Frequently Asked Questions

An earthquake is a trembling or shaking of the ground caused by a sudden slip on a fault. While the edges of faults are stuck together due to friction, and the rest of the surrounding blocks of rock is moving, the energy that would normally cause the blocks to slide past one another is being stored within the rocks as elastic energy. During an earthquake, this stored elastic energy is suddenly released and radiates away from the fault in all directions as seismic waves. These waves may be felt by people on Earth’s surface as shaking.

The point on a fault where an earthquake begins is called a hypocenter, which is typically underground. The epicenter is the point on Earth's surface directly above the hypocenter.

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 fault types are:

The lithosphere, which includes the Earth’s crust and upper part of the mantle, is comprised of several large rocky tectonic plates. These plates move relative to each other due to the convection currents in the Earth's mantle. They move at about the rate your fingernails grow, about an inch or two a year. Over hundreds of thousands or even millions of years, this adds up to miles of motion. Plates can spread apart from each other (divergent plate boundaries), collide into each other (convergent plate boundaries), or slide past each other (transform plate boundaries).

Plate tectonics contribute to mountain building, volcanism, and earthquakes. The movement of the plates, and the forces and stresses that consequently act along fault lines and continental margins, generate thousands of small and several large earthquakes every year.

The duration of shaking varies depending on the magnitude of the earthquake. Earthquakes may last seconds to minutes. While the shaking of small earthquakes typically lasts only a few seconds, strong shaking during large earthquakes, such as a megathrust Cascadia Subduction Zone earthquake, can last several minutes.

Foreshock and aftershock are relative terms. Foreshocks are smaller earthquakes which precede larger earthquakes in the same location. Foreshocks are no different than any other earthquake and can be recognized as “foreshocks” only after a mainshock has taken place. 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 mainshock. The frequency of aftershocks decreases with time. Historically, deep earthquakes (deeper than 30 kilometers below the Earth's surface) are much less likely to be followed by aftershocks than shallow earthquakes.

Earthquakes are relatively briefs, sudden events lasting only seconds to a few minutes. They can generate strong shaking that can be felt by people and damage structures. Episodic tremor and slip events (ETS) can occur over days or even weeks, and generate such low levels of shaking that only the most sensitive of instruments can detect it. ETS is also known as slow slip. Both earthquakes and ETS occur in response to large scale forces in the earth causing slip on faults, but they primarily differ in how quickly that slip occurs.

Magnitude measures the amount of energy released by an earthquake. There are multiple different magnitude scales (for example, the Richter scale and the moment magnitude scale), but in general they provide 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 about 32 times more energy released and about 10 times the level of ground shaking. Therefore, a M7.0 earthquake would release 32 times more energy than a M6.0 earthquake and 1,000 times more energy than a M5.0 earthquake.

The moment magnitude (Mw) scale is the most accurate way to measure an earthquake's size. An earthquake's moment is related to the area of the fault rupture, the amount of slip on the fault, and the strength of the surrounding rocks. Although many people have heard of the Richter scale, it is not commonly used today because it underestimates the size of large earthquakes. The Richter scale was developed by American seismologist Charles Richter in the 1930s and is also known as the local magnitude (ML).

Magnitude and Intensity

Magnitude refers to the amount of energy released during an earthquake. It is a fixed quantitative value. Intensity is a qualitative measure of an earthquake’s impact at a particular location based on reports of felt shaking and damage to buildings and other infrastructure. Although each earthquake has only one magnitude, earthquake intensity varies over a geographical area. Higher magnitude earthquakes are typically associated with a higher intensity of shaking, and people generally feel the strongest shaking closest to the earthquake's epicenter. However, earthquake intensity depends on many factors, including a location's distance from the epicenter, the type of soil and rock beneath that location, and even building construction methods. Therefore, maps of earthquake intensity commonly show complex patterns. Intensity is measured by the Modified Mercalli Intensity (MMI) scale, which ranges from "not felt" to "extreme."

