Geology Reference
In-Depth Information
Portland is on the down-thrown side of the Portland Hills fault shown here with the Tualatin Mountains
rising off to the west (left) (photo courtesy Delano Photographies).
The lack of ongoing strong seismic activity
along the boundaries of Pacific Northwest plates may
be due to an unlocked subduction zone with virtually
frictionless movement. However, this seems unlikely
when comparing Oregon's continental margin and
subduction zone to those of southern Chile where very
young oceanic crust is being subducted in both loca-
tions. Beneath Oregon the 10 million year old crust is
still warm and consequently very buoyant making it
difficult to subduct. Rocks of the warm descending
crust also tend to adhere to the overlying North
American plate, thus the boundary between the two
readily binds up and locks. As the plate collision
process continues at the rate of about 1 inch per year,
the bond will eventually break with catastrophic results.
In 1960 Chile experienced a 9.5 magnitude earthquake,
the largest of the 20th century.
In contrast to China where the record of
moderate and major quakes goes back 3,000 years or
California where Franciscan fathers began records in
1800, there is only scanty historical data on seismic
activity in Oregon. Indian legends of more than 200
years ago tell about massive earthquakes as well as
destructive tidal waves, but these stories cannot be
Fault pop-up or flower structure beneath Mt. Angel,
Oregon (after Werner, 1990; Yeats, et al., 1991).
