
is a massive fall of ice and snow sometimes caused by earthqua kes.

Body Wave
is a vibration from an earthquake that travels deep underground.

Creep
refers to the slow, usually continuous movement that takes place along many faults. Creep does not cause shaking.

Earthquake
is the shaking of the earth caused by a sudden movement of rock beneath the surface. Earthquakes cause seismic waves that travel from the point of rupture. These waves shake the earth as they travel.

Epicenter
is the point on the Earth's surface directly above the focus or hypocenter of the earthquake.

Fault
is a break in the Earth along which movement occurs during an earthquake. It is the sudden movement along a fault that causes an earthquake. Slow movement results in seismic creep. There are three types of faults- thrust, normal and strike-slip.

FEMA
is the Federal Emergency Management Agency. It is the agency responsible for identifying and mitigating natural and man-made hazards. FEMA also administers the National Flood Insurance Program.

Focus
or Hypocenter is the point within the Earth from which the earthquake originates.

Foreshock
is a small tremor that often precedes-the main shock from. a matter of a few seconds to weeks. It originates at or near the rupture zone of the primary earthquake.

Geologists
study rocks and geologic features.

Geotechnical Engineers
are civil engineers. Geotechnical Engineers also assess the risk to people and property from natural hazards such as landslides, sinkholes, earthquakes and soil liquefaction.

Hypocenter
is the same as the focus.

Intensity
is the measure of the shaking at a particular point. The intensity at any point depends upon both the strength of the earthquake (magnitude) and the distance from the epicenter. Intensity refers to the effect or severity of the earthquake.

Landslide
is a downward movement of soil, rocks and possibly mud, plant material, structures and roads, When the soil liquefies, it becomes a mudslide or mudflow. Landslides
can be very violent moving more than twenty miles per hour or may move as slowly as two to three inches in a year. Landslides can be triggered by earthquakes or heavy rains or may be caused by man.

Liquefaction
is the process that takes place when large earthquakes shake sandy soil until it starts to resemble liquid, allowing sand to "boil up" to the surface, structures to sink, and/or slopes to fail.

Magnitude
refers to the quantity of energy released by an earthquake as' compared to intensity which describes the effect at a particular point. Several earthquake magnitude scales exist including the Richter Magnitude Scale.

Mainshock
is the largest in a series of earthquake shocks. The mainshock is usually preceded by one or more foreshocks and followed by one or more aftershocks.

Mercalli Scale
is used to measure the intensity of earthquakes ranging from a scale of one to twelve. It is based upon the effects on people and buildings. A level of twelve equates to total destruction.

Mudflow
is similar to a landslide except that it occurs when a slope is so heavily saturated with water that it rushes down hill in the form of a muc{dy river spreading out at the base of the slope.

Plates
are giant slabs of underground rocks that are sometimes larger than continents.

Public Adjuster
is an insurance adjuster who represents an insured on a fee basis. A public adjuster is not licensed to practice law.

Retrofit
refers to strengthening existing structures to improve earthquake safety.

Richter Magnitude Scale
was developed in 1935 by Charles Richter as a mathematical device to compare the sizes of earthquakes. The magnitude of an earthquake is determined by an instrument called a seismograph. On the Richter Scale, magnitude is measured in whole numbers and decimals. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a ten fold increase in magnitude. For example, a magnitude 6 is ten times greater than a magnitude 5 earthquake. The scale is measured from one to ten. The Richter Scale is.also known as the local scale.

Ring of Fire
is the area around the Pacific Ocean where there is a high level of seismic activity taking place. The western coast of the Wnited States is included in this area.

Rupture Zone
is the area of fault movement. It may extend for hundreds of miles in length and mayor may not extend to the surface 9f the Earth.

SBA
is the Small Business Administration which is a federal agency that provides various types of loans for businesses, home-owners and renters in the event of disaster.

Seismic
means having to do with earthquakes.

Seismic Waves
are the vibrations from earthquakes that travel through the Earth. They are recorded and measured on instruments called seismographs. There are three types
of waves: P (fastest), S (slower) and surface waves (slowest).

Seismogram
is a graph showing ground motion over time.

Seismograph
is a scientific instrument used to detect and measure earthquakes.

Seismologist
is a scientist who studies earthquake causes and results.

Sinkhole
is a sudden subsidence of land, often relatively flat areas, underlain by limestone or similar materials.

Stress
refers to the intensity of two forces working against each other.

Strike-slip Fault
is generally a vertical fault along which the two sides move horizontally. The best known example is the large San Andreas Fault in California.is generally a vertical fault along which the two sides move horizontally. The best known example is the large San Andreas Fault in California.

Subsidence
is the vertical fall of soil caused by lack of adequate support.

Surface Wave
is a vibration from an earthquake that travels at ground level.

Tremors
are the shaking of the Earth caused by earthquakes.

Tsunami
is one or more waves caused by the rapid displacement of water by major earthquakes usually exceeding a magnitude of 7..5, or a significant under water landslide. In the open ocean they may travel at speeds exceeding 300 kilometers per hour. They are capable of crossing entire oceans and can exceed 30 meters in height when they reach the shoreline.

Vibration
is a continuous shaking movement.