A: There are many faults in the Pacific Northwest that can produce damaging earthquakes, including hard-to-identify faults that exist entirely underground and have not been identified at the earth's surface. At the same time, some mapped faults have been located that have not generated earthquakes in recent geologic time. New faults continue to be discovered as more field observations and earthquake data are collected.

There are three different sources for damaging earthquakes in the Pacific Northwest. The first of these is the "Cascadia Subduction Zone", a 1000 km long thrust fault which is the convergent boundary between the Juan de Fuca and North American plates and is the most extensive fault in the Pacific Northwest area. It surfaces about 50 miles offshore along the coasts of British Columbia, Washington, Oregon and northern California. No historic earthquakes have been directly recorded from this source zone. According to recent research, an earthquake estimated to be as large as 8.0 to 9.0 occurred in this zone in January of 1700.

The second source for damaging earthquakes is the Benioff Zone. This zone is the continuation of the extensive faulting that results as the subducting plate is forced into the upper mantle. The Benioff Zone can probably produce earthquakes with magnitudes as large as 7.5. Benioff Zone earthquakes are deeper than 30km.

The third source consists of shallow crustal earthquake activity (depths of 0 to 20 km) within the North American continental plate where faulting is extensive. Past earthquakes have revealed many shallow fault structures, including the Western Rainier Seismic Zone and the Mt. St. Helens Seismic Zone. Our best known crustal fault, the Seattle Fault, runs east-west through Seattle from Issaquah to Bremerton. This fault generated a very large earthquake approximately 1100 years ago. Other crustal faults have been located in the Puget Basin region. Click Here for a map of major faults located in the Puget Basin.

USGS Web page on Earthquake History of Washington - to 1973
USGS Web page on Earthquake History of Oregon - to 1973

A: Yes. Some of these are well known from geologic or geophysical surveys. Examples include the Seattle Fault and the Portland Hills Fault. How often earthquakes occur on these faults is not well known, but they are believed to have the potential to produce damaging earthquakes.

A: We are located at a convergent continental boundary, where two tectonic plates are colliding. This boundary is called the Cascadia Subduction Zone. It lies offshore and runs from British Columbia to northern California. The two plates are converging at a rate of about M 3-4 cm/year (1-2 inches/year), and the northeast-moving Juan de Fuca Plate is pushing into North America, causing stress to accumulate. Earthquakes are caused by the abrupt release of this slowly accumulated stress.

A: Typically, each year we locate over 1000 earthquakes with magnitude 1.0 or greater in Washington and Oregon. Of these, approximately two dozen are large enough to be felt. These felt events offer us a subtle reminder that the Pacific Northwest is an earthquake-prone region. As residents of the Pacific Northwest, we should be prepared for the consequences of larger earthquakes that could result in damage to the transportation systems and lifelines. There have been about 25 damaging earthquakes in Washington and Oregon since 1872. In the 20th century, about 17 people lost their lives due to earthquakes in the Pacific Northwest.

A: In the 20th century, there were eleven earthquakes of magnitude 5 or greater that have occurred near Puget Sound: in 1904 (M 5.3), 1909 (M 6.0), 1932 (M 5.2), 1939 (M 6.2), 1945 (M5.9) 1946 (M 6.4), 1949 (M 7.0), 1965 (M 6.5), 1995 (M 5.0), 1996 (M 5.3), and 1999 (M 5.1). Most of the events are associated with deep Benioff zone earthquake activity that effects the Pacific Northwest Region. The 1995 and 1996 events were shallow crustal events. Click Here for a drawing of the three different source zones for earthquake activity in the Pacific Northwest.

A: In this century there have been three significant earthquakes near Portland: in 1877 (M 5.3), 1962 (M 5.5), and 1993 (M 5.5). Additionally, Portland has been damaged by earthquakes that occurred in the Puget Sound region, such as the 1949 magnitude 7.1 event near Olympia, WA, and the 1965 magnitude 6.5 event located between Seattle and Tacoma.

A: Historical data and theory suggests that earthquakes only provoke other shocks within a limited area around the fault rupture. Distant earthquakes have no direct effect on Washington and Oregon. Earthquakes in California, Alaska and Japan are caused by the interaction of different plates than the earthquakes in the Pacific Northwest. However, the 1992 Landers earthquake in southern California caused an increase in tiny earthquakes in geothermal areas as far away as The Geysers in northern California.

