Don's Home Places California Bay Area Fault Zones - Northern

california plates, faults
Introduction San Andreas Fault: An Overview (pdf)
San Andreas Transform Fault Zone
Cascadia Subduction Zone
Earthquakes are caused by the the Pacific Plate moving north relative to the N. American plate. Earthquakes and volcanoes are caused by the Juan de Fuca plate system moving East and subducting under the the North American Plate.

The Pacific Plate grinds northwestward past the North American Plate at an average rate of about 2 inches per year [17 ft per century].
During the 1906 earthquake land on the west side of the rift jumped to the north/northwest as much as 20 feet in some places.
The displacement for the 1857 Ft. Tejon quake has been estimated at 30 feet in some places (Robert Wallace, Proceedings of the Conference on the San Andreas fault, 1965, pJ4).

san francisco bay area faults

30-year (2006-2036) Probabilities of M6.7 and larger Earthquakes in the San Francisco Bay Area
Fault Segment Probability
San Andreas 21%
Hayward/Rodgers Creek 31%
Calaveras 7%
Concord/Green Valley 3%
San Gregorio 6%
Greenville 3%
Mt. Diablo thurst 1%
Background 14%
Combined 63%
Source: Earthquake outlook for the San Francisco Bay region 2014-2043


Scientists Warn California Earthquakes Set To Trigger Catastrophic Mega-Quake, 2018 says,
"According to a map provided by USGS, 11 earthquakes ranging from 2.8 to 5.6 on the Richter scale all recently occured on the seabed of the Juan de Fuca tectonic plate, 6 miles below the surface of the Earth.""

"Don Blakeman, a geophysicist at the National Earthquake Information Center, says the plate's unusual motion is going to cause strain to build up "until the fault breaks and a few meters of Juan De Fuca slips under North America in a big earthquake."

Juan De Fuca plate stretches from Northern California to British Columbia and the Cascadia Subduction Zone stretches from N. Vancouver Island to Cape Mendicino, California. See map above.

If the entire 650-mile long Cascadia Subduction Zone (which includes the Juan de Fuca plate) were to experience a full rupture, it could not only trigger a 9.0 earthquake, but a tsunami as well.""

Earthquakes caused by the slipping of the Pacific Plate along the N. American Plate:
The Pacific Plate is moving North relative to the North American Plate
The primary boundary between these two plates is the San Andreas Fault. The San Andreas Fault is more than 650 miles long and extends to depths of at least 10 miles. Many other smaller faults like the Hayward branch from and join the San Andreas Fault Zone.

Parkfield, CA (Monterey County) lies about 1/4 mile from the fault. The area contains more seismometers than anywhere else in the world. Parkfield calls itself "The Earthquake Capitol of the World."
See Parkfield Area Seismic Observatory (PASO).

Past earthquakes: (Magnitude ≥ 6.2 and recent > 5)
1. Greater Bay Area, 2. Mendocino & Humbolt County, 3. Interior
1. Greater Bay Area Earthquakes - Santa Rosa to Parkfield (South of Monterey)
Date Where Magni-
Fault Deaths Property
Damage (M)
1817 San Rafael        
1836 East of Monterey Bay † 6.3      
1838 San Francisco area 6.8+ San Andreas NA  
1857 Ft. Tejon (near Parkfield - Central CA) 7.9 San Andreas 2  
1865 Santa Cruz Mountains 6.5     $0.500
1868 Hayward Fault 6.8 Hayward 30 $0.350
1897 Gilroy-San Felipe 6.3 Calavaras    
1898 Mare Island 6.3      
1901 Parkfield 6.4      
1906 San Francisco 7.8 ‡ San Andreas 3,000 $524
1955 Concord 5.4 Concord 1  
1957 Daly City 5.3 San Andreas 1  
1969 Santa Rosa 5.7 Rogers Creek 1  
1984 Morgan Hill (Santa Clara co.) 6.2     $8
1989 Loma Prieta, NE of Santa Cruz 7.1 San Andreas 1 67 $5,900
2000 Yountville (Napa Valley) 5.2      
2003 San Simeon 6.6   2  
2003 Muir Beach 3.4      
2007 Milpitas 5.6      
2. Mendocino and Humbolt Co. Earthquakes caused along the Pacific and Juan de Fuca plate systems. *
1898 Fort Bragg - Mendocino ~6.7      
1899 Eureka 7.0      
1922 Eureka 7.3      
1923 Off Cape Mendocino 7.2      
1932 Eureka 6.4   1  
1954 Eureka, Calif. 6.5   1 $2
1980 W. of Eureka (Humbolt Co) 7.4     $2
1991 NW of Eureka & Honeydew 7.1      
1992 Petrolia, Cape Mendocino area 7.2     $48
2003 Bayview 5.3      
2003 Humbolt Hill 5.3      
2005 Off the coast of Crescent City 7.2      
2005 Off the coast of N. Calif 6.6      
2008 Willow Creek 5.4      
3. Interior Northern and Central California Earthquakes from other fault systems.
1872 Owens Valley (Lone Pine) 7.4 Owens Valley 27 $0.250
1892 Vacaville 6.6   1 $0.225
1892 Winters 6.4      
1980 Mammoth Lakes 6.2     $1.5
1983 Coalinga (Fresno Co.) 6.4     $31
1966 Truckee 5.9      
See also Southern California Quakes
Tahoe faults

* Many of the earthquakes off the coast of Humbolt county are caused by the Juan de Fuca Plate system subducting under the North American plate and are about 100 miles out in the Pacific ocean, so damage is less than those in the more densely populated bay area.

