San Andreas Fault produces large earthquakes

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Why Should I Care? -- The Bay Area Is Your Home

All Bay Area Residents Live on an Active Plate Boundary Where Earthquakes Are Frequent Events!

We know that the San Andreas Fault produces large earthquakes and that many other Bay Area faults are also hazardous. However, even knowing this, it can be difficult to understand how to use this information to make us safer in our daily lives. Should we care only if we live near the San Andreas Fault, or is every place in the Bay Area just as dangerous?

This eight-page section describes where earthquakes occur in northern California. It also explains how earthquakes will shake the ground and cause damage in other ways, such as liquefaction and landslides (see pages 8 through 11). Technical terms used throughout this book are explained in the Glossary.

The idea or myth of California sliding into the Pacific Ocean in an earthquake and creating new beachfront property to the east appeals to those having a bit of fun at the Golden State's expense. Although part of the State west of the San Andreas Fault system is very slowly moving northward and in millions of years could become an island, earthquakes caused by this horizontal motion of the Earth's tectonic plates will not make California disappear into the sea, like fabled Atlantis.

Plate Motions Load the Faults

Deep beneath California, the Pacific and North American Plates relentlessly grind past one another, straining or "loading" faults in the Earth's rigid crust above. The horizontal ("strike slip") movement between these plates along the San Andreas Fault Zone is about 1.7 inches per year (40 mm/yr), about as fast as your fingernails grow. At this rate, Los Angeles will be west of San Francisco in about 12 million years.

JANUARY 1700 M 9

In this computer simulation, tsunami waves are radiating outward after a magnitude (M) 9 earthquake that occurred on the Cascadia Subduction Zone offshore of northern California, Oregon, and Washington on January 26, 1700. This view shows the waves 4 hours after the quake. Colors indicate wave heights -- red is highest. Along parts of the coast of the Pacific Northwest, 30-foot-high (9 m) waves rushed inland. Within 20 hours the tsunami did damage throughout the Pacific, and it is well documented in written records from Japan.

For more information go to
http://pubs.usgs.gov/circ/c1187/

"Orphan tsunami" Web address
http://pubs.usgs.gov/pp/pp1707/