Geology Reference
In-Depth Information
Structure of the Willamette Valley (after Yeats, et
al., 1991; Graven, 1990; Werner, 1990).
forms what is called a fault "pop-up" caught between a
series of intersecting fault blocks. With continuous
movement of both the Portland Hills and Mt. Angel
faults, the terrain trapped between them displays a
clockwise rotational movement.
Within the southern Willamette Valley, a
series of gentle north-south folds in the subsurface
separate parallel faults. Eastward from Corvallis a
network of normal and thrust faults extends in a criss-
crossed network across to the Western Cascades. These
faults were active as far back as Oligocene, but some
show signs of Pleistocene movement. The Corvallis
fault moved in two phases. In an earlier thrusting
phase during the Eocene, the western upper plate was
pushed eastward toward the valley while later tensional
forces saw the development of a normal fault here with
a down-dropped eastern block.
Earthquakes
Research into the relationship between move-
ment of crustal plates and earthquakes has led to
predictions that Oregon could experience an earth-
quake as high as magnitude 8 on the Richter scale. The
point of origin for such massive quakes would be
offshore along the subduction zone where the Juan de
Fuca plate is passing under the North American plate.
In the coastal region of Oregon good evidence for
catastrophic historic earthquake activity is found in the
form of sunken coastal bogs and distinctive turbidite
deposits offshore.
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