1. The type of motion for the Loma Prieta quake was not typical of the San Andreas fault and suggests that the earthquake occurred on a sub-parallel fault and not on the San Andreas itself.

Current seismic hazard models include two major earthquakes (M ~ 7) in the San Francisco Bay area that are close in space and time: an 1836 event on the northern Hayward fault and an 1838 event on the peninsula section of the San Andreas fault. Analysis and interpretation of the available historical accounts indicate that the 1836 event occurred east of Monterey Bay, far from the Hayward fault, and was of M ~ 6 1/4.

‡ - The "traditional" magnitude of 8.3 for the San Francisco earthquake was based on work by Richter [1958]. More recent research indicates that estimates in the range from 7.7 to 7.9 are more reliable.

Source: Historic United States Earthquakes

Historic United States Earthquakes
Bay Area Earthquakes this year
California Earthquake History at
USGS Earthquake Hazards Program-N. Calif.
Other Maps
California Historical Earthquakes - Bay Area

The Richter magnitude scale, also known as the local magnitude (ML) scale, assigns a number computed on a base-10 logarithmic scale of the horizontal amplitude of the largest displacement, so a magintude 8 quake is 10 time more powerful than a magnitude 7 quake. Though still widely used, the Richter scale has been superseded by the moment magnitude scale, which gives generally similar values.

5.0-5.9 Moderate
6.0-6.9 Strong - Damage in an area 100 mi. across
7.0-7.9 Major  - Serious damage over a large area
8.0-8.9 Great  - Major damage for several hundred miles
9.0-9.9 Great  - Major damage for thousands of miles
See Earthquake Glossary

Future Predictions: The chance of having a magnitude 6.7 or larger earthquake in the greater Bay area during the next 30 years (by 2037) is 63%. The likelihood of an even more powerful quake of magnitude 7.5 or greater in the next 30 years is 15%.

Earthquake outlook for the San Francisco Bay region 2014–2043

Uniform California Earthquake Rupture Forecast (UCERF)
  Map of Earthquake Probabilities
  Map Comparing Earthquake Probabilities, Current to Long-Term
Bay Area Earthquake Probabilities
Hayward Fault: Bay Area Earthquake probabilities at

A 2006 study shows that the fault has been stressed to a level sufficient for the next "big one"--an earthquake of magnitude seven or greater--and the risk of a large earthquake in this region may be increasing faster than researchers had believed. See "New Scripps Study Reveals San Andreas Fault Set for the 'Big One'" (2006)

The vibrations are of two types: compression waves and transverse or shear waves. Since compression waves, called "P" waves, travel faster through the earth, they arrive first at a distant point. The transverse or shear waves, called "S" waves, travel more slowly and therefore arrive later.

When an earthquake occurs, people may first notice a sharp thud, or blast-like shock; this marks the arrival of the P wave. A few seconds later, they may feel a swaying or rolling motion that marks the arrival of the S waves.
See - California Earthquake History 1769-Present
North Central California Faults

1906 San Francisco Earthquake:
The quake came at 5:12 in the morning. Strong shaking lasted for about 60 seconds. Land surfaces on either side of the fault were displaced horizontally up to 21 feet.

Figures at the time put the death toll at 700 to 800 people, although later estimates put the figure closer to 3000 deaths. Where the earth actually split open, in what is today Point Reyes National Seashore, there was only one casualty - a cow named Matilda - who, according to the official earthquake commission report at the time, "fell headfirst into the fault crack just before the earth closed in on her, so that only her tail remained visible."

The 1906 San Francisco earthquake destroyed more than 28,000 buildings. Looking north along Valencia Street are a sink hole (foreground) and the sunken Valencia Street Hotel (left). (Photo courtesy of the Bancroft Library, University of California, Berkeley.)

