Implications

Coastal Uplift and Locking of the Megathrust between Big Earthquakes

Measurements of uplift along coastal Washington and Oregon have been interpreted that the Cascadia megathrust is currently locked--typical of the deformation that occurs in the period between major earthquakes along a subduction zone megathrust. An important distinction must be made between long-term deformation (millions of years) such as in this study and deformation associated with the earthquake cycle (typically hundreds of years).

The long-term uplift along coastal Washington and Oregon is predicted by the model to be very low, supporting the interpretation that the measured uplift is almost solely due to strain accumulation between megathrust earthquakes. The predicted long-term uplift is significantly higher in coastal British Columbia and is ascribed to shortening of the continent perpendicular to the Queen Charlotte fault.

Patterns of Seismic Strain Release in the Pacific Northwest

The predicted rate of deformation of the continent in Oregon and southern Washington is relatively low compared to regions to the north and south. This is consistent with the rate of seismic strain release calculated from 80 years of earthquakes in the Pacific Northwest. Keep in mind, however, that significant hazards are associated with individual events within the low deformation rate regions such as the 1993 Scotts Mills earthquake: see The Pacific Northwest Seismograph Network for more information) and especially major earthquakes that may occur on the Cascadia megathrust.

Deciphering Historical Earthquakes

For some older earthquakes in the Pacific Northwest, we have insufficient data to determine the sense of movement and the style of faulting. Of particular interest is the 1872 North Cascades earthquake which was estimated to be a magnitude 7.4 event. The map in the results section shows that this earthquake is located in a region of dominant thrust faulting. Another important earthquake, the Milton-Freewater earthquake in southeastern Washington, is more difficult to decipher. It apparently is located in a complex transition between dominant normal faulting (Nevada), thrust faulting (central Washington) and strike-slip faulting (Oregon).

Basic Mechanics of the Cascadia Megathrust

From this study, the long-term basal shear traction along the Cascadia megathrust is estimated to be 15 MPa +/- 5 MPa. Given that the Cascadia subduction zone is in many respects unique among subduction zones, the basal shear traction is surprisingly similar to estimates of shear traction for other subduction zones around the world. The shear traction along the San Andreas fault too is thought to be less than 20 MPa, suggesting that many mature, plate boundary faults are weak (i.e., a low apparent coefficient of friction). An explanation for why these faults are weak is an area of intensive research. The important point to remember is that although these faults are mechanically weak, they are capable of generating major earthquakes.

Answers to Questions

1. N-S oriented compressive stress in the Pacific Northwest appears to be a result of the combined influence of basal traction along the Cascadia megathrust and the component of Pacific-North America relative plate motion perpendicular to the Queen Charlotte fault.
2. An increase in the basal shear traction along the Cascadia megathrust from south to north is not necessary to explain the clockwise rotation of the compressive stress axis that is associated with the Cascade Convergence Stress Province.