An earthquake is a trembling or shaking of the ground caused by a sudden slip on a fault. The sudden release of elastic energy stored in the rocks below the surface radiates as elastic waves from a fault along which movement has just taken place.
2. What is a fault and what are the different types?
A fault is a rock fracture in the Earth where the two sides have been displaced relative to each other. Faults are identified by how the two blocks on either side of the fault move. The four major faults are Normal fault, Reverse fault, Strike-slip fault, and Oblique-slip fault. IRIS provides a background page and animations that show the various fault types.
3. How long do earthquakes last?
How long earthquakes last varies depending on the size of the earthquake. Earthquakes may last seconds to minutes. While the shaking of small earthquakes typically lasts only a few seconds, strong shaking during moderate to large earthquakes, such as the 2004 Sumatra earthquake, can lasts couple minutes.
4. What are foreshocks and aftershocks?
"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 "main shock." 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. Foreshocks are no different than any other earthquake and can be recognized as “foreshocks” only after a “main shock” has taken place.
5. Why do the plates move?
The lithosphere, which is the Earth’s crust and upper part of the mantle, is comprised of several tectonic plates. These plates move around due to the convection currents in the Earth's mantle. They are constantly moving at about the rate your finger nails grow, about an inch or two a year. Over hundreds of thousands or even millions of years, this inch or two adds up to miles and miles of motion. In the short term time scale we see plate tectonics in action every time there is an earthquake. The movement of the plates, and the forces and stresses that build up along fault lines and continental margins generate many small and several large earthquakes every year. Click here to learn more about plate tectonics.
6. What is a tsunami?
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 (i.e. horizontal) 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.
7. What is liquefaction?
Liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading. Read this to learn more about liquefaction.
8. What is the difference between earthquake and ETS?
Earthquakes are brief sudden events lasting only seconds to a few minutes for the very large ones. They can generate strong shaking that can be felt or do damage to structures. ETS or “Episodic Tremor and Slip” also called “slow slip” lasts for many minutes to days to even weeks and generates such low level of shaking that only the most sensitive of instruments can detect it. Both of these types of events occur in response to large scale forces in the earth causing slip on faults. It is primarily a difference in how fast that slip occurs. Click here to learn more about ETS in the Pacific Northwest.
Measuring Earthquakes
1. What are a seismometer, seismograph, and a seismogram?
A seismometer is a sensor that measure vibrations of the Earth. A seismograph is an instrument that records these vibrations from a seismometer. A seismogram is the visual record of the Earth's vibrations produced by a seismograph. Click here to the PNSN seismogram page.
2. What is spectrogram?
A spectrogram is a visual way of representing the signal strength, or “loudness”, of a signal over time at various frequencies present in a particular waveform. For a complete description of spectrograms, with keys to their interpretation, including examples see the "What is a Spectrogram." Click here to view PNSN seismic spectrogram map.
3. How to measure an earthquake?
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 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 (M 5), large (M 6), major (M 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.
4. What is Richter Scale?
The Richter scale, also known as the local magnitude (ML) scale, was developed by Charles Richter in the 1930's. It is a base-10 logarithmic scale. This scale assigns a number to quantify the amount of seismic energy released by an earthquake.
5. What does a negative magnitude mean?
As magnitude calculations are based on a logarithmic scale, a ten-fold drop in amplitude decreases the magnitude by 1. Therefore, magnitude scales can be used to describe very small events expressed in negative numbers. These events are not felt by humans.
6. Why do some quakes show a 0.0 depth?
An earthquake cannot occur at depth of 0 km. But sometimes it could be just a very shallow event with poor depth resolution that was reported at a depth of 0 km. Most often it is not actually an earthquake, but a quarry blast or man-made activities near the surface. These explosions/activities are recorded by our instrument. After being reviewed by our seismologists they usually can be identified and labeled as explosions on our website.
7. What is the difference between an earthquake's magnitude and its intensity?
Magnitude is calculated from a measurement of either the amplitude or the duration of specific types of recorded seismic waves. Magnitude refers to the size (amount of energy release) at the earthquake’s source. Intensity is a qualitative measure of the earthquake’s effects at a particular place. Intensity reports are based on reports of shaking or interpretations of building damage. Thus an earthquake usually has only one estimate of its magnitude but can have many estimates of the shaking intensity over a geographical area. Click here to learn more about earthquake magnitudes and intensities.
8. What's the difference between an M 4 and an M 6 earthquake?
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.
