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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Do earthquakes occur in Antarctica?

Earthquakes do occur occasionally in Antarctica, but not very often. There have been some big earthquakes--including one magnitude 8--in the Balleny Islands. The boundary between the Scotia Plate and the Antarctic Plate just grazes the north tip of the Antarctic Peninsula (again, look "northwest" from the Pole toward South America). There is also...

Where can I find earthquake educational materials?

Learn more: Earthquakes for Kids Topics for Earthquake Education

Can we cause earthquakes? Is there any way to prevent earthquakes?

Earthquakes induced by human activity have been documented at many locations in the United States and in many other countries around the world. Earthquakes can be induced by a wide range of causes including impoundment of reservoirs, surface and underground mining, withdrawal of fluids and gas from the subsurface, and injection of fluids into...

What is surface faulting or surface rupture in an earthquake?

Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. NOT ALL earthquakes result in surface rupture.

At what depth do earthquakes occur? What is the significance of the depth?

Earthquakes occur in the crust or upper mantle , which ranges from the earth's surface to about 800 kilometers deep (about 500 miles). The strength of shaking from an earthquake diminishes with increasing distance from the earthquake's source, so the strength of shaking at the surface from an earthquake that occurs at 500km deep is considerably...

Why are there so many earthquakes in the Geysers area in Northern California?

The Geysers Geothermal Field is located in a tectonically active region of Northern California. The major seismic hazards in the region are from large earthquakes occurring along regional faults that are located miles away from the geothermal field, such as the San Andreas and Healdsburg-Rodgers Creek faults. However, activities associated with...

Foreshocks, aftershocks - what's the difference?

"Foreshock" and "aftershock" are relative terms. Foreshocks are earthquakes that precede larger earthquakes in the same location. An earthquake cannot be identified as a foreshock until after a larger earthquake in the same area occurs. Aftershocks are smaller earthquakes that occur in the same general area during the days to years following a...

Can the position of the moon or the planets affect seismicity? Are there more earthquakes in the morning/in the evening/at a certain time of the month?

Earthquakes are equally as likely to occur in the morning or the evening. Many studies in the past have shown no significant correlations between the rate of earthquake occurrence and the semi-diurnal tides when using large earthquake catalogs. Several recent studies, however, have found a correlation between earth tides (caused by the position of...
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Date published: March 1, 2013

"Living in Earthquake Country: Los Angeles and the Big One" - Dr. Lucy Jones presentation at LA Natural History Museum

"Living in Earthquake Country: Los Angeles and the Big One" - Dr. Lucy Jones presentation at LA Natural History Museum

Date published: April 8, 2011

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.   

Date published: February 10, 2005

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. 

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Map showing the Gulf of Alaska and epicenter of M7.9 earthquake
January 23, 2018

Gulf of Alaska M7.9 Earthquake

USGS map of the Gulf of Alaska earthquake

Building in Mexico City after Sept. 19, 2017 earthquake
October 3, 2017

Building in Mexico City after Sept. 19, 2017 earthquake

Building in Mexico City after Sept. 19, 2017 earthquake 

building damaged by an earthquake
December 31, 2011

Earthquake-damaged building

Building damanged by the February 2011 earthquake in Christchurch, New Zealand.

Attribution: Natural Hazards
Image shows an aerial view of the San Andreas Fault
November 30, 2000

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 illustrating tectonic convergent boundary and subduction of two oceanic plates

Block diagram of a subduction zone.

Block diagram of a subduction zone when two oceanic plates converge.

block diagram illustrating strike-slip fault

Block diagram of a Strike-Slip fault.

Block diagram of a Strike-Slip fault.

Interactive Quarternary Fault Database

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).