The San Andreas Fault line is perhaps the most famous (or infamous) fault line in the world. It is a continental transform fault and runs around 1,200 km or 750 through California. It marks the boundary between the Pacific Plate and the North American Plate. The San Andreas Fault is one of the main FAQs people have about visiting California.

While the fault line is a matter of concern and planning for those living there, it shouldn't be a worry for anyone wanting to discover this incredibly stunning state. There are innumerable things to and do in California - but one of its crowning jewels must be the glacial-carved Yosemite National Park.


What To Know About The San Andreas Fault

As the most famous fault in the United States, there is bound to be a lot of misunderstanding about this destructive faultline. For reliable information see the US Geological Survey's website about it.

  • Where: California
  • Length: 1,200 km or 750 miles
  • Identified: In 1895 by Professor Andrew Lawson of UC Berkeley
  • Age: The Faultline is 15-20 Million Years Old

The plates here are moving at a speed ranging from 20 to 35 mm (0.79 to 1.38 in) per year and even up to 2 inches in a year.

  • Movement: Around 20 to 35 mm or 0.79 to 1.38 inches and Up to 2 Inches

There are three ways a fault line can move - dip-slip (aka vertical - the most common), thrust faults, and strike-slips. The motion along this fault is right-lateral strike-slip (aka horizontal). That means if two people stand and face each other on opposite sides of the fault they would see each other move to the right (by about an inch a year).

The fault gained notoriety after the disastrous 1906 San Francisco earthquake and also because it happens to pass through the USA's most populated and wealthy state.

The fault forms a continuous narrow break in the Earth's crust and it extends from Cajon Pass near San Bernadino into northern California. Southeastward of Cajon Pass, it branches into several faults including the San Jacinto and Banning faults.

Related: The Ultimate Guide To Hiking In Yosemite National Park

Seeing The Faultline

The faultline can be easy to be seen from the air (the linear troughs with linearly aligned lakes, bays, and valleys is apparent). On the ground, it's more subtle to see the tell-tale signs. One can see it by distinctive landforms including long straight escarpments, narrow ridges, and small undrained ponds formed by the settling of small blocks within the zone.

During the earthquake, the faultline slipped by several yards. The most dramatic was a 21-foot offset (the maximum recorded) at Tomales Bay.

Over its 15-20 million-year history, it is thought to have moved some 350 miles.

When Will The Next "Big" One Happen

The San Andreas fault is an active faultline. Then too much pressure has built up in the fault, it will "snap" and move to a new position relieving the pent-up stress. That is an earthquake. It has been estimated that the massive 1906 earthquake was around a magnitude 8.3 on the Richter Scale (extremely high).

It can, will, and is generating earthquakes. According to the US Geologic Survey:

Scientists term these segments "seismic gaps" and, in general, have been successful in forecasting the time when some of the seismic gaps will produce large earthquakes. Geologic studies show that over the past 1,400 to 1,500 years large earthquakes have occurred at about 150-year intervals on the southern San Andreas fault."

  • Intervals: Around 150 Year Intervals

With that information, the last large earthquake in the southern San Andreas region occurred in 1857. So the southern section is liable for another major earthquake in the next few decades.

  • Southern San Andreas: Likely To Have A Major Earthquake In The Next Few Decades

San Francisco Bay would seem to have a slightly lower potential for a major earthquake as the last one happened in 1906. That being said, moderate-sized and potentially damaging earthquakes could happen at any time.

Related: Visiting The San Andreas Fault Line Is Easy, Here's Where You Can Go To See It

What Can Be Done About Earthquakes?

Can earthquakes be stopped or prevented? No. Nothing can stop earthquakes from happening. Instead, people much learn to live with them in an earthquake zone.

There are three major lines of defense against earthquakes when they happen to mitigate the inevitable damage. These include strict buildings codes, selective use of land, and prediction of earthquakes - this is still being worked on.

Lines of Defense according to USGS:

  • First Line of Defense: Strict Earthquake Building Codes and Refitting or Demolishing Older Buildings Not up To Code
  • Second Line of Defense: Selective Use of Land So That High-Occupancy or Critical Structures are Not On The Fault Or in Landslide Prone Zones
  • Third Line of Defense: Accurate Prediction of Earthquake - Scientists are Working On Ways To Predict Earthquakes

Next: A Massive "Boomerang" Earthquake Has Been Confirmed For The First Time Ever