|Don's Home Places California Bay Area Fault Zones - Northern
The Pacific Plate grinds northwestward past the North American Plate at an average rate of about 2 inches per year [17 ft per century].
Scientists Warn California Earthquakes Set To Trigger Catastrophic Mega-Quake, Planet-Today.com 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."
Past earthquakes: (Magnitude ≥ 6.2 and recent > 5)
* 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.
Historic United States Earthquakes
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.
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%.
Uniform California Earthquake Rupture Forecast (UCERF)
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.
1906 San Francisco Earthquake:
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:
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.
Southern California Earthquakes:
San Francisco Bay Area vs Greater Los Angeles Area
Volcanoes caused by Juan de Fuca Plate subduction:
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.
See: Cascade Range Volcanoes Compared The subduction also causes earthquakes.
See Earthquakes in Washington.
What do do during an Earthquake :
Return to California