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Glossary of earthquake terms

  • AB 300: The state Legislature approved AB 300 in 1999, requiring the Department of General Services to conduct a “collapse risk” inventory of the state’s K-12 school buildings. Although the report begins, “Public school buildings in California are the safest in the nation,” it ultimately concluded that 7,537 buildings, approximately 14 percent of the total square footage in the state’s public K-12 schools, were not expected to withstand future earthquakes and urgently needed further structural evaluation to gauge needed repairs.
  • Active fault: A fault where there is evidence of earth movement within the last 11,000 years.
  • Aftershocks: Smaller, weaker earthquakes that can follow major quakes as broken underground rocks settle.
  • Alquist-Priolo Earthquake Fault Zoning Act: After a devastating earthquake ripped through Los Angeles in 1971, the state Legislature passed the Alquist-Priolo Earthquake Fault Zoning Act, which restricts construction on or near fault lines. High-occupancy buildings, such as schools, hospitals and commercial projects, built since 1972 must be set back 50 feet from an active fault. Since June 1998, sellers of real estate must disclose to prospective buyers whether the property is within such a zone. California Watch found that the state has been changing the maps – making several earthquake zones smaller.
  • Bedrock: Relatively hard, solid rock that commonly underlies softer rock, sediment or soil.
  • Blind thrust fault: A thrust fault that does not rupture all the way up to the surface, meaning there is no evidence of it on the ground. It is buried under the uppermost layers of rock in the earth’s crust.
  • Division of the State Architect: The state regulatory office responsible for overseeing the Field Act. The division also is referred to as the state architect’s office.
  • Epicenter: The point on the planet’s surface above the underground origin of an earthquake.
  • Fault (fault line): A crack or break in the earth’s crust where two tectonic plates meet.
  • Field Act: Within a month of the 1933 Long Beach earthquake, the Legislature passed what is now called the Field Act, named after Assemblyman C. Don Field, R-Glendale. The law requires careful design and inspection of public school construction projects at K-12 schools and community colleges. Any person violating the act or making false statements in any verified report may be guilty of a felony.
    There are two types of letters issued by the Division of the State Architect for projects without Field Act certification. A Letter 4 is issued when a project has unresolved safety issues. A Letter 3 is issued when important documents are missing that could suggest safety issues.
  • Focus (or hypocenter): The underground place where an earthquake starts.
  • Foreshock: A smaller quake that precedes some major earthquakes.
  • Liquefaction: When water-saturated soil is shaken with enough force, the soil loses its strength and begins to act more like a liquid than a solid – essentially becoming like quicksand. The process can affect critical infrastructure, such as underground pipelines, airport runways, harbor facilities and road or highway surfaces. During the 1989 Loma Prieta earthquake, for instance, liquefaction caused severe damage to underground water pipes – hampering the fire department’s ability to control fires.
  • Liquefaction zones: The state has mapped areas where liquefaction has occurred during earthquakes, as well as areas of poorly compacted landfills. Any development within a liquefaction zone is supposed to be highly regulated under state law. Before a project can be permitted, builders must make sure the soil is safe enough for construction. Since 1998, the Natural Hazards Disclosure Act has required real estate sellers or their agents to inform buyers if a property is within one of these zones.
  • Magnitude: A measure of the amount of energy released by an earthquake; it does not measure the intensity of the shaking.
  • Seismic: Something that is caused by or related to an earthquake.
  • Stress: Pressure on underground rock caused when tectonic plates cannot move smoothly past one another.
  • Tectonic plate: One of the slabs of the earth’s crust.
  • Tsunami: A powerful, destructive sea wave from an undersea earthquake that travels at high speed across the ocean before smashing into land.
  • Valley fever: A sometimes-fatal flulike infection transmitted by airborne fungus; it can occur when the fungi are stirred into the air by anything that disrupts the soil, such as an earthquake. Billowing dust caused by the 1994 Northridge quake led to more than 200 reported cases. Dozens of people were hospitalized, and a 71-year-old Simi Valley man died. There are about 150,000 cases in the Southwest each year.


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