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Frequently Asked Questions
Earthquakes in Washington and Oregon
- Where are the major faults in the Pacific Northwest?
- What historic earthquakes have been important in Washington and Oregon?
- Are there faults near Seattle and Portland?
- Why does the Pacific Northwest have earthquakes?
- How often do earthquakes occur in the Pacific Northwest?
- How many and what size of earthquakes occur near
Seattle?
- How many and what size of earthquakes occur near
Portland?
- Could a big earthquake in California, Alaska, or
Japan cause earthquakes in Washington or Oregon?
- Could bridges collapse due to seismic activity in
the Pacific Northwest?
- Should I buy earthquake insurance for my house?
- Can earthquakes be predicted?
- Is one Seattle area neighborhood safer from earthquakes than
another?
- What fault lines pass under Mount Saint Helens?
- What is the best site for Volcano information?
- There seems to be a lot of activity on the Webicorders, but
there are not any events listed on the "Recent Events" list. What is going on?
General Questions About Earthquakes
- What are the different kinds of faults?
- What are foreshocks and aftershocks?
- What's the difference between an M 4 and an M 6
earthquake?
- What is a tsunami?
- Is any place safe from earthquakes?
- What is the biggest earthquake that has occurred
historically?
- What is the difference between an earthquake's
magnitude and its intensity?
- Where can I find teaching resources related to earthquakes?
- What is a good resource for learning more about P and S waves?
- What is a good site to learn more about recent earthquakes around the world?
- How is seismograph data interpreted?
Return to Pacific Northwest Earthquake Page
Return to Seismology Page
- Q: Where are the major faults in the Pacific Northwest?
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.
- Q: What historic earthquakes have been important in Washington and Oregon?
USGS Web page on Earthquake History of Washington - to 1973
USGS Web page on Earthquake History of Oregon - to 1973
- Q: Are there faults near Seattle and Portland?
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.
- Q: Why does the Pacific Northwest have 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.
- Q: How often do earthquakes occur in the Pacific Northwest?
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.
- Q: How many and what size of earthquakes occur near Seattle?
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.
- Q: How many and what size of earthquakes occur near Portland?
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.
- Q: Could a big earthquake in California, Alaska, or Japan cause earthquakes in Washington or Oregon?
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.
- Q: Could bridges collapse due to seismic activity in the Pacific Northwest?
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.
- Q: Should I buy earthquake insurance for my house?
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.
- Q: Can earthquakes be predicted?
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.
- Q: Is one Seattle area neighborhood safer than another?
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:
- Q: What fault lines pass under Mt. Saint Helens?
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.
- Q: What is the best web site for Volcano information?
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.
- Q: There seems to be a great deal of activity on the webicorders, but there are not any events listed on the "Recent Events" list. What is going on?
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.
- Q: What are the different kinds of faults?
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.
- Q: What are foreshocks and aftershocks?
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.
- Q: What's the difference between a magnitude 4 and a magnitude 6 earthquake?
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.
- Q: What is a tsunami?
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.
- Q: Is any place safe from earthquakes?
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.
- Q: What is the biggest earthquake that has occurred historically?
A: An earthquake in Chile in 1960 broke a fault over one
thousand miles long, and had a moment magnitude of 9.5.
- Q: What is the difference between an earthquake's magnitude and its intensity?
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.
- Q: Where can I find teaching resources related to earthquakes?
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.
- Q: What is a good resource for learning more about p and s waves?
A: For more information about the several types of seismic waves, we recommend the
following book:
- Earthquakes
- by Bruce A. Bolt
- Printed by: W.H. Freedman and Company
- New York, United States of America
The version we have in the seismology lab was printed in 1995, but there are
other versions, both older and newer.
- Q: What is a good site to learn more about recent earthquakes around the world?
A: Here at the PNSN, we are focused on Pacific Northwest Seismicity. For questions
about world wide seismic activity, we recommend the web site of the
National Earthquake Information Center. On the University of Washington Seismology Lab web site, we
have a page devoted to earthquake related links around the world.
The IRIS Web Page also has high quality worldwide seismicity
data.
- Q: How is seismograph data interpreted?
A: In order to determine how big and where an earthquake has occured, we must know exactly when
our seismographs recorded it. By having data from many seismographs, we can more accurately determine the
location and magnitude of an earthquake. Once we have this information, it is looked at by analysts and passed
on to emergemcy management agencies. For more information on how we analyze seismograph data, visit our Basic Info page.
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