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).
30-year (2006-2036) Probabilities of M6.7 and larger Earthquakes
in the San Francisco Bay Area
|Mt. Diablo thurst
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)
See also Southern California Quakes
1. Greater Bay Area, 2. Mendocino & Humbolt County, 3. Interior
* 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 . More recent research indicates that estimates in the range from 7.7 to 7.9 are more reliable.
Source: Historic United States Earthquakes
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%.
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 berkeley.edu
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.
pasadena.wr.usgs.gov/images/cahist_eqs.gif - California Earthquake History 1769-Present
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
A CRACK SPREADS ACROSS CALIFORNIA - 25 MILLION YEARS AGO
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
See Southern California Quakes.
|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
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:
* Have erupted since 1900.
- 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)
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 :
Putting Down Roots in Earthquake Country - Your Handbook for the San Francisco Bay Region
- 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.
What to do in an Earthquake (preparation, during, after)
UC Berkeley Seismological Laboratory
- Hayward Fault: Bay Area Earthquake probabilities at berkeley.edu
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 USGS.org
- 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 WhyFiles.org
Hayward fault at Jim Wanket Research Projects at Sacramento State U.
About the Hayward Fault of California at about.com
1906 San Francisco Earthquake at NASA.gov
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 AstronomyNotes.com
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 - AccuWeather.com
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last updated 6 July 2009