Magnitude and Intensity

A seismometer is a sensor that measures the 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.  A spectrogram is a visual method for representing the frequency content of a seismogram as it changes over time. It is a powerful tool for viewing the strength, or “loudness,” of a signal over time at various frequencies present in a particular waveform. This helps scientists distinguish and characterize different types of earthquakes.   

In order to determine an earthquake's magnitude and location, seismologists review seismograph data from multiple seismic monitoring stations. By knowing how quickly seismic waves travel through the Earth and the exact arrival time of wavefronts at various locations, seismologists can triangulate the waves' common origin point and identify the earthquake's location. To calculate magnitude, seismologists look at the amplitude of waves on a seismogram and calculate the force needed to generate the recorded waves. This is called the seismic moment, which is a function of the rock's rigidity, the distance that one block slips relative to the other, and the area that ruptured between the rocks. The seismic moment can be determined by mathematical modeling of seismograms.

Megathrust earthquakes at subduction zones produce the largest earthquakes on Earth. The largest earthquake recorded instrumentally was the Great Chilean Earthquake on May 22, 1960, estimated to have a moment magnitude of M9.5. This earthquake ruptured along a 1,000-mile-long subduction zone fault. Shaking lasted for up to 10 minutes in some locations, and tsunami waves impacted much of the Pacific Basin.

The Pacific Northwest is located at a convergent plate boundary called the Cascadia Subduction Zone. The Cascadia Subduction Zone is located 70 to 100 miles off the coast and is 600 miles long, stretching from Vancouver Island to Cape Mendocino. Here, the oceanic Juan de Fuca Plate is spreading away from the Pacific Plate and is moving toward the continental North American Plate at a rate of 1 to 2 inches per year. Because oceanic crust is heavier and denser than continental crust, the Juan de Fuca Plate is slowly subducting (or sinking) beneath the North American Plate. The plates are locked together by friction, causing elastic strain to accumulate and resulting in extensive faulting. Earthquakes are caused by the abrupt slipping on a fault, which suddenly releases this slowly accumulated stress.

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 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 a megathrust earthquake along the Cascadia Subduction Zone, the 1,000-kilometer-long fault off the coast of the Pacific Northwest that is the convergent boundary between the Juan de Fuca Plate and the North American Plate. An earthquake rupture could occur along the entire length of the fault or over only part of it. Although megathrust subduction zone earthquakes occur relatively infrequently, they have high magnitudes and can cause catastrophic damage. The last megathrust Cascadia Subduction Zone earthquake occurred on January 26, 1700 and was likely a M9.0 event. According to the U.S. Geological Survey (USGS), in the next 50 years there is a 37% of another M7.0+ megathrust Cascadia Subduction Zone earthquake. These events typically occur every few hundred years.

However, most earthquakes in the Pacific Northwest are deep intra-slab earthquakes and shallow crustal fault earthquakes. The crust of the Juan de Fuca Plate is itself placed under strain as it subducts beneath North America and into Earth’s mantle. Extensive faulting in this subducting slab produces deep intra-slab earthquakes, also known as Benioff Zone earthquakes. The 2001 Nisqually earthquake is one example. The Benioff Zone can produce earthquakes with magnitudes as large as M7.5. These earthquakes typically occur on faults deeper than 30 kilometers. Deep earthquakes are the most common damaging earthquakes in Washington and Oregon, historically occurring around every 30 years. The USGS estimates there is an 84% chance of another M6.5+ deep earthquake striking the region in the next 50 years.

Shallow crustal fault earthquakes occur within the North American Plate, where faulting is extensive. These earthquakes typically have depths of 0 to 20 kilometers. Although they are typically moderate in size and are less frequent than deep earthquakes, they can still be very damaging to local cities because they occur so close to the Earth's surface. The Pacific Northwest's best known crustal fault, the Seattle Fault, runs east-west through Seattle from Issaquah to Bremerton. This fault generated a very large earthquake and tsunami approximately 1,100 years ago. Examples of other crustal faults include the Portland Hills Fault, the Western Rainier Seismic Zone and the Mt. St. Helens Seismic Zone.