A: Yes, even modern bridges have sustained damage during earthquakes, leaving them unsafe for use. More rarely, some bridges have failed completely due to strong ground motion. Several collapsed in the Northridge earthquake in January 1994, even though they had been strengthened. The January, 1995 Kobe, Japan earthquake also caused many bridges to fail. It is important to note that both of these earthquakes produced accelerations far exceeding the design criteria used in the design of the failed structures. Because the bridges in our urban areas vary in their size, materials, siting, and design, they will be affected differently by any given earthquake. Major bridge design improvements occurred in the 1970's. Bridges built before the mid 1970's have a significantly higher risk of suffering structural damage during a moderate to large earthquake compared with those built after 1980. The 1970's was a decade of evolution for bridge design, so bridges built during this time may or may not have these improvements. Much of the interstate highway system in the Pacific Northwest has been built in the mid to late 1960's. The Washington State Department of Transportation should be consulted for further information about the seismic resistance of individual structures maintained by the state. Many other bridges are under other jurisdictions, but most have been evaluated.

A: That is an individual decision, which depends on the risk that homeowners are financially willing to take. It also depends on their confidence in the quality of their homes, since there is quite a large deductible on most policies. Commonly the policies only pay for damage exceeding 5 to 10% of the value of a house. Some seismologists do have earthquake insurance.

A: Although scientists have long tried to predict earthquakes, no reliable method has been discovered. Seismicity in the Pacific Northwest has only been extensively studied for a couple of decades, and seismologists are still trying to understand the frequency and hazards of earthquakes in our region. Click Here for a more in depth discussion on earthquake prediction.

A: There is no Seattle area neighborhood that is immune from possible earthquake damage. The age of the structure and the type of geology in the area are two factors that will affect the vulnerability to earthquakes. There are ways to perform a seismic retrofit on older homes. The Project Impact web page has information on home retrofits. Another valuable resource is the Cascadia Regional Earthquake Workgroup (CREW). The American Red Cross has a variety of earthquake preparedness publications. We encourage you to visit the following page for more information about earthquake-related risks in the Seattle area:

A: Mt. Saint Helens is located on the St. Helens Fault Zone (SHZ). This is a strike-slip fault. Right at Mount St. Helens there is a gap and a step in the SHZ. This step causes the crust to pull apart inside the gap, creating a zone of weakness where volcanic material can more easily reach the surface. It will help you to understand this if you draw some pictures of a step in a strike-slip fault, with arrows to show the direction ov movement. Many volcanos are found in similar circumstances. The St. Helens Fault Zone was not discovered until after the eruption of Mt. St. Helens (1980). In 1981 a magnitude 5+ earthquake on the SHZ had thousands of aftershocks which "lit up" the fault.

A: The best site for information about volcanoes are at the Cascade Volcanic Observatory (CVO). In addition, the University of Washington has a volcanoes web page.

A:Only earthquakes with magnitudes greater than 1.5 are on the list of recent events. It is possible that several earthquakes have taken place that were all of magnitude less than 1.5. Also, it takes the seismology lab time to analyze each earthquake and properly determine its magnitude. Some smaller events in the magnitude 2 range may not be posted on the list until three days after they occur. There is also the large possibility that the activity on webicorders is not seismic. Weather conditions such as wind and heavy rain will cause plenty of spikes and glitches. The instrument that is producing unusual signals may be broken. Outages in our network can last hours, days, or months, depending on the cause of the failiure and our ability to access the instrument. Some instruments are at high elevations or remote locations, and fixing them takes longer than other, easier to access instruments. The PNSN has more than 150 stations. Temporary problems with a few stations at any given time will not interfere with our ability to identify and analyze seismic activity in the Pacific Northwest.

A: Faults are identified by how the two blocks on either side of the fault move. Some types of faulting are driven by extension, others by compression. Animations show the various fault types.

