The San Andreas Fault



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The Great San Francisco Earthquake of 1906 

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The presence of the San Andreas fault was brought dramatically to world attention on April 18, 1906, when sudden displacement along the fault produced the great San Francisco earthquake and fire. This earthquake, however, was but one of many that have resulted from episodic displacement along the fault throughout its life of about 15-20 million years. 
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1906_SFEarthquake_330_224_c1_c_t The Great San Francisco Earthquake of 1906 was one of the deadliest earthquakes of recent times. The quake ruptured the 296 miles of the north San Andreas Fault in California, displacing land along the rupture zone. The quake caused multiple aftershocks, fire throughout San Francisco, shaking damage across the affected area, and a high death toll, particularly in San Francisco, but also in surrounding regions.
The earthquake is considered one of the most devastating natural disasters in American history.

A look back at the Great San Fransisco Earthquake 1906 >>

For more information regarding the Great San Francisco Earthquake of 1906, watch the USGS video Shockwaves: 100 years after the 1906 Earthquake.

Significant Bay Area Earthquakes

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The San Andreas Fault 


San Andreas Fault Interactive Map

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The San Andreas Fault is a continental transform fault that extends roughly 1300 km (810 miles) through California.
It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizontal). The fault divides into three segments, each with different characteristics and a different degree of earthquake risk, the most significant being the southern segment, which passes within about 35 miles of Los Angeles.

The fault was first identified in 1895 by professor Andrew Lawson from UC Berkeley who discovered the northern zone. It is named after San Andreas Lake, a small body of water that was formed in a valley between the two plates. Following the 1906 San Francisco Earthquake, Lawson concluded that the fault extended all the way into southern California.

In 1953, geologist Thomas Dibblee astounded the scientific establishment with his conclusion that hundreds of miles of lateral movement could occur along the fault. A project called the San Andreas Fault Observatory at Depth (SAFOD) near Parkfield, Monterey County, is drilling into the fault to improve prediction and recording of future earthquakes.

Probabilistic Seismic Hazard Map

Earthquake Shaking Potential Map for California

A probabilistic seismic hazard map is a map that shows the hazard from earthquakes that geologists and seismologists agree could occur in California. It is probabilistic in the sense that the analysis takes into consideration the uncertainties in the size and location of earthquakes and the resulting ground motions that can affect a particular site.

The maps are typically expressed in terms of probability of exceeding a certain ground motion. For example, the 10% probability of exceedance in 50 years maps depict an annual probability of 1 in 475 of being exceeded each year. This level of ground shaking has been used for designing buildings in high seismic areas. The maps for 10% probability of exceedance in 50 years show ground motions that we do not think will be exceeded in the next 50 years. In fact, there is a 90% chance that these ground motions will NOT be exceeded. This probability level allows engineers to design buildings for larger ground motions than what we think will occur during a 50-year interval, which will make buildings safer than if they were only designed for the ground motions that are expected to occur in the next 50 years.

View / Download a copy of Earthquake Shaking Potential map shown:
PDF version     JPEG version
View / Download a copy of Regional Earthquake Shaking Potential maps similar to above: (pdf files)
North Coast        San Francisco Area       Central Coast        Los Angeles Area        San Diego Area

When Could the Next Large Earthquake Occur Along the San Andreas Fault?

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Notable earthquakes in the San Francisco Bay Region before and after the 1906 earthquake. Source

Along the Earth’s plate boundaries, such as the San Andreas fault, segments exist where no large earthquakes have occurred for long intervals of time. 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. As the last large earthquake on the southern San Andreas occurred in 1857, that section of the fault is considered a likely location for an earthquake within the next few decades. The San Francisco Bay area has a slightly lower potential for a great earthquake, as less than 100 years have passed since the great 1906 earthquake; however, moderate-sized, potentially damaging earthquakes could occur in this area at any time.

New data suggest megaquakes in California might be slightly more common than previously thought 

The 800-mile San Andreas Fault, which runs from northern California to Mexico, has been the source of the state’s biggest earthquakes. Known as the ‘sleeping giant’, it is one of more than 350 faults that are found across the state.

Scientists now predict that the risk of a mega quake in the next 30 years is higher than was previously thought.

Scientists are gathering vastly more data about faults than ever before, allowing them to refine earthquake predictions. The data suggest megaquakes in California might be slightly more common than previously thought. A new report from the United States Geological Survey points to how scientists are refining earthquake predictions as huge volumes of new data allow them to understand more deeply how fault networks work.

The Third Uniform California Earthquake Rupture Forecast (UCERF3), published in March 2015, includes newly discovered fault zones and accounts for the possibility of an earthquake jumping between them. This could result in multiple faults shaking in a simultaneous mega quake (magnitude-8), releasing enough energy to cause massive destruction.

The report says that, while there is a lower likelihood of moderate-sized earthquakes, the odds of a mega quake occurring in the next 30 years have increased from 4.7 per cent to 7 per cent.
Meanwhile, the likelihood of moderate-size earthquakes (magnitude 6.5 to 7.5) decreased by about 30 percent, from an average of one per 4.8 years to about one per 6.3 years.

Earthquakes are nothing new for Californians. The state experiences 1,000 quakes a year, but most are too small to be felt. While the San Andreas Fault has experienced massive earthquakes in the central and northern segments — Fort Tejon in 1857 and San Francisco in 1906 — the southern section has not had a large quake for more than 300 years.


EarthScope Chronicles: The Great ShakeOut Earthquake Simulation 

Read more about The Great California ShakeOut


More info

The San Andreas Fault wiki
The 1906 San Francisco Earthquake wiki
Google Map 

A New Earthquake Forecast for California’s Complex Fault System

The Great 1906 San Francisco Earthquake 5:12 AM – April 18, 1906 USGS
A look back at the Great San Fransisco Earthquake 1906 USGS
1906 Earthquake website
Re-evaluation of the 1836 “Hayward Fault” and the 1838 San Andreas Fault Earthquakes
Interactive Map San Andreas Fault
San Andreas Fault Homepage
USGS – San Andreas Fault
SAFOD | EarthscopeSAFOD (San Andreas Fault Observatory at Depth)
UCERF3: A New Earthquake Forecast for California’s Complex Fault System.pdf USGS, March 2015
San Andreas Faults Facts
The Great California ShakeOut
Geology.com – San Andreas Fault

Earthquake Information

California Earthquake Information
Recent Earthquakes in California USGS Interactive Map
Recent California Earthquakes SCEC
Southern California Earthquake Data Centrum SCEDC
Recent Eartquakes on Google Maps – SCEDC
Probabilistic Seismic Hazard Assessment Maps
How Earthquakes and their effects are measured 

Magnitude 6.5 and greater (or destructive) California Earthquakes
Magnitude 6.5 and greater (or destructive) California Earthquakes Sorted by Month
California Earthquake History and Catalogs
Southern San Andreas Shakeout Scenario | ShakeOut Videos
Southern California Earthquake Center

California Volcano Observatory

Movies 

San Andreas 2015San Andreas (2015 – about | watch online)

In the aftermath of a massive 9.1 earthquake in California, a rescue-chopper pilot makes a dangerous journey across the state in order to rescue his estranged daughter.

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San Andreas QuakeSan Andreas Quake (2015 watch online)

When a discredited L.A. Seismologist warns of an impending 12.7 earthquake, no one takes her seriously. Now on her own, she races desperately to get her family to safety before the earthquake breaks Los Angeles apart from the mainland.

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