USGS/Cascades Volcano Observatory, Vancouver, Washington
DESCRIPTION:
Cascade Range Earthquakes and Seismicity
- Cascade Range
- Pacific Northwest Seismic Network
- Oregon Earthquakes and Seismicity
- Washington State Earthquakes and Seismicity
From:
Swanson, et.al., 1989,
Cenozoic Volcanism in the Cascade Range and Columbia Plateau,
Southern Washington and Northernmost Oregon:
AGU Field Trip Guidebook T106.
-
The
Cascade Range
has been an active arc for about 36 million years as a result of
plate convergence. ...
The southern Washington Cascades are
seismically active.
Most
earthquakes
occur along the 100-kilometer-long, north-northwest trending
St. Helens seismic zone,
where most focal mechanisms show dextral slip parallel to the trend of the zone
and consistent with the direction of plate convergence. Other crustal
earthquakes concentrate just west of Mount Rainier and in the
Portland (Oregon) area. Few earthquakes occur north of
Mount Rainier or south of
Mount Hood.
From:
University of Washington, Geophysics Program Website, 1998
-
Information on Pacific Northwest earthquake activity and hazards is
provided by the Pacific Northwest Seismograph
Network (PNSN) which operates seismograph stations and locates
earthquakes in Washington and Oregon. The PNSN is
operated jointly by the University of Washington, the
University of Oregon, and Oregon State University, and is funded by the
U.S. Geological Survey, the Department of Energy, and
the State of Washington. The PNSN is based at the Geophysics
Program of the University of Washington in Seattle, and
is a member of the Council of the National Seismic Systems (CNSS),
a group of regional network operators who cooperate to locate and
catalog earthquakes throughout the United States.
-
Pacific Northwest Seismograph Network
-- Information, Maps, Links, etc.
From:
D.R. Sherrod, L.G. Mastin, W.E. Scott, and S.P. Schilling, 1997,
Volcano Hazards at Newberry Volcano, Oregon:
USGS Open-File Report 97-513
-
Earthquakes occur when rocks break suddenly in response to various
geologic forces. Magma moving in the Earth's crust may create sufficient
force to produce volcanic earthquakes. More common, however, are tectonic
earthquakes, which periodically strike parts of Oregon. These earthquakes,
the result of fault movements driven by regional crustal stresses,
typically have no direct connection to magma movement. Regardless
of type, earthquake size is reported by magnitude, and many scientists
and media describe earthquakes by the well-known Richter magnitude
scale. ...
-
Tectonic earthquakes occur periodically in south-central and southeast
Oregon, and they are capable of exceeding the magnitude of volcanic
earthquakes. An example is the Klamath Falls earthquakes, a swarm
that began in September 1993 with two large earthquakes of magnitude
5.9 and 6.0 that killed two people and and caused $7.5 million in
property damage. Aftershocks as large as magnitude 5.1 continued to
disturb residents for as much as six months. These earthquakes had
no connection with volcanic processes.
Information courtesy University of Washington's Geophysics Program, 1998 and 2001
-
Information on Pacific Northwest earthquake activity and hazards is provided by
the Pacific Northwest Seismic Network (PNSN) which operates
seismograph stations and locates earthquakes in Washington and Oregon. The
PNSN
is operated jointly by the University of Washington, the University of Oregon,
and Oregon State University, and is funded by the U.S. Geological Survey, the
Department of Energy, and the State of Washington. The PNSN
is based at the Geophysics Program of the University of Washington in Seattle,
and is a member of the Council of the National Seismic Systems (CNSS), a group
of regional network operators who cooperate to locate and catalog earthquakes
throughout the United States.
-
In addition to locating regional earthquakes, the Pacific Northwest Seismic Network
(PNSN), in cooperation with the Cascades Volcano Observatory,
is also responsible for monitoring seismic activity at volcanoes in the Pacific Northwest.
The PNSN currently operates seismometers on or near
Mount Adams,
Mount Rainier,
Mount St. Helens,
Mount Hood,
Mount Baker,
Three Sisters, and
Crater Lake.
From:
Washington State Department of Natural Resources, Division of Geology and Earth Resources
Website, 2002
-
Washington is situated
at a convergent continental margin,
the collisional boundary between two tectonic plates.
The Cascadia subduction zone, which is the
convergent boundary between the North America plate and the Juan de Fuca
plate, lies offshore from northernmost California
to southernmost British Columbia. The two plates are converging at a rate
of about 3-4 centimeters per year (about 2 inches per year);
in addition, the northward-moving Pacific plate is pushing the
Juan de Fuca plate
north, causing complex seismic strain to accumulate.
Earthquakes are caused by the abrupt release of
this slowly accumulated strain.
From:
Swanson, et.al., 1989,
Cenozoic Volcanism in the Cascade Range and Columbia Plateau,
Southern Washington and Northernmost Oregon:
AGU Field Trip Guidebook T106.
-
The southern Washington Cascades are seismically active. Most earthquakes occur
along the 100-kilometer-long, north-northwest trending St. Helens seismic zone,
where most focal mechanisms show dextral slip parallel to the trend of the zone
and consistent with the direction of plate convergence. Other crustal
earthquakes concentrate just west of Mount Rainier and in the Portland
(Oregon) area. Few earthquakes occur north of
Mount Rainier or south of Mount Hood.
