Nine west-northwest-trending faults on the continental margin of
Oregon and Washington, between 43° 05'N and 470 20'N latitude, have been
mapped using seismic reflection, sidescan sonar, submersibles, and swath
bathymetry. Five of these oblique faults are found on both the Juan de Fuca
and North American plates, and offset abyssal plain sedimentary units left-laterally
from 2.0 to 5.5 km. These five faults extend 8-18 km northwestward
from the deformation front. The remaining four faults, found only on the North
American plate, are also inferred to have a left-lateral slip sense. The age of
the Wecoma fault on the abyssal plain is 600±50 ka, and has an average slip
rate of 7-1 0 mm/year. Slip rates of the other four abyssal plain faults are 5.5 ±
2 - 6. 7 ± 3 mm/yr. These faults are active, as indicated by offset of the
youngest sedimentary units, surficial fault scarps, offsets of surficial channels,
and deep fluid venting. All nine faults have been surveyed on the continental
slope using SeaMARC 1A sidescan sonar, and three of them were surveyed
with a high-resolution AMS 150 sidescan sonar on the continental shelf off
central Oregon. On the continental slope, the faults are expressed as linear,
high-angle WNW trending scarps, and WNW trending fault-parallel folds that
we interpret as flower structures. Active structures on the shelf include folds
trending from NNE to WNW and associated flexural slip thrust faulting; NNW to
N trending right-lateral strike-slip faults; and WNW trending left-lateral strike-slip
faults. Some of these structures intersect the coast and can be correlated
with onshore Quaternary faults and folds, and others are suspected to be
deforming the coastal region. These structures may be contributing to the
coastal marsh stratigraphic record of co-seismic subsidence events in the
Holocene.
We postulate that the set of nine WNW trending left-lateral strike-slip
faults extend and rotate the forearc clockwise, absorbing most or all of the arc
parallel component of plate convergence. The high rate of forearc
deformation implies that the Cascadia forearc may lack the rigidity to generate
M > 8.2 earthquakes. From a comparison of Cascadia seismogenic zone
geometry to data from circum-Pacific great earthquakes of this century, the
maximum Cascadia rupture is estimated to be 500 to 600 km in length, with a
150-400 km rupture length in best agreement with historical data. / Graduation date: 1994
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/36664 |
| Date | 31 January 1994 |
| Creators | Goldfinger, Chris |
| Contributors | Yeats, Robert S. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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