SEISMICITY AND STRUCTURE OF THE COLUMBIA BASIN: PRELIMINARY CORRELATIONS

The distribution of earthquakes in the Columbia Basin suggests superimposed thick- and thin-skinned contraction. Some clear alignments of hypocenters cut the entire seismogenic crust (to 20-25km depths); these presumed faults appear to be associated with broad basement-cored folds such as the Wenatchee Mountains. The Yakima fold belt, however, appears to be controlled by a decollement at 5-12 km, from which mostly south-dipping reverse faults ramp to the surface. Broadly listric thrusting and/or shear fault bend folding can account for the gentle backlimbs of folds such as the Horse Heaven Hills. Many earthquakes appear to correspond to axial surfaces (some very subtle), not major faults. The great local complexity of some folds appears to be due to secondary kink and fault-bend folding within the Columbia River basalts. This presumably accommodates additional shortening taken up by more distributed strain at depth, (e.g. by pressure solution).

The present manifestation of the Olympic Wallowa line (OWL) is a compound, anticlinal uplift that is wedge- or tulip-shaped in cross section. This uplift appears from stratigraphic evidence to be mostly latest Miocene and younger. Focal mechanisms and striations on young faults in and near the OWL suggest active contraction directed about N15-20E, characteristic of the entire southwest half of Washington. Regional NNE-directed contraction implies a small sinistral component of slip on the reverse faults that control the Yakima folds, and a dextral component of slip on OWL-parallel structures.

From the Oligocene (?) through the mid-Miocene the OWL was well oriented for dextral wrenching: dikes and fold axes suggest NNW-directed contraction across the Columbia Basin. Mid-late Eocene slip on steep SSW-dipping faults, on which the young OWL uplift is superimposed, appears to have been reverse/sinistral. Raisz' (1945) speculation that the OWL originated as a sinistral fault zone is consistent with regional fault patterns.

Near the margins of Columbia basin, radially-directed strain can be attributed to steep topographic gradients. Earthquakes in the Chelan area specifically may be related to the relatively rapid uplift of the North Cascades above the arch in the Juan de Fuca slab, inducing tension above the crest of the arch and contraction towards its flanks.