Earthquake Basics

What are earthquakes? An earthquake is caused by sudden slip on a fault. Stresses build on a fault (usually due to the relative motions between tectonic plates) but a fault remains ‘stuck’ or motionless until the stress level exceeds the strength of the fault. Stresses may build for hundreds or thousands of years. Once the strength is exceeded, the fault slips suddenly, releasing energy in waves that travel through the rock to cause the shaking that we feel during an earthquake. The slippage in an earthquake occurs within a fraction to several hundred seconds.  Some faults relieve stresses by slipping very slowly, or by 'creep' lasting hours to weeks. Sudden displacements are more damaging to structures because they produce seismic waves, which cause shaking at large distances from the fault.  Creeping faults do not send out seismic waves.

What is a fault? A fault is a thin zone of weakness between two larger blocks of rock. It can be any length, from centimeters to hundreds of kilometers. It is a fracture in the Earth where rocks on one side may move relative to those on the other side. When an earthquake occurs, the rock on one side of the fault slips with respect to the other. The fault surface can be vertical, horizontal, or at some angle to the surface of the Earth. The slip direction can also be at any angle. Because of this, three different types of fault motion can occur (or some combination of them):

• Normal, dip-slip fault. The fault plane dips and the upper block slides downward. Faulting occurs in response to stretching, or tensional stresses. 
• Reverse, dip-slip fault. The fault plane dips and the upper block slides upward.  Faulting occurs in response to squeezing, or compressional stresses. 
• Strike-slip fault. Crustal blocks slide past each other horizontally, usually on an approximately vertical fault plane. 

Will I always see the fault where the earthquake occurred or could occur? Not all earthquakes result in faulting that cuts the Earth's surface.  Some faults are many tens or hundreds of miles/kilometers beneath the surface.  Earthquakes almost always occur on preexisting faults, which are zones of weakness.

Can earthquakes be prevented? No, but their impacts can be mitigated. With proper planning now, you can avoid damage to your home, business and yourself when an earthquake strikes. To learn more about protecting your home or business from an earthquake, please see the Reducing Impacts from Earthquakes section of our site.

How are earthquakes measured? Earthquakes are measured in a variety of ways.  Earthquakes are commonly detected and measured using networks of seismographs, particularly for earthquakes that are too small or deep to see faulting at the surface. The rapid slip of one block of rock over another in an earthquake releases energy in the form of seismic waves. Each seismic station in the network measures the movement of the ground at the site as the seismic waves pass by. Networks are comprised of groups of stations from which recordings are analyzed together, to measure various attributes of the earthquake.

The instruments used to record the motion of the ground during earthquakes are called seismographs. First developed during the 1890′s, these instruments are installed in the ground throughout the world. At the heart of the seismograph is a seismometer, which senses the motion of the ground beneath it.  Mechanical seismometers often consist of a pendulum or a mass mounted on a spring, and which moves when seismic waves shake the ground beneath them. 

 Two mass-and-spring seismometers (model Geotech S-13) that measure weak to moderate amplitude motions with rapid oscillation periods (a few tenths of seconds to tens of seconds).  The smaller one is more sensitive to more rapid motions.  Picture from http://www.pnsn.org/EDHOME/SENSORS/sp_desc.html

 A recording of the ground motion sensed by a seismograph is called a seismogram (the squiggly lines that almost everyone has seen). Here is a seismogram of the waves from the January 17,1994 earthquake in Northridge, CA and one of its aftershocks. Seismograms show how the ground moves with the passage of time. The horizontal axis shows time (measured in seconds) and the vertical axis shows ground motion, usually measured by its velocity (or sometimes displacement or acceleration). When there is no earthquake, the seismogram usually shows a straight line except for small wiggles caused by local disturbances or “noise.”

The size of an earthquake typically is measured using a metric called ‘magnitude’.  Magnitude measures the energy radiated in the seismic waves (bigger earthquakes send out bigger waves).  There are several types of magnitude scales; one type is the Richter Magnitude Scale.  The impact or effects of an earthquake are measured using the Modified Mercalli Intensity Scale; intensities are not a measure of earthquake size but rather the impact of the earthquake.

For more background information about earthquakes

• Earthquake Frequently Asked Questions  - http://earthquake.usgs.gov/learn/faq/?categoryID=2&faqID=118/index.html
• Glossary of common earthquake terms -- http://earthquake.usgs.gov/learn/glossary/
• U.S. Geological Survey Earthquake Hazards Program - http://earthquake.usgs.gov/
• The Geological Survey of Canada - http://earthquakescanada.nrcan.gc.ca/index-eng.php
• Cool visuals, presentation materials about specific earthquakes, seismic data: http://www.iris.edu/hq/
• More cool visuals, presentation materials about specific earthquakes, GPS and other data measuring slow deformation: http://www.unavco.org/
• NOAA Pacific Marine Environmental Lab - http://nctr.pmel.noaa.gov/
• U.S. Geological Survey Cascades Volcano Observatory- http://vulcan.wr.usgs.gov/