-
From tomography, Rasmussen and Humphreys (1988) interpret the subducted
Juan de Fuca plate
as a quasi-planar feature dipping about 65 degrees to about 300 kilometers under
the southern Washington Cascades. The plate is poorly defined seismically,
however, owing to a lack of earthquakes within it. Guffanti and Weaver
(1988) show that the present volcanic front of the Washington Cascades, defined
by the westernmost young vents, parallels the curved trend of the subducting
plate reflected by the 60 kilometer-depth contour. The front trends northwest in
northern Washington -- where Glacier Peak, Mount Baker, and the
volcanoes of southern British Columbia occur along a virtually straight line --
and northeast in southern Washington. A 90-kilometer gap free of young
volcanoes between Mount Rainier and Glacier Peak is landward of
that part of the subducting plate with the least average dip to a depth of 60
kilometers. South of
Portland,
the volcanic front is offset 50 kilometers
eastward and extends southward into California, probably still parallel to the
trend of the convergent margin.
From:
Noson, Qamar, and Thorsen, 1988,
Washington State Earthquake Hazards:
Washing State Department of Natural Resources,
Washington Division of Geology and Earth Resources Information Circular 85
-
Washington is earthquake country. More than 1,000 earthquakes are
recorded in the state each year; a dozen or more of these produce significant
shaking or damage. Large earthquakes in 1949 and 1965 killed 15 people and
caused more than $200 million (1984 dollars) property damage.
-
Earth scientists believe that most earthquakes are caused by slow movements
inside the Earth that push against the Earth's brittle, relatively thin outer
layer, causing the rocks to break suddenly. This outer layer is fragmented into
a number of pieces, called
plates.
Most earthquakes occur at the boundaries of these plates. In Washington, the
small
Juan de Fuca plate
off the coast of Washington, Oregon, and northern
California is slowly moving eastward beneath a much larger plate that includes
both the North American continent the land beneath part of the Atlantic Ocean.
Plate motions in the Pacific Northwest result in shallow earthquakes widely
distributed over Washington and deep earthquakes in the western parts of
Washington and Oregon. The movement of the Juan de Fuca plate beneath the North
America plate is in many respects similar to the movements of plates in South
America, Mexico, Japan, and Alaska, where the world's largest earthquakes occur.
-
We cannot predict precisely where, when, and how large the next destructive
earthquake will be in Washington, but seismological and geological evidence
supports several possibilities. Large earthquakes reported historically in
Washington have most frequently occurred deep beneath the Puget Sound region.
The most recent and best documented of these were the 1949 Olympia earthquake
(magnitude 7.1)
and the 1965 Seattle-Tacoma earthquake (magnitude 6.5).
The pattern of earthquake occurrence
observed in Washington so far indicates that large earthquakes similar to the
1965 Seattle-Tacoma earthquake are likely to occur about ever 35 years and large
earthquakes similar to the 1949 Olympia earthquake about ever 110 years. Such
large earthquakes deep beneath the Puget Sound area will happen again.
-
The largest earthquake reported in the state did not occur in the Puget Sound
region, but rather at a shallow depth under the North Cascade Mountains. Recent
studies in the southern Cascades near Mount St. Helens indicate that
other areas in the Cascades may produce large, shallow earthquakes, comparable
in size to the 1949 and 1965 Puget Sound earthquakes. The average interval of
time between occurrences of such earthquakes in the Cascade Mountains is
uncertain because they have occurred infrequently. However, the 1872 North
Cascade earthquake and earthquake activity in the southern Cascades are
reminders that Puget Sound is not the only region in Washington having
significant earthquake hazards. ...
-
Washington has features typical of convergent boundaries ...
A zone of deep earthquakes near the probable boundary between the
Juan de Fuca plate
and the North America plate.
The 1949 magnitude 7.1 Olympia earthquake and the
1965 magnitude 6.5 Seattle-Tacoma earthquake occurred within this deep zone.
... In sum, The subduction of the Juan de Fuca plate
beneath the North America plate is believed to directly or indirectly cause most
of the earthquakes and young geologic features in Washington and Oregon.
Information courtesy University of Washington's Geophysics Program, 1998 and 2001
-
Information on Pacific Northwest earthquake activity and hazards is provided by
the Pacific Northwest Seismic Network (PNSN) which operates
seismograph stations and locates earthquakes in Washington and Oregon. The
PNSN
is operated jointly by the University of Washington, the University of Oregon,
and Oregon State University, and is funded by the U.S. Geological Survey, the
Department of Energy, and the State of Washington. The PNSN
is based at the Geophysics Program of the University of Washington in Seattle,
and is a member of the Council of the National Seismic Systems (CNSS), a group
of regional network operators who cooperate to locate and catalog earthquakes
throughout the United States.
-
In addition to locating regional earthquakes, the Pacific Northwest Seismic Network
(PNSN), in cooperation with the Cascades Volcano Observatory,
is also responsible for monitoring seismic activity at volcanoes in the Pacific Northwest.
The PNSN currently operates seismometers on or near
Mount Adams,
Mount Rainier,
Mount St. Helens,
Mount Hood,
Mount Baker,
Three Sisters, and
Crater Lake.
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10/25/08, Lyn Topinka