9. What is the biggest earthquake that has occurred historically?
An earthquake in Chile in 1960 broke a fault over one thousand miles long, and had a moment magnitude of 9.5.
10. How is seismograph data interpreted?
In order to determine how big and where an earthquake has occurred, we must know exactly when our seismographs recorded it. By having data from many seismographs, we can determine the location by knowing how fast the seismic waves travel through the earth finding their common point of origin. The amplitude of the traces on the seismograms is used to determine its magnitude. Once we have this information, it is plotted on maps and passed on to emergency management agencies. This video made by the USGS illustrates how to interpret seismic records on a seismogram. Here is a guide from IRIS on seismic signatures and how to read a seismogram.
Earthquakes in Washington and Oregon
1. Why does the Pacific Northwest have earthquakes?
We are located at a convergent continental boundary, where two tectonic plates are colliding. This boundary is called the Cascadia Subduction Zone. It lies just offshore and runs from British Columbia to northern California. The oceanic plate is converging under the North American plate at a rate of about 3-4 cm/year (1-2 inches/year). This convergence causes elastic stress to accumulate in the rocks around the zone. Earthquakes are caused by the abrupt slipping on a fault that suddenly releases this slowly accumulated stress.
2. Where are the major faults in the Pacific Northwest?
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 faults mapped at the surface 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 major 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. Few and only very small historic earthquakes have been directly recorded from this source zone. According to recent geologic 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, also known as deep earthquakes, are deeper than 30km, which have been the most common damaging earthquakes in Washington and Oregon. The Nisqually Earthquake in 2001 is an example of deep earthquakes. They have historically occurred about every 30 years. The USGS estimates there is an 84% chance of another deep earthquake, of Magnitude 6.5 or greater, striking the region sometime in the next 50 years. 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.
3. Are there faults near Seattle and Portland?
Yes. Some of these are well known from geologic or geophysical surveys. Examples include the Seattle Fault and the Portland Hills Fault. How often large earthquakes occur on these faults is not well known but is less frequent than the Benioff zone earthquakes. A large earthquake on one of these faults has the potential to produce extensive damage to local cities.
4. How often do earthquakes occur in the Pacific Northwest?
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, lifelines and buildings. 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.
5. How many and what size of major earthquakes occur near Seattle and Portland?
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), 1999 (M 5.1), and 2001 (M 6.8). Most of the events are associated with deep Benioff zone earthquake activity that affects the Pacific Northwest Region. The 1995 and 1996 events were shallow crustal events. 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 M 7.1 near Olympia, WA in 1949, and the M 6.5 located between Seattle and Tacoma in 1965.
6. Could a big earthquake in California, Alaska, or Japan cause earthquakes in Washington or Oregon?
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.
7. Could bridges collapse due to seismic activity in the Pacific Northwest?
Yes, even modern bridges have sustained damage during large 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 in that city 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 1970s 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 and Oregon 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 and some of the older bridges have been retrofit to improve their resistance to earthquake shaking.
8. What historic earthquakes have been important in Washington and Oregon?
9. When was the last Cascadia Subduction Zone earthquake?
The last known CSZ earthquake was on January 26, 1700, just over 300 years ago. The Pacific Northwest experienced the 1700 Cascadia earthquake and tsunami which had an impact as far away as Japan. Geological evidence indicates that such great earthquakes have occurred at least seven times in the last 3,500 years, a reoccurrence interval of 300 to 600 years. The next major CSZ earthquake could strike the PNW at any time or still be hundreds of years away.
10. What is earthquake early warning? Do we have EEW in the Pacific Northwest?
Earthquake early warning (EEW) is a technology that detects earthquakes and estimates their location and magnitude quickly enough to alert people in the affected areas before they feel dangerous shaking. The ShakeAlert Earthquake Early Warning System, managed by the U.S. Geological Survey and partners including the Pacific Northwest Seismic Network. PNSN seismic stations contribute real-time data to the ShakeAlert System. ShakeAlert is available in Washington, Oregon, and California and can provide seconds to tens of seconds of early warrning. More information is available on the PNSN Earthquake Early Warning page and at ShakeAlert.org.
Volcanoes in Washington and Oregon
1. Do volcanoes produce different kinds of earthquakes?
Yes, because of the highly variable nature of the rocks and fluids in volcanoes earthquakes generated within them also come in many different kinds but most are fundamentally due to faulting.
