Geology Reference
In-Depth Information
Although it may seem that the Willamette
Valley and Portland have not had recent earthquakes,
the contrary is true. Beneath Portland and the northern
Willamette Valley a network of faults exist which could
become active at any time. In 1990 earthquakes record-
ed beneath Mt. Hood were similar to quakes occurring
there in 1974, 1982, and 1989. The Mt. Angel fault in
the central valley was the apparent source of dozens of
small quakes in 1990. After 1882 Portland experienced
a recorded quake on the average of once every 5 years.
Brick buildings swayed, windows rattled, and terrified
inhabitants rushed to the street during the tremor of
February 3, 1892, although there was no damage. In
1962 and again in 1968 magnitude 3.7 and 5.0 quakes
repeatedly struck Portland from epicenters on the
eastern edge of the city. Shocks were due to movement
of a fault as much as 10 to 15 miles deep. Centered in
the Gorda plate off the southern coast, an earthquake
of July 12, 1991, registering 6.6 was felt in many
communities of western Oregon. A series of quakes on
July 22, 1991, measuring 3.5 were focused in the
Tualatin Mountains.
Because of the lack of recorded quake activity,
few of the larger buildings in the Willamette Valley
and Portland were designed to sustain powerful seismic
shaking. Many of the building foundations in the
northern valley are on water saturated sediment. These
soils continue to shake even after the quake has
stopped somewhat like a bowl of jello on a table. This
type of shaking accentuates the destructive effects.
Additionally, the lack of cohesion of many of the
Portland and northern valley soils would cause them to
flow during a quake. This slope failure, or soil liquifac-
tion, not only destroys structural foundations, but the
motion triggers disasterous mud slides and flows that
spread destruction laterally. Combined with the inevita-
ble movement along the subduction zone and the dense
population of the valley, there is the potential for a
disaster of unprecedented size when the predicted
strong earthquake occurs.
Recorded earthquake activity is low in the
eastern sections of the state. In May and June of 1968
earthquakes caused rockslides and building damage in
southcentral Oregon where the epicenter was located
between Crump Lake and Hart Lake in the Warner
Valley. The largest quake on May 29 registered 5.1 on
the Richter scale. Although not well understood,
seismic processes in central Oregon are related to
stresses within the Basin and Range province. A
slightly milder earthquake of magnitude 4.8 centered
northwest of Maupin in the Deschutes Valley, shaking
central Oregon on April 12, 1976. Inhabitants stated
that their houses shook and swayed although there was
no serious damage. The Blue Mountain province, an
area of little known earthquake activity, experienced a
shock north of Baker registering 3.6 on August 14,
1969. Earthquake history of this region is very brief
with only two other noted shocks in 1906 and 1916.
The proximity of subduction faults along with
loose sediments and unconsolidated soils make the
Willamette Valley particularly susceptible to quakes.
Below the Cascades and eastern Oregon, the great
depth of the subduction zone tends to reduce risk,
whereas the coastal area close to the quake sources is
perhaps at greatest peril.
Mining and Mineral Industry
Bauxite
The residual iron and aluminum ores in the
Willamette Valley developed as deposits on top of
basalts of the Columbia River series where much of the
surface of the Miocene lava is deeply weathered. As
rock decomposition proceeds, soluble chemicals are
leached out leaving a clay soil with the highly insoluble
bauxite enriched with iron and aluminum. Limited
ferruginous bauxite deposits in the Pacific Northwest
are located in scattered sections of Columbia, Washing-
ton, Multnomah, Yamhill, Clackamas, and Marion
counties and in Cowlitz and Wahkiakum counties in
adjoining Washington state. The thickest deposits are
in Washington and Columbia counties thinning toward
Salem.
Iron
Within the sequences of Columbia River
basalts, the iron oxide mineral limonite developed in
bogs along undrained depressions. Several attempts
were made to mine and process the limonite near
Scappoose and Lake Oswego before 1867 when furnac-
es of the Oregon Iron and Steel Company produced
the first iron west of the Rocky Mountains. Unfortu-
nately financial problems and difficulties with stock-
holders hampered the operation throughout its history,
and even the construction of an adjacent pipe foundry
utilizing pig iron failed to keep the furnace going.
Despite its stormy history, the company produced
83,400 tons of pig iron from 1867 to 1894. Several
thousand feet of old tunnels still exist under the Lake
Oswego Country Club and nearby Iron Mountain. The
furnace used for smelting the iron ore, located near the
outlet of Lake Oswego, is now part of the Lake Oswe-
go City park.
Features of Geologic Interest
Buttes
Although most of the Willamette Valley is flat
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