California Frequently Asked Questions

Q: Will California eventually fall off into the ocean?

A: No. The San Andreas Fault System, which crosses California from the Salton Sea in the south to Cape Mendocino in the north, is the boundary between the Pacific Plate and North American Plate. The Pacific Plate is moving in northwest with respect to the North American Plate at approximately 46 millimeters per year (the rate your fingernails grow). The strike-slip earthquakes on the San Andreas Fault are a result of this plate motion. The plates are moving horizontally past one another, so California is not going to fall into the ocean. However, Los Angeles and San Francisco will one day be adjacent to one another!

Q: Where can I buy a Richter scale?

A: The Richter scale is not a physical device, but a mathematical formula. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded on a seismogram at a certain period. See next question and answer.
For further information, see: The Richter Magnitude Scale.

Q: Why are there no faults in the Great Valley?

A: The Great Valley is a basin, initially forming some ~100 million years ago as a low area between the subducting ocean plate on the west (diving down under the North American plate) and the volcanoes to the east (now the Sierra Nevada mountains). Since its formation, the Great Valley has continued to be low in elevation. Starting about 15 million years ago the tectonics changed in California and instead of the ocean plate diving down under the North American plate, it began to slide along it, with the ocean plate moving northward. This movement occurs along the San Andreas fault and the many other faults that are roughly parallel to it.

The faults on the east side of the Great Valley, mostly in Nevada, are the result of the North American plate pulling apart there, in a different tectonic setting that results in the linear mountain ranges and long valleys you can see there. The faults just to the east of the Great Valley are mostly old faults and may or may not still be active today. So there is movement of faults in two separate regimes: sideways motion along the San Andreas system to the west-southwest, and pull apart motion along the faults mostly in Nevada to the east-northeast of Sacramento. (Heidi Stenner)

Q: What do the descriptions of earthquake location quality mean on the Recent Earthquakes maps?

A: Here is what the words, excellent, good, fair and poor mean:

See the Earthquake Hazards Program FAQ for all other questions.