A: "Foreshock" and "aftershock" are relative terms. Foreshocks are earthquakes which precede larger earthquakes in the same location. Aftershocks are smaller earthquakes which occur in the same general area during the days to years following a larger event or "mainshock." As a general rule, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the main shock. The magnitude 5.0 Robinson Point earthquake of January 28, 1995 that occurred in the Seattle - Tacoma region was preceded by two "unfelt" foreshocks of magnitudes 0.7 and 1.8. Similarly, roughly twenty five "unfelt" aftershocks less than magnitude 2.0 occurred in the region after the M 5.0 earthquake. The frequency of these aftershocks decreases with time. Historically, deep earthquakes (>30km) are much less likely to be followed by aftershocks than shallow earthquakes.

A: Each step of one in magnitude is an increase of ten times the amount of ground motion amplitude, corresponding to thirty-two times the amount of 'elastic' energy in the form of seismic waves. So a magnitude 6 quake has over 1,000 times as much energy as a magnitude 4 quake, and a 100 fold increase in ground motion amplitude. Above magnitude 6.0, the ground motion amplitude can no longer increase, and the excess energy is expressed as a longer duration of shaking.

A: Tsunamis are sea waves generated by an abrupt displacement of large volumes of water. Large subduction zone earthquakes of magnitude 7.5 or greater are the most frequent cause of tsunamis, as the vertical displacement of the sea floor along the subduction zone fault results in displacement of the water above. A tsunami can also be generated by other types of submarine faults, as well as by large coastal or submarine landslides. Not all submarine earthquakes will cause tsunamis. A submarine earthquake with pure strike-slip motion may not produce a tsunami, because water is less likely to be displaced unless the ocean bottom is rough. Click here for a more in depth discussion on tsunamis.

A: No place is completely safe from natural hazards. We choose what kinds of hazards we are willing to live with, and to prepare for. Regions of the U.S. that have almost no earthquake hazard, like the midwest, may instead have hazards from floods, tornados, or hurricanes.

A: An earthquake in Chile in 1960 broke a fault over one thousand miles long, and had a moment magnitude of 9.5.

A: Magnitude is calculated from a measurement of either the amplitude or the duration of specific types of recorded seismic waves. Magnitude is determined from measurements made from seismograms and not on reports of shaking or interpretations of building damage. In general, the different magnitude scales (for example, local or Richter magnitude and surface wave magnitude) give similar numerical estimates of the size of an earthquake, and all display a logarithmic relation to recorded ground motion. That means each unit increase in magnitude represents an increase in the size of the recorded signal by a factor of 10. Therefore, a magnitude 7 earthquake would have a maximum signal amplitude 10 times greater than that of a magnitude 6 earthquake and 100 times greater than that of a magnitude 5 earthquake. Seismologists sometimes refer to the size of an earthquake as moderate (magnitude 5), large (magnitude 6), major (magnitude 7). The Richter magnitude of an earthquake is calculated by measuring the amplitude of the maximum wave motion recorded on the seismogram.

Intensity of an earthquake is a measure of the amount of ground shaking at a particular site, and it is determined from reports of human reaction to shaking, damage done to structures, and other effects. The Modified Mercalli Intensity Scale is now the scale most commonly used to rank earthquakes felt in the United States.

If magnitude is compared to the power output of a radio broadcasting station, then the intensity of an earthquake is the signal strength at a particular radio receiver. In practice, an earthquake is assigned one magnitude, but it may give rise to reports of intensities at many different levels.

The magnitude 6.5 April 29, 1965, Seattle-Tacoma earthquake produced intensity VII to VIII damage near its epicenter, intensity V damage 150 kilometers away, and intensity I and 11 (barely felt) 300 to 500 kilometers from the epicenter. Although the greatest damage, and thus highest intensity, is usually near the earthquake's origin, damage to buildings depends on many factors, such as the type of construction, distance from the epicenter, and type of soil beneath the building. Therefore, maps of earthquake intensity commonly show complex patterns.

For more about earthquake magnitudes and intensities, visit UNR's Seismology Lab.

Also see "HOW ARE EARTHQUAKES MEASURED? Taken from " Washington State Earthquake Hazards" by Linda Lawrance Noson, Anthony Qamar, and Gerald W. Thorsen.

A: We have a special web page just for teachers.
The web page has seismology resources for teachers, earthquake resources, fault animations, virtual geology labs, a build-your-own seismograph, and an earthquake seismology link.

A: For more information about the several types of seismic waves, we recommend the following book: