2D INTEGRATED GEOPHYSICAL MODELING OVER THE JUAN DE FUCA PLATE

In the Cascadia Subduction Zone, Juan de Fuca (JdF) plate is the most intriguing one as it shows an unusual seismicity pattern in the continental domain. The two other plates – the Explorer and the Gorda – are associated with a large number of earthquakes along the subduction zone. In contrast, some portion of the JdF is seismically quiescent, so the inevitable and potentially devastating megathrust earthquake is expected in this region. The unusual seismicity pattern may be governed by variations in overall crustal architecture of the margin as well as in physical properties (densities and magnetic susceptibilities) of the rocks. To test this hypothesis, we performed integrated geophysical modeling spanning from oceanic spreading center to onshore domain through regions of different seismicity. In our analysis, we have integrated multiple geophysical data (i.e., potential fields, seismic reflections and refractions) from public sources. The southern model ends in the low seismicity zone in Central Oregon, while the northern model extends up to the high seismicity zone in Southern Washington.

Both models require lower densities of mantle rocks on the western side of the profiles, which is consistent with the location of spreading ridges. However, this region of less dense mantle rocks is wider in the Oregon model, which can be explained by the presence of Cobb Hotspot underneath Axial Seamount at the spreading center. Our modeling suggests that physical properties of the corresponding rocks are same for both models. In contrast, there are notable differences in the crustal architecture of the continental domain between the Oregon and Washington models, such as the width and thickness of the Siletz terrane, and the dip of the subducting slab. In addition, the oceanic crust in both models requires a number of lower density zones to satisfy the observed gravity anomaly. Some of the lower density zones correlate to previously identified pseudofaults, while others are interpreted as newly mapped pseudofaults. We interpret the decrease in density as evidence that pseudofaults represent the zones of weakness within the JdF plate. Therefore, pseudofault zones that are being subducted underneath the North American continental plate, may influence the overall subduction process and hence impact the observed seismicity pattern.