What are Earthquake hazards?

An earthquake is a general term to describe when two segments of the Earth’s crust shift in relation to one another along a fault, or fracture on the Earth’s surface. The movement between the two sides of the fault can be slow or sudden. Sudden slips can cause the ground to shake, resulting in injury, death and property damage.

These are descriptions of earthquake and geotechnical hazards. We’ve added one other category to include tsunamis that can result from underwater earthquakes:

  • Surface ruptures occur when movement along the fault breaks through to the earth’s surface, causing ground displacement.
  • Other types of earthquake hazards happen below the earth’s surface. Liquefaction occurs when sandy, loose soils that are saturated by water are subjected to ground shaking. This shaking causes the soil to lose its structure and strength.
  • Earthquake-induced landslides are a downslope movement of bedrock, debris, or earth caused by ground shaking. Common types of earthquake-induced landslides include mud flows, also known as debris flows, and rock falls. Landslides can also be induced by other causes, such as heavy rains.
  • Tsunamis are a series of waves generated by undersea landslides, earthquakes (magnitude 8 or greater), volcanic activity or meteors that strike the ocean, causing a mound of water to swell from the ocean floor towards the ocean surface. This mound of water can travel across the ocean surface at speeds up to 500 miles per hour, slowing and growing taller when the ocean floor becomes more shallow until it crashes onshore as a series of large waves.

Two regulatory acts address reducing loses from earthquake hazards. The Alquist-Priolo Earthquake Fault Zoning Act set up regulatory zones around surface faults in California, prohibiting development within a minimum distance from the fault.

The Seismic Hazards Mapping Act addresses other, sub-surface earthquake hazards such as liquefaction, earthquake-induced landslides and amplified ground shaking.

Which parts of Los Angeles County are affected by earthquakes?

California is rated at “very high risk” for earthquakes by the Federal Emergency Management Agency. Many fault systems run throughout Los Angeles County which can lead to both surface and subsurface earthquakes of all sizes.

In general, valleys and alluvial floodplains with loose, sandy soils, ground water levels less than 40 ft from the surface, and nearby active faults are prone to liquefaction. Past earthquake-induced liquefaction in Los Angeles County include the San Fernando Earthquake (1971) and the Northridge Earthquake (1994).

Earthquake-induced landslides are a risk on steep hillsides that are situated near active faults and are comprised of soils made of loose and highly fractured rocks.

How do I find out if I am in a earthquake hazard area?

The California Earthquake Hazards Zone Application (EQ Zapp) shows earthquake fault and seismic hazard zones in relation to any parcel.

How do earthquakes affect residents and communities?

Knowing what seismic hazards in your area and having an emergency plan ready can help you prepare for, respond to, and recover from earthquakes.

California law requires the seller to disclose to the buyer when the property is located in an earthquake fault zone.

What is Los Angeles County doing to address earthquakes?
Regional PlanningNo development on fault zones,
Required 50 ft setback from fault zones for adjacent development
What can I do to prepare for earthquakes?

Create an emergency plan by following guidelines set out in the Los Angeles County Emergency Survival Guide.

Tsunamis can happen within minutes and result in flooding that lasts for several hours, recognizing signs that a tsunami when one is on the coast can be life-saving. When strong ground shaking occurs, ocean water recedes from the beach exposing the ocean floor, and an unusual roar emanates from the ocean, evacuate inland and to higher ground immediately.

Check out these resources

My Hazards Interactive Map – Enter your address to discover what hazards are in your area (earthquake, flood, fire, and tsunami (created by the California State Office of Emergency Services)

Los Angeles County Emergency Survival Guide – How to prepare for, respond to, and recover from disasters in Los Angeles County

The California Earthquake Hazards Zone Application (EQ Zapp) – Allows one to enter a property address and determine whether it is in an earthquake hazard zone.

Earthquake Terms and Definitions – Common terms (Los Angeles County Chief Executive Office)

History of Major County of Los Angeles Earthquakes – Notable earthquakes from 1857 to present (Los Angeles County Chief Executive Office)

The Alquist-Priolo Earthquake Fault Zoning Act  – Created earthquake hazard zones to minimize losses by prohibiting building within a buffer around active faults. This act only addresses surface ruptures. Other earthquake hazards are addressed by the California Seismic Hazard Map (below)

California Seismic Hazard Map – Interactive map showing seismic hazard zones, including liquefaction and earthquake-induced landscape zones (created by the California Department of Conservation) Note: The statewide map of seismic hazard has been developed using regional information and is not appropriate for site specific structural design applications

U.S. Landslide Inventory – Interactive map showing locations where landslides are likely (created by the United States Geological Service)

Los Angeles County Tsunami Inundation Maps (2009)

California Dept of Conservation Interactive Tsunami Map

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