ONGOING DEFORMATION AND STATE OF STRESS IN THE COLUMBIA RIVER FLOOD BASALTS

Over thirty years of instrumental seismic monitoring indicates ongoing deformation in the area of eastern Washington occupied by the Columbia River Flood Basalts. Most of the observed seismicity is associated with the Yakima Fold Belt, a series of east-west reverse faults and folds. Focal mechanisms of micro-earthquakes in the basalt show that these events are a result of north-south-oriented maximum compressive stress, consistent with the long-term formation of these geologic structures. The association of seismicity and the Yakima Fold Belt is of a regional nature rather than correlating with known faults or showing significant planar alignments that would indicate subsurface faults. Focal mechanisms for micro-earthquakes in the sub-basalt sediments and basement rocks also indicate this state of stress even though the vertical overburden stresses are greater. In the western part of the Yakima Fold Belt and the Cascade Range, there is a tendency for an increased element of (presumably right-lateral) strike-slip focal mechanisms, where the minimum compressive stress is oriented east-west rather than vertical.

Seismicity in the basalts typically occurs as earthquake swarms, a series of tens to hundreds of micro-earthquakes that gradually increase in frequency and magnitude over periods of weeks to months, but without including an outstanding large event. This may indicate that the heterogeneous strength of the multiple basalt layers may lead to clusters of relatively small events within individual layers, separated in time due to slow plastic deformation and/or fluid flow in the weaker zones between them.

Seismicity in the basalt layers is higher than in the sub-basalt layers, and the basalt group may form a rigid stress guide above the sub-basalt sediments. Variation in thickness of the basalt flows, or the existence of pre-existing fractures developed during growth of the Yakima Folds, does not seem consistent with the current seismicity pattern. The strength and thickness of sub-basalt layers may affect the stresses in the basalts. A basement high to the north may produce a buttress effect that affects the stresses and the structures of the Yakima Fold Belt.

Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for the U.S. Department of Energy under Contract DE-AC06-76RL01830.