Creation of the San Andreas Fault:
65 Million years ago then North America was bordered on its west side by the subducting Farallon plate. Sometime between then and 40 million years ago the Farallon plate had entirely subducted under the North American plate, which was now overrunning a seafloor spreading center. The plate boundary began turning into a transform boundary separating the Pacific and the North American plates. Over the next 35-40 million years, this transform plate boundary developed into the San Andreas Fault System (SAFS).
See The tectonic initiation of the San Andreas transform system

The relative rate of motion between the North American plate and the Pacific plate is approximately 3.5 to 4.6 cm (1.4 - 2.4 in) per year (about the same rate that your fingernails grow), most of which (2.0 to 3.5 cm per year) is accounted for by horizontal displacement along the San Andreas fault zone. The remainder is expressed by displacement along other, subparallel faults such as the Imperial and the San Jacinto fault zones in southern California.
In about 10-15 Million years Los Angeles will be suburb of San Francisco.
See: San Andreas Transform Fault Zone, 2008

Southern California Earthquakes:
In 1971 65 people died in a 6.6 earthquake in the San Fernando Vly.
51 people died in the Northridge earthquake near Los Angeles in 1994.
Since the last large earthquake in the southern region occurred in 1857, that section of the fault is considered a likely location for a major earthquake in the near future.

San Francisco Bay Area vs Greater Los Angeles Area
What Bay Area LA area
1900-49 1950-2008 1900-49 1950-2008
Mag 6-6.9 5 3 4 4
Mag ≥7 1 1 2 2
Fatalities 3000 72 137 158
Probability of ≥6.7
in next 30 yrs
63% 67%
See Southern California Quakes.

Volcanoes caused by Juan de Fuca Plate subduction:
The Juan de Fuca Plate is moving East and is subducting under the northerly portion of the western side of the North American Plate at the Cascadia subduction zone.

The Cascade Volcanic Arc is a continental island arc that extends from northern California to the coastal mountains of British Columbia. The arc consists of a series of Quaternary (Pleistocene to Holocene) age stratovolcanoes that grew on top of pre-existing geologic materials that ranged from Miocene volcanics to glacial ice.
The major peaks from south to north include:

  • Lassen Peak (1914-17)*, Mt. Shasta and Medicine Lake (1000) (California)
  • Three Sisters, Mt. Jefferson, Crater Lake (Mazama), Mt. Hood (Oregon) Mt. Adams, St. Helens (1980)*, Mt. Rainier, Glacier Peak, Mt. Baker (Washington)
  • Garibaldi and Mt. Meagher (British Columbia)
* Have erupted since 1900.
See: Cascade Range Volcanoes Compared The subduction also causes earthquakes.
See Earthquakes in Washington.

Earthquakes that occur offshore west of Eureka in the Cascadia Subduction Zone can also cause tsunamis. In 1700 an earthquake offshore of northern California, Oregon, and Washington caused 30 foot high waves rushing inland and damage throughout the Pacific all the way to Japan.

What do do during an Earthquake :

  • If you're indoors, stay there. Get under -- and hold onto --a desk or table, or stand against an interior wall. Stay clear of exterior walls, glass, heavy furniture, fireplaces and appliances. The kitchen is a particularly dangerous spot. If you're in an office building, stay away from windows and outside walls and do not use the elevator.
  • If you're outside, get into the open. Stay clear of buildings, power lines or anything else that could fall on you.
  • If you're driving, move the car out of traffic and stop. Avoid parking under or on bridges or overpasses. Try to get clear of trees, light posts, signs and power lines. When you resume driving, watch out for road hazards.
  • If you're in a mountainous area, beware of the potential for landslides.
    Likewise, if you're near the ocean, be aware that tsunamis are associated with large earthquakes. Get to high ground.
  • If you're in a crowded public place, avoid panicking and do not rush for the exit. Stay low and cover your head and neck with your hands and arms.
Putting Down Roots in Earthquake Country - Your Handbook for the San Francisco Bay Region

Other Maps
Earthquake Glossary
What to do in an Earthquake (preparation, during, after)
UC Berkeley Seismological Laboratory
  - Hayward Fault: Bay Area Earthquake probabilities at
USGS Earthquake Hazards Program-Northern California
  - Geologic History of the San Andreas Fault System at the USGS,org
  - Tectonic Summary CENTRAL CALIFORNIA
  - Bay Area Earthquake Probabilities at
  - California Earthquake History 1769-Present
  - Introduction San Andreas Fault: An Overview (pdf)
Uniform California Earthquake Rupture Forecast (UCERF) Southern California Earthquake Center (SCEC)
  Map of Earthquake Probabilities
San Andreas Transform Fault Zone
Historical California Earthquakes - Bay Area
Juan de Fuca Plate at Wikipedia
Undersatnding Quakes at
Hayward fault at Jim Wanket Research Projects at Sacramento State U.
About the Hayward Fault of California at
1906 San Francisco Earthquake at
Re-evaluation of the 1836 "Hayward Fault" and the 1838 San Andreas Fault Earthquakes - Bulletin of the Seismological Society of America
Mount Hood--History and Hazards of Oregon's Most Recently Active Volcano
Subduction diagrams at
A Crack Spreads Across California - 25 Million Years Ago
San Andreas Transform Fault Zone
Long-Term Global Forecast? Fewer Continents
Measuring Earthquakes at USGS
Worldwide Historical Earthquakes - here - USGS
Preparing for the unpredictable: Megaquake, tsunami may forever alter the Pacific Northwest in next 50 years -

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last updated 6 July 2009