THE
EARTHQUAKE OF 28 FEBRUARY 2001 IN THE STATE OF WASHINGTON, U.S.
George
Pararas-Carayannis
Introduction
On 28 February 2001,
a strong earthquake occurred in the State of Washington, U.S..
The quake was the most powerful to strike this region since 1949.
It did not result in any deaths but it caused significant property
damage.
Earthquake Epicenter,
Origin Time, Magnitude, Aftershocks and Source Mechanism
According to the USGS's
National Earthquake Information Center (NEIC), the earthquake
had a magnitude of 6.8. Its origin time was 10:54:32 AM Pacific
Standard Time (PST) , Wednesday, February 28, 2001 (18:54:32
UTC). It occurred near the Seattle - Tacoma area, with epicenter
at 47.2 N 122.7 W, approximately 11 miles ( 20 km) NE of Olympia,
15 miles (24 km) from Tacoma, and 36 miles( 57 km) from Seattle.
The epicenter was very near to that of the 7.1 magnitude, April
13, 1949 quake. The focal depth was 52.4 km (32.6 miles).
Quake intensity in
the immediate epicentral region was much as IX or X on the Modified
Mercalli Scale. Ground motions were felt as far away as Salt
Lake City. Numerous aftershocks followed the main quake.
The epicenter, the
focal depth and preliminary tensor analysis indicate that the
mechanism for this earthquake was due to tensional (normal) faulting
which occurred along the subducting Benioff seismic zone of the
Juan de Fuca tectonic plate.
(Historical
Earthquakes in the Puget Sound Area (Modified USGS graphic)
Damages
The quake was the
worst in 52 years, with only one third the energy of the 1949
event. Because of its depth (52.4 km), its destructiveness was
not extensive. It did not result in any deaths but at least 250
people were injured in the Olympia-Seattle area. There was no
damage to bridges but strong ground motions toppled walls, rocked
skyscrapers and ignited fires. U.S. Highway 101 buckled in several
places. There was temporary loss of power throughout the affected
region and the Seattle-Tacoma International Airport was closed
for several hours.
Tectonic
Setting - Geological Instability of the Pacific Northwest
The
Pacific Northwest is a region of geologic instability controlled
by the convergence and collision of the northeast-moving Juan
de Fuca Plate pushing into the North American Plate along the
tectonic boundary known as the Cascadia Subduction Zone.
This zone is the the
most extensive fault system in the Pacific Northwest. It lies
offshore and runs for about 1,000 km (600-miles) from British
Columbia to northern California. The two tectonic plates converge
at a rate of about 3-4 cm/year (1-2 inches/year), which causes
stresses to accumulate along different, but parallel, seismic
belts. Abrupt releases of slowly accumulated stresses along the
Cascadia Subduction Zone have been responsible for major, shallow
earthquakes in the region in the past. Although rare, quakes
along this zone constitute the greatest threat for the Pacific
Northwest. They are known as megathrust events.
(USGS graphic)
The second source
for damaging earthquakes in the region is that known as the Benioff
Zone. It parallels the offshore Cascadia Subduction Zone and
it is the continuation of extensive faulting that results from
the same subducting San Juan de Fuca plate as it forces further
its way into the earth's upper mantle, underneath the North American
plate. The Benioff Zone can probably produce earthquakes with
magnitudes as large as 7.5 , most having focal depths deeper
than 30 km.
A
third source for shallow crustal earthquake activity (depths
of 0 to 20 km) in the Pacific Northwest occurs within the North
American continental plate itself where surface faulting is extensive
due to crustal buckling.
Past earthquake activity
has revealed many shallow fault structures, including the Western
Rainier Seismic Zone and the Mt. St. Helen's Seismic Zone.
(Canadian Geologic Survey
graphic)
One of the best known
crustal faults is the Seattle Fault, which runs east-west through
Seattle from Issaquah to Bremerton. Geologic evidence reveals
that this fault was responsible for a very large earthquake approximately
1100 years ago. The U.S. Geological Survey has mapped several
other crustal faults in the Puget Basin region, some of which
have not generated earthquakes in recent geologic time. It is
suspected that many more, still unknown, faults exist in the
region.
