GEOMORPHIC AND SEISMIC EVIDENCE FOR RECENT DEFORMATION IN THE YAKIMA FOLD BELT BETWEEN ELLENSBURG AND YAKIMA, WA

The Yakima fold belt between Ellensburg and Yakima, WA is a zone of E-W to SE-NW trending anticlinal ridges formed in Columbia River Basalt and cut through by the south-flowing Yakima and Columbia rivers. Regional stress patterns indicate that the area is under N-S compression, while abundant local seismicity suggests active deformation. Because of relatively scattered earthquake foci, previous studies have not tied seismicity to specific structures at the surface, while structural evidence alone has been insufficient to demonstrate that the ca. 16 Ma Yakima structures are presently active. On the basis of geomorphic data and the patterns of well-located earthquake hypocenters (rms error < .15), we maintain that the fold belt is actively deforming, and we identify two anticlines with apparently strong coupling between seismicity, surface structure and fluvial geomorphology. Our analysis reveals that the Yakima River and its tributaries steepen and narrow as they flow downstream across both the Yakima Ridge anticline and the Umtanum Ridge anticline. These mapped anticlines have relatively undissected topographic surfaces and overlie high angle, E-W oriented, relatively planar clusters of earthquake hypocenters at depth. The planes defined by the hypocenters also project to meet the locally oversteepened river reaches at the ground surface. This pattern is consistent with the presence of at least two high-angle thrust faults that terminate in active fault propagation folds, as has been previously suggested for the Yakima folds. Our analysis illustrates how integrated geomorphic and seismic observations can help assess the activity of structures where evidence for recent deformation is relatively cryptic or equivocal.