Earthquakes 101

The Cascadia Region, from northwestern California to British Columbia, has been struck by earthquakes and related hazards in the past. Such events are likely to happen again. The following FAQ will introduce you to earthquakes and provide links to more information about these events and how to plan for them.

What are earthquakes? An earthquake is caused by a sudden slip on a fault, which is a thin zone of weakness between two larger blocks of rock. As stress builds on a fault (usually due to the relative motions between tectonic plates), the fault will remain stuck or motionless until the stress level exceeds the strength of the fault. Stresses may build for hundreds or thousands of years. Once the stress level exceeds the strength of the fault, the fault slips suddenly and releases energy in waves that travel through the rock to cause the shaking that we feel during an earthquake. Learn more about where earthquakes come from. 

What is a fault? A fault is a fracture in the Earth where rocks on one side may move relative to those on the other side. It can be a few centimeters to hundreds of kilometers in length. An earthquake occurs when the rock on one side of the fault slips. 

What happens after an earthquake? Earthquakes cause a variety of earthquake hazards, including ground shaking, landslides, liquefaction (when soil liquefies during shaking) and tsunami.  Learn more about earthquake hazards.

When will the next earthquake occur?  If you live in Cascadia, you will probably experience a serious, damaging earthquake in your lifetime. For example, scientists estimate that within 50 years, there is an 84% chance of a magnitude 6.5 (M6.5) or higher earthquake in the Puget Sound region. The odds are less for Oregon and northern California, but still significant. Learn more about the different kinds of earthquakes.

Can earthquakes be prevented? No, but we can lessen the damage they cause. With proper planning, you can avoid injury and damage to your home or business when an earthquake strikes. To learn more about protecting your home or business from an earthquake, please see What You Can Do.

How are earthquakes measured? The rapid slip of one block of rock over another in an earthquake releases energy in the form of seismic waves. Seismic waves are detected and measured using networks of seismographs. Seismographs are devices installed in the ground throughout the world that can sense the motion of the ground beneath them. 

A recording of the ground motion sensed by a seismograph is called a seismogram (the squiggly lines that almost everyone has seen).

What is magnitude? 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). For example, the Northridge, Calif. earthquake  (1994) was a magnitude 6.7 earthquake. On the magnitude scale, each whole number increase represents an earthquake ten times greater in size. A M5 event will rattle people but probably create mostly minor damage. A M7, which would be 100 times greater than a M5 event, could cause deaths and devastate cities. Earthquakes are usually felt if they are at least magnitude 3 to 4. Learn more about how magnitude feels during an earthquake using the Modified Mercalli Intensity Scale.

Will I always see the fault where an 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 kilometers beneath the surface. Earthquakes almost always occur on pre-existing faults.

How can I reduce the impact of an earthquake? For help getting started and other online resources, please see What You Can Do.

Where can I learn more about earthquakes and related hazards?

Please see More Earthquake Resources and Links.