Between earthquakes, when the oceanic and continental
plates are locked, internal stress stored by the interacting plates
slowly deforms the land, pushing it upward and inland. When the locked plates
slip, the toe of the subduction zone moves seaward and up, and the uplifted land drops
to a lower position.
Two features of coastal Washington demonstrate the subsidence characteristic
of subduction zone earthquakes: buried lowlands and sequences of soil
horizons.
Buried Lowlands
When the land elevation falls in a coastal region, some fresh-water lowlands
drop below sea-level. Vegetation in these areas is exposed
to seawater and dies. There are many areas that show this kind of devegetation
along Washington’s
southern coast, from Grays Harbor to the lower Columbia River.
The most obvious of these buried lowland areas are the standing groves
of dead Western red cedar common along the coast between the Copalis
and Columbia rivers.
Soil Horizons
Soils also record the environment of the lowlands in their sediment layers.
When the land surface drops abruptly during great earthquakes,
large number of freshwater coastal plants die and leave
evidence in the form of dark soils, called "peat".
At low tide, the soil layers are exposed in riverbanks. Here, on Oregon's Salmon River,
the dark layer of vegetable matter is viewed from the side as a
"peaty horizon". It contains freshwater plant fossils, and is
overlain by tsunami sands, which are then covered by grey-green layers of marine
plants and sediments. Two campfire pits with fire-cracked rocks, from
Native American encampments, are also buried beneath the tsunami and marine deposits.
Over time, accumulated vegetation and sediment rebuild the land surface, and fresh water
plant assemblages reestablish.
Multiple cycles of fresh-water peaty horizons, overlain by tsunami and marine deposits, are found
in coastal lowland soils of Washington and Oregon,
Brian Atwater, of the USGS, has studied and dated these layers at many places along our coasts. He finds evidence for seven cycles of sudden submergence during the last 3,500 years - an average of 500 years between events, with the most recent
horizon dated by Carbon 14 to between 320 and 410 years ago.
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