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Earthquake & Landslide Glossary

This glossary of earthquake and landslide terms was prepared by the law firm of Michael T Chulak & Associates for non-attorneys to assist them in understanding earthquakes, landslides and mudslides. This glossary of earthquake and landslide terms is general information only and is not intended to be legal advice. If you need legal advice, you should consult with an attorney licensed in your state.


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 earthquakes.

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

iis 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 muddy 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 strengthing 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 United States is included in this area.

Rupture Zone

is the area of fault movement. It may extend for hundreds of miles in length and may or may not extend to the surface of 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.



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