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An earthquake is a sudden motion or trembling caused by a release of stress accumulated within or along the edge of the earth‘s tectonic plates. Earthquakes usually occur without warning and, after just a few seconds, can cause massive damage and extensive casualties. The severity of ground motion generally increases with the amount of energy released and decreases with distance from the fault or hypocenter of the earthquake.

Coincidental events manifested by earthquake activity include:

Surface Rupture:
Surface rupture occurs when the ground surface ‘breaks’ due to fault movement during an earthquake. The Alquist-Priolo Earthquake Fault Zoning Act (APEFZA) was passed in 1972 to attempt to mitigate the hazard of surface rupture to structures for human occupancy. The State has not designated any special study zones for Portola or the immediate surrounding areas indicating that Portola is not in a fault zone.

Ground Shaking:
Ground shaking is a general term that refers to all aspects of motion of the earth‘s surface resulting from an earthquake and is usually considered the most important of all seismic hazards because all the other hazards are triggered by ground shaking. The amount of ground shaking experienced varies with each earthquake and each fault system. Structural design, construction materials and supporting soils/bedrock also significantly influence how the structure responds to the event and how much damage occurs. Portola is considered in an area of moderate ground shaking potential.

Liquefaction & Lateral Spreading:
Some of the most infamous events associated with earthquake damage deal with liquefaction, the point during a seismic event when soils lose their strength and begin to act as fluids, and spread laterally or liquefy. The greatest potential for lateral spreading would follow the river corridor where significant changes in grade/elevation occur. Liquefaction can cause severe damage to structures, bridges, roadways, and buried utilities as well. Liquefaction typically manifests in somewhat isolated zones and areas. An area can be the same seismically and geologically, yet liquefaction may only impact a small portion of the area or an isolated structure.

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