What is an earthquake and what causes them to happen?
An earthquake is caused by a sudden slip on a fault. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. When the stress
on the edge overcomes the friction, there is an earthquake that
releases energy in waves that travel through the earth's crust and
cause the shaking that we feel.
In California there are two plates - the Pacific Plate and the
North American Plate. The Pacific Plate consists of most of the Pacific
Ocean floor and the California Coast line. The North American Plate
comprises most the North American Continent and parts of the Atlantic
Ocean floor. 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 (Northern California) and the San Jacinto (Southern
California) branch from and join the San Andreas Fault Zone. The
Pacific Plate grinds northwestward past the North American Plate at a
rate of about two inches per year.
Parts of the San Andreas Fault system adapt to this movement by constant "creep"
resulting in many tiny shocks and a few moderate earth tremors. In
other areas where creep is NOT constant, strain can build up for
hundreds of years, producing great EQs when it finally releases.
To see locations of current major earthquakes, go the Hazards Viewer and click on one of the Earth globes on the right hand side.
Poll
1. | Tokyo | 68 |
2. | Los Angeles | 25 |
3. | Manila | 18 |
4. | Bandung | 17 |
5. | Santiago | 16 |
6. | Nagoya | 9.4 |
7. | Osaka | 9.4 |
8. | Jakarta | 9 |
9. | Lima | 8 |
10. | Chengdu | 7.5 |