San Francisco Bay Region Geology and Geologic Hazards

About Quaternary Faults > What Makes a Fault Break?

What Makes a Fault Break?

Where the Earth's tectonic plates collide, pull apart, or slide by each other, they form and drive faults.

Although the Earth seems very stable on a human time scale, over geologic time it is a very dynamic system. Rock bodies are continuously being made and destroyed. Mountains are pushed up and ground down. Huge slabs of the Earth's surface are sliding and grinding past and over one another.

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Fossilized fragments of a marine animal called crinoid, or sea lily (left), have been found near the top of Mount Everest (right), evidence of the dynamic forces that drive faults.

The powerful forces that drive this system cause huge slabs of the Earth's crust, called tectonic plates, to grind and push against one another. The rocks along the boundaries of these plates are continuously being squeezed and sheared, causing them to bend and break. California is cut by one such plate boundary, between the Pacific and North American plates, at the San Andreas Fault system. All the Quaternary-active faults in the San Francisco Bay region are part of the San Andreas Fault system.

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A map of the world's earthquakes, shown as red dots. Note how most earthquakes occur at plate boundaries, as the plates grind against each other to drive faults and generate earthquakes.

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A map of tectonic plates that make up the Earth's crust. Note how most earthquakes occur at plate boundaries, as the plates grind against each other to drive faults and generate earthquakes.

 

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