UNT Research, Volume 16, 2006 Page: 15
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unt. edu/untre s e arch
Although the concept of a tsunami
striking the United States may seem
strange, the possibility is far from fiction.
Because of its position along the Cascadia
subduction zone - the area where the
undersea Juan de Fuca plate is being
shoved, sometimes violently, under the
continental North America plate - the
Pacific Northwest coast is highly suscep-
tible to submarine earthquakes, the primary
source of tsunamis.
Often incorrectly referred to as a
tidal wave, a tsunami is actually a sequence
of waves - called a wave train - that
resembles a massive, onrushing tide. It was
this rapid surge of water that devastated
Native American groups in the Pacific
Northwest, inspiring the Thunderbird
and Whale stories.
Geological data suggest that the last
major tsunami in the region occurred in
1700 A.D., a year that matches up with
many of the stories. Generated by a power-
ful earthquake (estimated at magnitude
9.0), the tsunami caused flooding and
damage as far away as Japan.
"The event was probably similar
in size to the one that hit South Asia
in 2004," says Williams. "And," he
adds, "there's a good chance it will hap-
To better predict future tsunami
events, Williams looks to the past. Using
a hollow, metal cylinder called a soil
corer, he tunnels down several feet through
the sediments of the tidal marshes near
Discovery Bay, going farther backward in
time the deeper he digs. The retrieved col-
umn of soil serves as a chronological record
of events, with layers of sand indicating
the incidence and approximate time of
"A tsunami can pick up sand from the
lower tidal zone, or even offshore, and
carry it into the high marsh," says Williams.
"The result is an anomalous sand layer that
becomes buried in the marsh."
Gathering this type of data is no easy
task. Williams takes hundreds of soil cores
from marshes in northwest Washington and
on southwest Vancouver Island in British
"For every site where I've found tsunami
sand layers, I've probably dug into 10 other
marshes and found nothing," he says. "But
the effort is worth the excitement of dis-
covering a tsunami sand layer that no one
else has found before."
Williams determined that the tsunamis
that hit the Discovery Bay area in the past
2,500 years had multiple origins. Sources
included subduction zone earthquakes
generated by the Cascadia subduction zone
and, to a lesser extent, crustal earthquakes
originating from shallow faults and subma-
Williams also found that the shape of
Discovery Bay - wide near the open ocean
and tapering to a point further inland -
causes tsunamis to become amplified. As
water from the ocean is funneled into the
bay, it becomes increasingly confined, as the
same volume of water is forced into a
smaller space. The result is a wave train
with taller waves and a greater potential to
While he admits it's difficult to know
exactly how high the ancient tsunamis'
waves were, Williams guesses they reached
at least 6 feet and might even have achieved
heights of 20 feet.
Geographer Harry Williams, who studies sediments to uncover the patterns of ancient
tsunamis, now pians to examine storm surge deposits left by hurricanes. His research
may contribute to improved preparedness in the future.
"Clearly, if I'm right," he says, "a
house within 6 feet of high-tide level is
Fortunately, most of the homes in the
area today are perched on hillsides, out of
harm's way. But a few stores along Highway
101, beyond the marsh, are below the 6-
foot high-tide level.
"There are some big cities in the
Pacific Northwest, but not many people
live that close to the coast, so the loss of
life would be a lot less than it was during
the recent tsunami in South Asia," says
Williams. "But," he adds, "even one life
lost is too many."
The tsunami isn't the only member
of the natural disaster family that may leave
behind clues in coastal marsh sediments.
"Much of my expertise on tsunami
deposits can be adapted to studying hurri-
cane deposits," explains Williams, who says
that last year's hurricane season inspired
him to study hurricanes.
"I think hurricanes leave a geologic
record in coastal marshes, so the potential
exists to study them with similar applica-
tions used to study tsunamis," he says.
With funding from the National
Science Foundation, Williams plans to
examine storm surge deposits left by
Hurricane Rita in the coastal marshes of
He will identify the source and com-
position of the deposits and then
will resurvey the area 18 months later to
find out how much of the deposits were
incorporated into the marsh. The work
will contribute to an understanding of
Williams hopes that results of this
study, together with investigations he is
planning of ancient hurricane storm
surge deposits in Louisiana, will help
determine which areas are at high risk
for damage from hurricane sediment
UNT RESEARCH 2006
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University of North Texas. UNT Research, Volume 16, 2006, periodical, 2006; Denton, Texas. (digital.library.unt.edu/ark:/67531/metadc29777/m1/15/: accessed September 24, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting University Relations, Communications & Marketing department for UNT.