Active deformation of the Cascadia forearc : implications for great earthquake potential in Oregon and Washington

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