Earthquake
An
earthquake is the sudden
release of stored
energy; most earthquakes
occur along a fracture
within the earth, called
a fault. The shaking
caused by this sudden
shift is often very
small, but occasionally
large earthquakes
produce very strong
ground shaking. It is
this strong shaking and
its consequences –
ground failure,
landslides, liquefaction
– that damages buildings
and structures and
upsets the regional
economy.
Washington, especially
the Puget Sound basin,
has a history of
frequent earthquakes.
More than 1,000
earthquakes occur in the
state each year. A dozen
or more are strong
enough that people feel
ground shaking ;
occasionally,
earthquakes cause
damage. Large
earthquakes in 1946
(magnitude 5.8), 1949
(magnitude 7.1) and 1965
(magnitude 6.5) killed
15 people and caused
more than $200 million
(1984 dollars) in damage
throughout several
counties. The state
experienced at least 20
damaging events in the
last 125 years.
The
Nisqually earthquake on
February 28, 2001, was a
deep, magnitude 6.8
earthquake 10 miles
northeast of Olympia.
One person died of a
heart attack, more than
700 people were injured,
and various estimates
place damage at between
$1 billion and $4
billion; exact figures
are not available , as
insurance claims
information is not
available.
The
earthquake threat in
Washington is not
uniform. While most
earthquakes occur in
Western Washington, some
damaging events, such as
the 1872 magnitude 6.8
(est.) quake, do occur
east of the Cascades.
Geologic evidence
documents prehistoric
magnitude 8 to 9.5
earthquakes along the
outer coast, and events
of magnitude 7 or
greater along shallow
crustal faults in the
urban areas of Puget
Sound.
Washington’s earthquake
hazards reflect its
tectonic setting. The
Pacific Northwest is at
a convergent continental
margin, the collision
boundary between two
tectonic plates of the
Earth’s crust. The
Cascadia subduction
zone, the fault boundary
between the North
America plate and the
Juan de Fuca plate, lies
offshore from northern
California to southern
British Columbia. The
two plates are
converging at a rate of
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. The abrupt
release of this slowly
accumulated strain
causes earthquakes.
