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 earthquakes when it finally releases.
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"Living in Earthquake Country: Los Angeles and the Big One" - Dr. Lucy Jones presentation at LA Natural History Museum
California Can Learn From Recent Large Earthquakes
A new report issued by the American Red Cross and the U.S. Geological Survey documents the Chilean response and recovery efforts following the Feb. 2010 magnitude 8.8 earthquake and the lessons that California should learn from this disaster.
2004 Deadliest in Nearly 500 Years for Earthquakes
According to the U.S. Geological Survey (USGS), 2004 was the deadliest year for earthquakes since the Renaissance Age, making it the second most fatal in recorded history, with more than 275,950 deaths reported from the magnitude 9.0 earthquake and subsequent tsunami that hit the Indian Ocean on Dec. 26.
Building in Mexico City after Sept. 19, 2017 earthquake
Building in Mexico City after Sept. 19, 2017 earthquake
Earthquake-damaged building
Building damanged by the February 2011 earthquake in Christchurch, New Zealand.
San-Andreas Fault
Aerial photo of the San Andreas Fault in the Carrizo Plain. By Ikluft - Own work, GFDL, https://commons.wikimedia.org/w/index.php?curid=3106006
Block diagram of a subduction zone.
Block diagram of a subduction zone when two oceanic plates converge.
Interactive Quarternary Fault Database
This database contains information on faults and associated folds in the United States that demonstrate geological evidence of coseismic surface deformation in large earthquakes during the Quaternary (the past 1.6 million years).