Earthquake Sources in the Pacific Northwest

The Pacific Northwest Seismic Network typically locates over 1,000 earthquakes with magnitude 1.0 or greater in Washington and Oregon each year. 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, critical infrastructure, and community lifelines.

Since 1872, there have been about 25 damaging earthquakes in Washington and Oregon. In the Puget Sound region, eleven earthquakes of magnitude 5 or greater have occurred in this time: 1904 (M5.3), 1909 (M6.0), 1932 (M5.2), 1939 (M6.2), 1945 (M5.9) 1946 (M6.4), 1949 (M7.0), 1965 (M6.5), 1995 (M5.0), 1996 (M5.3), 1999 (M5.1), and 2001 (M6.8). Most of these events were deep earthquakes, although the 1995 and 1996 earthquakes were shallow crustal events. There have been three significant earthquakes near Portland in this time as well: 1877 (M5.3), 1962 (M5.5), and 1993 (M5.5). Overall, 17 people lost their lives due to earthquakes in the Pacific Northwest during the 20th century.

The last known megathrust Cascadia Subduction Zone earthquake occurred on January 26, 1700. This date is known through a combination of Native American oral histories, geologic evidence, and Japanese written records, which described an orphan tsunami that traveled across the Pacific Basin to Japanese shores. 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 earthquake could strike the Pacific Northwest at any time or still be hundreds of years away.

In addition to ground shaking and surface rupture, earthquakes can cause secondary hazards that also impact communities.

Tsunamis are extremely long waves caused by an abrupt displacement of large volumes of ocean water. Tsunamis are most frequently caused by large megathrust earthquakes at subduction zones (M7.5+). These events vertically displace the sea floor, which results in displacement of the water column above. Tsunamis travel quickly across the ocean, but slow as they approach coastal shores, which causes wave amplitude to increase and form walls of fast-moving turbid water that can be dozens of meters high. Tsunamis can be generated by other types of submarine faults as well as by large coastal or submarine landslides. However, submarine earthquakes with pure strike-slip (i.e. horizontal) motion may not produce a tsunami because water is less likely to be displaced vertically.

Liquefaction is a phenomenon in loose water-logged soil looses its stiffness and strength due to strong earthquake shaking or other rapid loading. Liquefaction can cause major damage to buildings and other infrastructure due to subsidence and horizontal sliding at the ground surface.

Earthquakes also contribute significantly to landslides by creating stresses that cause steep weak slopes to fail. Strong earthquake shaking increases the likelihood of landslides where the landscape is already susceptible to ground failure, particularly if the ground is saturated with water following heavy rainfall.

In addition to megathrust, deep intraslab, and shallow crustal fault earthquakes, volcanic earthquakes also occur in the Pacific Northwest. As the Juan de Fuca Plate subducts, it partially melts, generating magma that rises to form volcanic arcs. Volcanic earthquakes are triggered by magma moving within and beneath volcanoes and can provide clues about potential volcanic eruptions. However, they are generally too small to be felt by people.

Earthquakes are brief, sudden events caused by abrupt slip along a fault that last only seconds to minutes. Volcanic tremor (like the tremor associated with episodic tremor and slip or slow slip) is a relatively continuous low-level shaking that may go on for minutes to hours with little change. There are many possible sources for volcanic tremor, such as:

Volcanoes present a variety of hazards beyond explosive eruptions and lava flows. In the Pacific Northwest, the greatest volcanic hazards are pyroclastic density currents, lahars, and ashfall. Lahars are debris flows that originate on the slopes of volcanoes and and move rapidly down valleys, potentially impacting communities many miles away. Lahars can begin through the rapid melting of mountain snow and ice during eruption and by erosion of fresh volcanic ash deposits during heavy rains. Pyroclastic flows are hot, fast-moving clouds of gas, ash, and rock debris known as tephra. They are denser than the surrounding air, and so they flow downhill close to volcanic slopes inside of creating an airborne plume. Their impacts are mostly limited to the immediate area surrounding a volcano. Ashfall has the widest geographic impact during a volcanic eruption.