Crossection
of the Subducting Juan De Fuca Plate showing the distribution
of deeper earthquake hypocenters (and temperatures) along the
Benioff zone and the shallower hypocenters on the northern extensions
of the Western Rainier and Mt. St. Helen's Seismic Zones.
(Canadian Geologic Survey
graphic)
Historical
Earthquakes in the Pacific Northwest
On the basis of recently
found geological evidence (paleo-tsunami deposits and submerged
ghost forests inside areas that are now tidal marshes) - extensively
spread over large areas of the Pacific Northwest - claims have
been made that great earthquakes (with magnitude of as much as
8 to 9) and very long ruptures (of as much as 1,000 Km or more)
must have occurred along Cascadia's Pacific coast . Further claims
have been made that such earthquakes have repeatedly struck the
Pacific Northwest in the past several thousand years - the most
recent about 300 years ago, on January 26, 1700. Whether the
Cascadia Subduction Zone can indeed generate earthquakes of such
great magnitudes and ruptures is not known with certainty. The
Zone has been relatively quiet for many years. Whether it is
locked - as claimed - or "lubricated" by sediments
from the Columbia river, is not known. Although no "great"
earthquakes have occurred since 1700 on the Cascadia megathrust
, more than 1000 earthquakes of much lesser magnitudes occur
each year in the inland Pacific Northwest region. Most of these
are too small and only about a dozen are large enough to be felt.
However, since 1872, there have been about 25 damaging earthquakes.
In the 20th century, about 17 people have lost their lives from
the larger events. The following table lists historical earthquakes
of magnitude 5 or greater that have occurred near Puget Sound
or Portland since 1877.
Historical
Earthquakes (Magnitude >5) since 1877 |
| Year |
Mo/Day |
Mag |
Location |
| 1877 |
|
5.3 |
Near Portland |
| 1904 |
|
5.3 |
Puget Sound |
| 1909 |
|
6.0 |
Puget Sound |
| 1932 |
|
5.2 |
Puget Sound |
| 1939 |
|
6.2 |
Puget Sound |
| 1945 |
|
5.9 |
Puget Sound |
| 1946 |
February 23 |
5.0 |
SE of Olympia |
| 1946 |
|
6.4 |
Puget Sound |
| 1946 |
February 15 |
5.8 |
N of Olympia |
| 1949 |
April 13 |
7.1 |
ENE of Olympia (8
deaths) |
| 1962 |
|
5.5 |
Near Portland |
| 1965 |
April 29 |
6.5 |
Seattle /Tacoma
(31 injured) |
| 1993 |
|
5.5 |
Near Portland |
| 1995 |
January 28 |
5.0 Shallow |
Puget Sound (Seattle
- Tacoma) |
| 1996 |
|
5.3 Shallow |
Puget Sound |
| 1999 |
|
5.1 |
Puget Sound |
| 2001 |
February 28 |
6.8 |
Puget Sound |
Most of the events
were associated with deep Benioff zone earthquake activity (deeper
focal depth). The 1995 and 1996 quakes were the only exceptions
in that they were shallow crustal events.
The Pacific
Northwest's Earthquake Risk 
According to recent
reports the earthquake risk for the Pacific Northwest is much
greater than what was previously assessed. As reported above,
claims have been made that earthquakes with moment magnitudes
of 8 or 9 occur along the Cascadia Subduction Zone (Cascadia
Megathrust). Recent geological studies have concluded that 13
great earthquakes (known as megathrust events with magnitudes
of 8 to 9) have occurred along the Cascadia Subduction Zone in
the last 6000 years.
If indeed such rare
great, shallow earthquakes occur as postulated, they are capable
of releasing 30 to 1,000 times more energy than quakes of magnitude
7. If they recur - as claimed - along the Cascadia's Subduction
Zone, they will be by far more destructive than the inland earthquakes
of February 28, 2001, the 1949, or 1965 (in the Seattle-Tacoma-
Puget Sound area). Also, the Seattle Fault and the Portland Hills
Fault are two major faults near large metropolitan areas with
the potential of producing damaging earthquakes of up to 7 or
even 7.5 in magnitude.
