Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
The subduction of the Farallon Plate under the North American Plate has caused several volcanoes along the coastline in the Cascade Mountain Range. By viewing wear the Juan de Fuca Plate is passing under the North American Plate (pictured above) you can see why there are so many volcanoes in the Cascade Mountains (pictured below).
Now a small part of California rests on a portion of the Pacific Plate that is above sea level. As you can see from the diagram below that the Pacific Plate surfaces just around San Francisco and goes down to the Gulf of Baja California. The San Andreas fault, the borders wear the Pacific Plate and the North American Plate meet, is wear a majority of seismic activity (earthquakes) take place in California. The area between the plates sliding horizontally past one another is called a transform-fault boundary. Because the two plates are moving past one another (as shown in the diagram below) it creates earthquakes.
Now the San Andreas fault zone, which is about 1,300 km long and in places tens of kilometers wide, slices through two thirds of the length of California. The Pacific Plate has been grinding horizontally past the North American Plate for 10 million years at about 4 inches a year. Land on the west side of the fault zone, on the Pacific Plat, is moving in a northwesterly direction relative to the land on the east side of the fault zone which is moving in a southeasterly direction, on the North American Plate. Now some of these earthquakes caused by the movement by these two plates will cause visual cracks on the surface called surface ruptures. The article below explains what a surface rupture is in more detail.
What is "surface rupture" in an earthquake?
Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. Surface ruptures associated with the 1992 Landers Earthquake, in San Bernardino County, extended for 50 miles with displacements of an inch to 20 feet. Not all earthquakes result in surface rupture. The Loma Prieta Earthquake of 1989 caused major damage in the San Francisco Bay Area but the movement deep in the earth did not break through to the surface. Fault rupture almost always follows preexisting faults, which are zones of weakness.
Now that we know that it is possible to see the earth move during an earthquake I asked the U.S.G.S. if the huge fissure type openings that were possible.
A popular cinematic and literary device is a fault that opens during an earthquake to swallow up an inconvenient character. But unfortunately for principled writers, gaping faults exist only in movies and novels. The ground moves parallel to a fault during an earthquake, not away from it. If the fault could open, there would be no friction. Without friction, there would be no earthquake. Shallow crevasses can form during earthquake induced landslides, lateral spreads, or other types of ground failures. Faults, however, do not gape open during an earthquake.
Thank you for taking the time to read the research that I took the time to put together. I hope that after reading this article you now understand why we have mountains, volcanoes, and earthquakes in California and that although parts of it are moving it is not falling into the ocean. Now I will leave you with this. While doing my research I came across something you east coast and Midwesterners should be worried about.
Survivors reported that the
earthquakes caused cracks to open in the earth's surface, the ground to
roll in visible waves, and large areas of land to sink or rise. The crew
of the New Orleans (the first steamboat on the Mississippi, which was on
her maiden voyage) reported mooring to an island only to awake in the
morning and find that the island had disappeared below the waters of the
Mississippi River. Damage was reported as far away as Charleston, South
Carolina, and Washington, D.C.
These dramatic accounts
clearly show that destructive earthquakes do not happen only in the
western United States. In the past 20 years, scientists have learned
that strong earthquakes in the central Mississippi Valley are not freak
events but have occurred repeatedly in the geologic past. The area of
major earthquake activity also has frequent minor shocks and is known as
the New Madrid seismic zone.
Earthquakes in the central or eastern United States affect much larger areas than earthquakes of similar magnitude in the western United States. For example, the San Francisco, California, earthquake of 1906 (magnitude 7.8) was felt 350 miles away in the middle of Nevada, whereas the New Madrid earthquake of December 1811 (magnitude 8.0) rang church bells in Boston, Massachusetts, 1,000 miles away. Differences in geology east and west of the Rocky Mountains cause this strong contrast.