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The February 28, 2001 Nisqually (Seattle) earthquake struck
the south Puget Sound area at 10:55 am (PST) causing minor
building damage over portions of western Washington. This
earthquake, with a moment magnitude Mw = 6.8, was slightly
smaller than the 1949 earthquake in the Olympia area which
measured 7.1. Damage from the Nisqually earthquake was also
less than the 1965 Seattle-Tacoma earthquake, which had a
magnitude of 6.5. Because the recent Nisqually earthquake
occurred at a depth of 32 miles (52 km), ground shaking and
associated structural damage were relatively limited, although
the earthquake was felt throughout most of the populated areas
of western Washington. It is postulated that the fault responsible
for the earthquake is 18.5 x 6 miles (30 x 10 km) in area
and had approximately 3 feet (1 meter) of vertical movement.
This movement occurred within the descending ocean crust,
which acts like a conveyor belt transporting the ocean floor
under the North American continent. Maximum accelerations
were approximately
0.3 g in the Olympia and Seattle areas compared to a maximum
of approximately 0.93 g in the 1994 Northridge earthquake
and 0.64 g in the 1989 Loma Prieta earthquake. Ground shaking
from the earthquake tended to be strongest in river
valleys draining into Puget Sound. Liquefaction, which
occurs when strong shaking in loose, water-saturated sandy
soils causes the soils to liquefy much like quick sand, resulted
in sand boils and lateral spreading that was usually found
in low-lying areas around Puget Sound. Numerous areas experienced
landslides, and there are reports of at least one rockslide
in the Cascades and an underwater slide in Lake Washington.
Structural damage was most prevalent in older unreinforced
masonry buildings and non-structural members, such as chimneys
and parapet walls. Failure of masonry walls, parapets, and
chimneys occurred in Olympia, Seattle, Tacoma, and their outlying
environs. Minor damage was found in wood frame, concrete,
and steel frame structures. Much of the damage was to nonstructural
portions of buildings, including contents and architectural
finishes. Some of the more widely reported damage, such as
the control tower at the Seattle-Tacoma International Airport,
was a result of its age and design; for the most part steel
and concrete frame buildings fared well. Minor structural
damage has also been observed on a number of the area’s
bridges, such as spalled concrete on support columns and flaked
paint where steel members yielded, but only four of the area’s
older bridges received significant damage. Fortunately damage in the Puget
Sound area from deep-seated earthquakes over the past 50 years
has been relatively limited, however historical records suggest
that larger and more damaging earthquakes associated with
subduction zone and crustal earthquakes have occurred throughout
history. Read more about how the
Nisqually earthquake compares with other major earthquakes.
Dr. John D.
Osteraas, P.E., a Principal Engineer and Practice
Director of Exponent's Civil/Structural
Engineering practice based in Menlo
Park, California, specializing in issues related to structures,
soils, earthquakes and construction, including structural
safety and damage assessment, structural and construction
failure analysis, building design, and construction technology.
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