In response to this
potential threat, the Uniform Building Code was revised in 1994
for the states of Washington and Oregon. This revision of the
Code was an important first step toward meeting the purported
great earthquake threat in the Pacific Northwest. The new Code
has extended the higher level hazard zoning to include parts
of Washington and Oregon that are near potential sources of great
earthquakes. All new structures are now designed to resist earthquake
forces 50% stronger than they were under the old code.
The Tsunami Risk of
the Pacific Northwest.
The February 28, 2001
earthquake did not produce a noticeable tsunami in Puget Sound
or on the open coast because of its deeper focus and because
it occurred inland, along the Benioff seismic zone.
Earthquakes in the
Benioff zone do not produce tsunamis of any significance. However,
surface seismic waves can induce seiche activity within Puget
Sound itself. Also, surface waves can induce ground liquefaction
of heavily sedimented coastal areas of Puget Sound and thus trigger
subearial or subaqueous landslides which, in turn, can generate
destructive waves.
Major, shallow earthquakes
occurring on the Seattle Fault, have also the potential to generate
damaging tsunami activity within Puget Sound - although shallow
crustal events, the earthquakes of 1995 and 1996 - in the Seattle/Tacoma
area - had magnitudes (5 and 5.1 respectively) which were too
low for tsunami generation.
Diagram
showing the Gorda and Explorer tectonic plates and the locked
portion of the Juan De Fuca Subduction Zone where great megathrust
events can occur and large tsunamis generated. (Canadian Geologic Survey graphic)
Past earthquakes along
the Cascadia Subduction Zone probably generated significant tsunamis
along the coastal areas of Vancouver, Washington, Oregon, and
northern California. The most significant of these was presumably
the one generated by a megathrust event on the Cascadia Subduction
Zone more than 300 years ago, sometime in the winter in late
1699 or early 1700. According to some scientists this Cascadia
earthquake generated a Pacific-wide tsunami that was destructive
in Japan. Whether there is indeed a connection between the earthquake
in Cascadia and the tsunami in Japan has not been established
with certainty because no tsunami was observed or reported anywhere
else. It is very possible that the observed tsunami in Japan
had a different source.
Future megathrust
quakes on the Cascadia Subduction Zone can be expected to involve
large vertical crustal displacements of the ocean floor along
an offshore rupture zone which may be extensive - depending on
the magnitude of the earthquake. However, it should be pointed
out that the Cascadia Subduction Zone is highly fragmented and
this may limit rupture lengths and earthquake magnitudes. Thus,
major earthquakes in the region are possible and have the potential
of generating destructive tsunamis which could affect the entire
coastal area from British Columbia to northern California, as
well as Puget Sound. For example, even an intermediate size earthquake
on the southern segment of the Cascadia Subduction Zone could
result in a local tsunami which could be somewhat damaging not
only in Washington and Oregon but , particularly at Humboldt
and Del Norte Counties in Northern California. Coastal towns
like Eureka, Arcata, Petrolia, Fortuna and Crescent City could
definitely be affected. Tsunami waves would reach coastal communities
in Vancouver Island and in Washington, Oregon and northern California
within minutes after an earthquake, most likely without warning
(Pararas-Carayannis 2000). It is difficult to estimate the recurrence interval
of tsunamis since larger earthquakes on the Cascadia Megathrust
are infrequent. Most of the recent earthquakes which occurred
somewhat west of the Cascadia Subduction Zone, had magnitudes
of about 7.2 and generated relatively small local tsunamis.
Reference
Pararas-Carayannis,
George, The Big One - The Next Great California Earthquake, Aston Forbes Press, 345 pp. 2000
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©1982 - 2011 Dr. George Pararas-Carayannis - All rights reserved
Last update: OCT 3, 2011 |
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