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Pacific Coastal & Marine Science Center

Coastal and Marine Earthquake Studies

 

The Cascadia Megathrust and Tectonic Stress in the Pacific Northwest

Results

Pacific Northwest Stresses and Faults See full-size image (60 kb)

Explanation

Red Symbols: Orientation of faults predicted by the model. Rectangle represents normal faulting, line with diamond ends represents thrust faulting, and X represents two possible orientations of strike-slip faulting (see faults).

Shaded Blue Regions: Regions deforming at a higher rate than the interior part of North America.

Yellow Dots: Recent and important historical earthquakes in the Pacific Northwest.

The model shown above provides the best fit to the present-day stress and deformation field. For this model, the basal shear traction along the Cascadia megathrust is 15 MPa. Sensitivity analysis demonstrates that basal shear traction lower or higher than 15 MPa result in a greater misfit to the stress data and style of faulting. Conversely, the present-day stress field appears to be insensitive to shear traction less than 30 MPa along the transform margins. (Many independent studies indicate that the long-term traction along the San Andreas is less than 20 MPa.)

Tectonic Cause of Deformation in the Pacific Northwest

If shear traction along the San Andreas fault is not causing the deformation observed in the Pacific Northwest then what is? A clue is apparent by looking at the vector velocity field for the Pacific Northwest which can best be described as a broadly clockwise rotating velocity field:

Shear Stresses

The highest velocity is in coastal British Columbia and is in a direction approximately parallel to the component of Pacific-North America plate motion that is perpendicular to the Queen Charlotte Fault. Plate motion studies demonstrate that the motion between the Pacific and North American plates is not exactly parallel to the Queen Charlotte fault, resulting in compressive stress perpendicular to the fault. This study suggests that this compressive stress contributes to deformation in the Pacific Northwest

Another way to view the major plate bounding faults is that although the Queen Charlotte, Cascadia megathrust, and San Andreas faults are weak in terms of overall shear strength, small components of compressive stress perpendicular to the fault plane can lead to significant deformation within the continental plate.

It is important to note that a minimum basal shear traction is required along the Cascadia megathrust to explain the observed deformation. If the basal shear traction is less than 10 MPa, then Basin-and-Range normal faulting that is typically mapped in Nevada would occur throughout the Pacific Northwest--clearly contrary to the observed style of faulting. Spreading of the continental crust in the Basin-and-Range region, however, may also be driving deformation in the Pacific Northwest.

Good Predictions

The N-S orientation of horizontal compressive stress (resulting in E-W oriented thrust faulting--see faults) is generally reproduced by the model. Also, the predicted clockwise rotation of the compressive stress axis towards coastal Oregon and Washington is consistent with stress data within the Cascade Convergence Stress Province, suggesting a smoothly varying stress field not ascribed to N-S changes in shear traction along the subduction zone (magnitude of basal shear traction specified as constant in the model).

The predicted style of faulting is consistent with geologic mapping of recent faulting in the Pacific Northwest. That is, E-W thrust faulting in NW Washington (such as the Seattle fault); thrust faulting mixed with N-S strike-slip faulting (such as observed in the St. Helens region, and the 1993 Scotts Mills earthquake: see the Pacific Northwest Seismic Network for more information) in NW Oregon and SW Washington; and normal faulting in eastern Oregon and Idaho.

Bad Predictions

The model results do not correctly predict long-term deformation local to the triple-junctions. Specifically, in the northern Vancouver Island region and near Cape Mendocino. The mismatch between the predicted and observed stress and deformation suggests that the imposed boundary conditions are not correct for the Explorer plate (offshore Vancouver Island) or the Gorda deformation zone. The model does not predict extensional deformation in southeastern Oregon, site of the 1993 Klamath Falls earthquake sequence (see the Pacific Northwest Seismic Network for more information).