Ask A Scientist General Science Archive

Question:  Has there been any predicrtion made on how long it will take
for California to break off from the continent?  I just heard that
approx. every 20 years a mag. 6 quake comes along.  These are pretty small
compared to "the" quake that is predicted.  What about the east coast?
I've been hearing for a long time that we are also in for it.
 wildman jackson

Answer:
I hope this won't disappoint you, but California is not going
to break off from the North American continent.  It is true that
California is cut in half by the San Andreas Fault that runs from
the Salton Sea in the Mojave desert, bends north around Los Angeles,
passes just west of San Francisco, and finally extends
out into the Pacific Ocean where it ends off of the coast near Mendocino.
 
The San Andreas Fault is a boundary between two plates of the earth's crust.
The earth's crust is split up into a number of plates that "float"
on a semi-molten layer beneath them.  The San Andreas Fault
is the major fault separating the Pacific and North American
Plates.  The part of California to the west of the San Andreas
Fault is part of the Pacific Plate--this includes, for example,
San Diego, Los Angeles, and Santa Barbara.
California to the east of the San Andreas Fault is part of
the North American Plate.  These plates are moving horizontally past each
other along the San Andreas Fault at a rate of about 3.5 centimeters/year.
Thus, Los Angeles is moving north with respect to San Francisco.  At the
present rate--in another fifteen million years--Los Angeles and
San Francisco will lie next to each other.

As for the frequency of magnitude six earthquakes, it depends what area
you are talking about.  If you are referring only to the Los Angeles
Basin, one magnitude six or larger earthquake every twenty years
seems about right. For all of California, I can think of seven
magnitude six or larger earthquakes that have occurred just in
the last decade.  In the written history of California (which only
extends back about 200 years), there have been three
great earthquakes--magnitude eight or larger shakers.  These are
the 1857 Central California earthquake, the 1872 Lone Pine
earthquake, and the 1906 San Francisco earthquake.  The 1857 and
1906 earthquakes occurred along the San Andreas Fault; the 1872
earthquake occurred on the Sierra Nevada Fault at the eastern
end of the Sierra Nevada mountain range.
 
-Grant
 
P.S.  For eastern United States earthquakes, see the response to no. 142.