Puget Sound Tsunami
The Hammons Preserve provides a sweeping southward view of
Cultus Bay. Though it looks idyllic now, geologists have uncovered
evidence of a tsunami that occurred there more than a thousand
years ago.
Tsunami is a Japanese word that means "harbor wave."
Earthquakes, volcanic eruptions, landslides, and other mass
movements above or below water all have the potential to generate a
tsunami. The best way to imagine one is to picture yourself jumping
into a bathtub filled with water. Your body will displace so much
of the water that it will cause a large wave to slosh over the
side.
In the case of Cultus Bay, the tsunami was thought to have been
created by an earthquake along the Seattle Fault. This shallow
fault is located 40 km south of Cultus Bay and crosses underneath
the Puget Sound in an east-to-west direction from Seattle to
Bainbridge Island (fig. 1). Geological studies have shown that a
significant earthquake with an estimated magnitude of 7 or higher
occurred along this fault approximately 1,100 years ago. The
evidence includes terraces in the Seattle area that were abruptly
uplifted. At Restoration Point, located across the Puget Sound from
Seattle, the land was raised 7 meters. Carbon-14 dating of organic
material from these sites confirmed the approximate date of the
earthquake.
Figure 1
In 1992, Andrew Moore and Brian Atwater of the University of
Washington looked for evidence of a tsunami generated by this
earthquake. Specifically, they looked for the presence of sand
sheets, which tsunamis commonly deposit as they inundate coastal
lowlands. Moore and Atwater chose Cultus Bay as a possible site for
such evidence, because it was likely to have been in the path of a
tsunami created by movement along the Seattle Fault.
Since prehistoric times, a tidal saltwater marsh at Cultus Bay
has slowly built wetland peat. The researchers augured 150 cores of
sediment samples from a 100 by 200 meter area of the marsh and also
examined more than 100 meters of a drainage ditch that was dug more
recently. They confirmed that a layer of sand up to 15 cm thick was
interbedded with the peat (fig 2). Furthermore, as they followed
the sand sheet landward, they found that median grain size of the
sand particles decreased. This strongly suggested that the sand was
deposited by a tsunami and was not the result of a flood or storm
carrying sand from the land into the bay. The sand sheet extends
landward to a height of 4.5 m above present-day low tide and
contains microscopic marine fossils, as well as fossilized plants
such as arrowgrass. Radiocarbon dating of these organic remains
showed that the sand sheet was deposited approximately 1,100 years
ago, consistent with the date of the Seattle Fault
earthquake.
Figure 2
When looking out over the water from the Hammons Preserve,
it’s easy to forget the potential for devastating earthquakes
and tsunamis in the Puget Sound. But buried on the shore of Cultus
Bay is testimony that such cataclysmic events have occurred in the
past and will again.
There’s no reason to fear tsunamis, though. Just bear this
simple rule in mind: if you’re on a Puget Sound beach
and feel an earthquake, move to higher ground. Stay there
until the National Oceanic and Atmospheric Administration (NOAA)
gives an "all clear" via local media.
About the Hammons Preserve
Long-time Whidbey Island resident Alvin Hammons was a veteran of
World War II and a retired railroader. He lived on his beloved
Whidbey Island family farm for 40 years. There, Al found beauty and
serenity in its gently sloping agricultural fields, orchard,
forested edges and sweeping unobstructed views of Cultus Bay.
Al’s wish was to keep this land as a place where others could
come and enjoy the quiet beauty, raptors and deer that gave him so
much pleasure during his lifetime. His dream was to have his farm
turned into a wildlife refuge where all are welcome to come and
“rest their souls awhile.” His family donated his farm
to the Whidbey Camano Land Trust and the community.
After several years of volunteer restoration efforts that
included removing dilapidated farm structures, restoring a wetland,
and controlling blackberries, it’s easy to see that the
community has continued to put love and energy into this beautiful
place. We are proud to continue Al’s legacy and hope you
enjoy your time at the Hammons Preserve.
The Whidbey Camano Land Trust’s mission is to protect the
Islands’ most important natural habitats, scenic vistas and
working farms and forests in partnership with landowners and our
island communities. It is a 501c3 nonprofit organization.
References
Atwater, B.F. and Moore, A.L. (1992): A tsunami about 1000 years ago in Puget Sound,
Washington. Science 258:1614-1617.
Moore A.L. (2001): Grain-size trends in a Holocene tsunami deposit
from Cultus Bay, Puget Sound, Washington. International Tsunami
Society 2001 Proceedings, Session 3, Number 3-6:503-509.
Koshimura S., Mofjeld H.O., Gonzalez F.I., and Moore A.L.
(2002): Modeling the 1100bp paleotsunami in Puget Sound,
Washington. Geophysical Research Letters 29:9-1 to 9-4.
To claim this EarthCache
- Look for two small plaques on the side of the footbridge facing
the bay. Identify who built the bridge.
- Answer these questions:
- 1,100 years ago, the Puget Sound region was sparsely populated,
and the tsunami did relatively little damage. How would a tsunami
of the same size impact the structures around Cultus Bay
today?
- Computer modeling of this earthquake and tsunami (ref. 3)
suggest that it would take twenty minutes for the tsunami to reach
Cultus Bay from the Seattle Fault. From this, calculate the speed
of the tsunami expressed in kilometers per hour.
While it is not a logging requirement, we'd love to see photos
of you enjoying the Hammons Preserve.
The Hammons Preserve is owned by the Whidbey Camano Land
Trust, which is committed to protecting our Islands' natural
habitats, scenic views, and working farms. Please consider becoming
a member of the Land Trust and helping us protect the places we all
cherish most.
If you see anything that requires our attention, please call us
at 360.222.3310 or email our site stewards at hammons@whidbeycamanolandtrust.org.
The Whidbey Camano Land Trust extends its heartfelt thanks
to John Boone for preparing this EarthCache and to Grant Heiken for
reviewing and contributing to it.