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
There is strong evidence that great
earthquakes (magnitude 8 to 9) have repeatedly struck the Pacific
Northwest in the past several thousand years - the most recent
about 300 years ago, on January 26,1700. Although great earthquakes
such as this are relatively rare, more than 1000 earthquakes
occur each year in the 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 
The earthquake risk for the Pacific
Northwest is much greater than what was previously assessed.
Contrary to common belief, earthquakes occurring in the region
are not limited to magnitude 7 or less.
Recent geological studies have concluded
that, although infrequent, great earthquakes with magnitudes
of 8 or 9 can occur. There is preponderance of geologic evidence
documenting 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.
Although relatively rare, these great,
shallow earthquakes are capable of releasing 30 to 1,000 times
more energy than quakes of magnitude 7. When they recur, 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 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 tsunami 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)
There is also conclusive geological
evidence that past megathrust earthquakes generated large tsunamis
along the coastal areas of Vancouver, Washington, Oregon, and
northern California. The latest of the great historical tsunamis
in the Pacific Northwest was generated by a megathrust event
on the Cascadia Subduction Zone more than 300 years ago, in January
of 1700. Another, similarly large and potentially destructive
tsunami can be expected in the future.
Future megathrust quakes on the Cascadia
Zone are expected to involve large vertical crustal displacements
of the ocean floor along an offshore rupture zone which may extend
for hundreds of miles - depending on the magnitude of the earthquake.
Such earthquakes 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 tsunami
which could be 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 would definitely be affected. Tsunami waves
would reach coastal communities in Vancouver Island and in Washington,
Oregon and northern California within minutes after a megathrust
earthquake, most likely without warning. (Pararas-Carayannis
2000).
However, it is difficult to estimate
the recurrence interval of such a historical tsunamis since megathrust
earthquakes are infrequent and have occurred at irregular intervals
separated by hundreds of years. A detailed analysis of the tsunami
risk for the Pacific Northwest and documentation of the great
tsunami of 26 January 1700, will be posted soon at the author's
Tsunami
Page
Reference:
Pararas-Carayannis, George, The Big One - The Next Great California
Earthquake, Forbes Press, 345 pp.
2000
RETURN TO
The Next Great
California Earthquake
© 2001 George Pararas-Carayannis
/ all rights reserved
|