Geology Reference
In-Depth Information
FIGURE 11.8
Lake Erie annual mean water levels, in meters,
1860-2005.
(Modified from NOAA)
PART C. TSUNAMI HAZARDS
ON THE WEST COAST
Tsunamis
The tsunami of December 2004, in the Indian Ocean,
reminded the world about the destructive impacts
these waves can have. This tsunami, triggered by a
magnitude 9.1 earthquake, killed an estimated 200,000
people. The waves reached heights of 30 meters
(approximately 100 feet), and destroyed many build-
ings and communities.
The size of the rupture zone that was associated
with the Indian Ocean earthquake has been approxi-
mated by study of the distribution of aftershocks. This
rupture zone, when overlaid on a map of the Pacific
Northwest (Figure 11.9), shows that a similar earth-
quake could be anticipated adjacent to the United
States. This local earthquake could also trigger a local
tsunami. In January 1700, the last time a major earth-
quake shook the northwest, a tsunami traveled across
the Pacific Ocean and was recorded in Japan.
The Pacific coast of the United States is also sub-
ject to tsunamis that have been generated by earth-
quakes elsewhere in the Pacific Ocean. A tsunami that
was triggered by the 1964 Alaskan earthquake caused
damage along the west coast of the United States. In
addition to the amount of energy released by the earth-
quake and the proximity to the coast, the height of a
tsunami at a specific place is influenced by the tidal
stage when the waves hit, tidal currents, topography of
the land (how much low-lying land is along the shore),
changes in topography due to earthquake-related
uplift or subsidence, and the bathymetry (depth and
shape) of the seafloor immediately offshore.
Coastal Processes
When tsunamis impact barrier islands, spits, and broad,
low-sloping coasts, or where they travel far in kind
along rivers the destruction of engineered structures
and modifications of landforms can be rapid and very
significant. Tsunamis can be survived, if individuals
are educated about warning signs and actions to take
(Atwata and others, 1999). Communities must also be
prepared (Samant and others, 2008), for example by
having clearly marked evacuation routes.
The plate tectonic setting of the coast of Washing-
ton, Oregon, and northern California is generally simi-
lar to the plate tectonic setting of the Sumatra
earthquake of 2004. In the area of Sumatra, the India
Plate is subducting beneath the Burma microplate. In
the Pacific Northwest, the Juan de Fuca plate is sub-
ducting beneath the North America plate. The Sumatra
earthquake had a magnitude of 9.1, and it is antici-
pated that the Pacific Northwest could have a similar
size event, which could also unleash a major
tsunami.