The Cascades Volcano Observatory is the authoritative source of information about volcanoes in Oregon and Washington.

The Washington Geologic Survey provides volcanic hazard maps in Washington.

The Oregon GEOHub provides volcanic hazard maps in Oregon.

During an earthquake, seconds matter. As soon as you feel shaking or receive an earthquake early warning alert, Drop, Cover, and Hold On (DCHO) to protect yourself.

Practice these protective actions at work and at home at least twice a year. You can also register for ShakeOut, the world's largest earthquake drill that occurs annually on the third Thursday of October.

Earthquake early warning (EEW) systems use a dense sensor network to rapidly detect earthquakes and alert people in impacted areas before dangerous shaking arrives. Depending on their distance from the earthquake epicenter, individuals can receive seconds to tens of seconds of early warning. By giving people time to protect themselves, EEW can save lives.

Earthquake early warning has been implemented in many locations around the world, including the United States, Canada, and Japan. ShakeAlert is the earthquake early warning system for the United States. It is managed by the U.S. Geological Survey in partnership with the Pacific Northwest Seismic Network, the California Integrated Seismic Network, and other state and federal partners. ShakeAlert is currently available in Washington, Oregon, and California. Learn more at ShakeAlert.org. 

Earthquake Early Warning

We cannot predict or prevent earthquakes, but earthquake early warning, personal preparedness, and earthquake mitigation can save lives, reduce damage, and speed recovery. Earthquake risk reduction strategies include:

Preparedness

Earthquakes are a worldwide problem. Across the United States, over 143 million people are exposed to potentially damaging earthquakes. Most of our nation's earthquake risk (taking into account the overlap of natural hazards and population exposure) is concentrated in Washington, Oregon, and California. And in general, earthquake hazards are concencentrated at the boundaries of tectonic plates. But no place is completely safe from earthquakes. The National Seismic Hazard Model can help people understand earthquake hazards in their region. 

During an earthquake, the safest places to be are underneath a sturdy table, or outside, away from buildings or things that can collapse.

No, earthquakes cannot be predicted. An earthquake prediction must accurately define a future earthquake's date, time, location, and magnitude. Although humans have long tried to predict earthquakes, no reliable method has been discovered. Statements that claim to predict earthquakes are not based on scientific evidence and may be so general that there will always be an applicable earthquake. Scientists can only calculate the probability that a significant earthquake will occur in a specific area within a certain time frame. Learn more about earthquake prediction, earthquake probabilities, earthquake forecasting, and earthquake early warning here.  

Earthquake Prediction

No, the best way to protect yourself during an earthquake is by practicing Drop, Cover, and Hold On (DCHO). Most earthquake-related injuries in the United States result from falling objects, flying glass, or people trying to move more than a few feet during shaking, with the main source of injury being moving during shaking. The door frames of old unreinforced masonry structures may once have been safer during earthquakes, but doorways in most modern American homes are not stronger than other parts of the house and may be potentially life threatening. 

Earthquake swarms are sequences of small earthquakes with no identifiable mainshock. They can occur over days, weeks, or even months over the same geographic location. A series of small earthquakes may be a sign of increased likelihood of a future mainshock event. But earthquake swarms are not useful predictors of future earthquakes, because in most cases no large earthquake occurs. Most swarms are associated with geothermal activity.

A popular cinematic and literary device is to show a fault opening up during an earthquake to swallow up people and buildings. However, faults do not open during an earthquake. The ground moves along a fault during an earthquake, not away from it. If the fault could open, there would be no friction and therefore no earthquake.

Still, shallow crevasses can form during earthquake-induced landslides, lateral spreads, or other types of ground failures. Some lateral displacement may be visible along a fault.