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Catastrophic San Francisco Earthquake: A detectable earthquake occurs every 30 seconds. Fortunately, severe and damaging earthquakes are far less frequent. Since 1897 there has been an average of 16 severe earthquakes a year; that is, earthquakes with a rating.
The year 1967 was unusual in that only six major earthquakes occurred, and up until May 1968 there had been no earthquakes of intensity greater than 8 on the Richter scale since the catastrophic Alaskan earthquake of March 27, 1964, which caused the deaths of 121 people.
S. Breiner and R. Kovach of Stanford University reported in October 1967 that during the previous two years local changes in the earth's magnetic field occurred some tens of hours before the beginning of abrupt creeping movements on the San Andreas fault at Hollister, Calif. The San Andreas fault is a zone of fractures and rifts stretching for about 800 mi (1,288 km) in a southeasterly direction from Cape Mendocino, Calif. A sudden movement of only a few feet along one of the fractures in this fault belt was responsible for the catastrophic San Francisco earthquake of 1906.
Whether a catastrophic event—a great earthquake, or the impact of a meteorite, or natural processes inside the earth's core—causes the reversal remains a mystery.
At the same time that evidence for magnetic reversal was accumulating, the theory that the sea Floor was spreading began to have currency among geophysicists. According to this theory, lava issues forth from mid-oceanic fractures, creating new crust on the sea floor. Repeated opening of the fractures must therefore produce spreading. |
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