FROM GRAVITY ANOMALIES TO GRADED STREAMS: ASSESSING EARTHQUAKE HAZARDS IN CENTRAL WASHINGTON STATE WITH GEOPHYSICAL, GEOLOGIC, AND GEOMORPHIC CONSTRAINTS

Earthquakes in the Cascadia backarc of central Washington are recognized as potential hazards to regional population centers and major infrastructure, including a large nuclear waste facility and hydroelectric dams. Resolving fault geometries and slip rates along individual structures is imperative to seismic hazard assessment. Here we use geomorphic and geophysical mapping, stream profile inversion, and balanced cross sections to constrain fault geometries and slip rates in the Cascadia backarc.

We focus primarily on the Yakima fold province (YFP), where an array of reverse faults accommodate active distributed NNE-SSW shortening. Stream profiles extracted from 2m-resolution LiDAR imagery show clear knickpoints that are not collocated with mapped structures. We interpret knickpoints as possible transients that record temporal variability in deformation rate. We exploit LiDAR data to analytically solve for the rate of relative uplift along several active fault-cored anticlines using stream profile inversion techniques. To constrain the fault geometries at depth and the long-term magnitude of deformation, we constructed a line-balanced cross section across the folds. In concert with cross section analysis, we forward model magnetic and gravity anomaly data to further constrain our structural models.

From stream profile inversion, we see an increase of incision rates in Pliocene time and suggest that the increase is tectonically controlled. Independent estimates of cumulative uplift along each anticline from geomorphic and structural methods are in agreement, suggesting that we capture the majority of the deformation history within our geomorphic solutions. From our analyses, we estimate modern slip rates of 0.4 and 0.5 mm yr^-1 accommodated on reverse faults that core the Manastash and Umtanum Ridge anticlines, respectively, and that these faults reactivate and invert older normal faults in the basement. Finally, we use fault slip rate and geometry parameters to calculate the time required to accumulate sufficient strain energy for a large magnitude earthquake (M≥7) along individual faults. Results show that the Yakima folds likely accommodate large magnitude earthquakes and that it takes several hundred to several thousand years to accumulate sufficient strain energy for a M≥7 earthquake.