2. What is volcanic tremor, and how does it differ from earthquakes?
Volcanic tremor (like tremor associated with ETS or slow slip) is a relatively continuous low-level shaking that may go on for minutes to hours with little change. Earthquakes are brief sudden events caused by the breaking of rock lasting for only seconds. There are many possible sources for volcanic tremor such as a continuous set of tiny earthquakes one after another producing a continuous vibration. Fluid flow in conduits or gas escaping through cracks can also generate continuous low-level vibrations (i.e. tremor). During eruptions the violent expulsion of magma and gas from a volcanic vent can generate continuous “eruption tremor”.
3. What kinds of hazards are associated with volcanic eruptions?
Volcanoes present a variety of hazards including lava flows, gas, lahars, landslides, pyroclastic flows, and ash/ tephra fall. Visit USGS Volcano Hazard Program website to learn more about the types of volcano hazards and Cascades Volcano Observatory website for volcano hazard maps for Washington and Oregon.
4. What fault lines pass under Mount Saint Helens?
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 of movement. Many volcanos are found in similar circumstances. The St. Helens Fault Zone was not discovered until after the eruption of Mt. St. Helens in 1980. In 1981 a magnitude 5+ earthquake on the SHZ had thousands of aftershocks which "lit up" the fault.
5. What is the best site for Cascadia volcano information?
The best site for information about volcanoes is at the Cascade Volcanic Observatory (CVO). In addition, PNSN Volcano Seismicity page provides current seismic info of volcanoes in Oregon and Washington and Volcanic Hazards page basics about these hazards.
Common Myths and Misconceptions
1. Can earthquakes be predicted?
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 attempts at earthquake prediction.
Changes in animal behavior cannot be used to predict earthquakes. Even though there have been reported cases of unusual animal behavior prior to earthquakes, a reproducible connection between a specific behavior and the occurrence of an earthquake has not been made. Because of their finely tuned senses, some animals might feel the earliest arriving and smallest waves from an earthquake before the humans around it can. This feeds the myth that the animal knew the earthquake was coming. But animals also change their behavior for many reasons, and given that an earthquake can shake millions of people, it is likely that a few of their pets will, by chance, be acting strangely before an earthquake. Read more on USGS website.
3. Is any place safe from earthquakes?
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.
4. Should I stand in a doorway during an earthquake?
No! That is outdated advice. In the old days, the door frames of old unreinforced masonry structures and adobe homes may have been safer during an earthquake. However, doorways in modern homes are no stronger than any other parts of the house and they may be potentially life threatening. The best way to reduce your chance of injury is to “DROP, COVER, and HOLD ON”. (From Earthquake Country Alliance)
5. Do many small earthquakes prevent larger earthquakes?
No! This is a popular misconception. In fact, based on past examples, an increase in small earthquakes in a region may very slightly increase the likelihood of a larger event. (From Earthquake Country Alliance)
6. Are earthquake swarms a sign of a bigger earthquake?
Not necessarily. Swarms may be a sign of a slight increase in stress or minor changes in hydrology. But it is far from being useful for a specific prediction of a coming earthquake since in most cases no large earthquake occurs.
7. Does the ground open up during an earthquake?
A popular cinematic and literary device is a fault that opens during an earthquake to swallow up an inconvenient character. The ground moves along a fault during an earthquake, not away from it. If the fault could open, there would be no friction. Without friction, there would be no earthquake. Shallow crevasses can form during earthquake induced landslides, lateral spreads, or other types of ground failures. Faults, however, do not open during an earthquake. (From USGS)
Natural Disaster Preparedness
1. Is one Seattle area neighborhood safer from earthquakes than another?
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 to help reduce possible damage in older homes.
2. What should I do during an earthquake?
In most situations, you should “DROP, COVER, and HOLD ON” to protect yourself.
- If you are indoors, stay there. Drop to the floor, get under a sturdy table or desk, cover your head and neck, and hang on to it until the shaking stops, or get against an inside wall.
- If you are outdoors, get into an open area away from buildings, windows, walls, and power lines and tall trees.
- If you are driving, pull over to the side of the road and stop carefully. Avoid bridges or overpasses, and power lines. Stay inside your car until the shaking is over.
- If in a crowded public place, do not rush for the doors. Crouch and cover your head and neck with your hands and arms.
Practice the “DROP, COVER AND HOLD” method at work and at home at least twice a year. You can also register the Great ShakeOut to practice earthquake drill.
4. Should I buy earthquake insurance for my house?
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. The USGS has a list of factors you should consider when deciding whether or not to get earthquake insurance.