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What causes earthquakes and where do they occur? The earth is divided into three main layers – a hard outer crust, a soft middle layer and a center core. The outer crust is broken into massive, irregular pieces called “plates.” The plates have been moving very slowly for billions of years, driven by energy deep within the earth. The movement has shaped mountains, valleys, plains and plateaus. Earthquakes occur when these moving plates grind and scrape against each other.
In California, two of the plates meet – the Pacific Plate and the North American Plate. The primary boundary between them is the San Andreas Fault. It is more than 650 miles long and extends 10 miles deep. Many smaller faults branch out from the San Andreas Fault. Experts suspect there are many faults that have not yet been discovered.
The Pacific Plate moves northwestward, past the North American Plate, at a rate of about two inches per year. Some areas along a fault adapt to movement with a constant “creep,” resulting in frequent, moderate earth tremors. In other areas, however, movement and strain can build up for hundreds of years, resulting in strong earthquakes when released.
The damage caused by earthquakes depends on their intensity and proximity to population centers. An earthquake of less than three degrees on the Richter sale can cause cracks and broken windows. But an earthquake of 6 can cause landslides and severe structural damage, like the legendary San Francisco earthquake of 1906. It was between 7.7 and 7.9 on the Richter scale and caused 3,000 deaths and destroyed at least half of San Francisco.
Because there is no way to predict an earthquake, it is best to learn how to protect your family and home in case of an earthquake.
A message from the University of California Cooperative Extension and the California Office of Emergency Services |