Date of this Version
Cascadia Subduction Zone (CSZ) has a long history of devastating earthquakes as the Juan de Fuca plate subducts beneath North America, where an imminent megathrust earthquake is expected. Compared to other subduction zones, CSZ exhibits unusually low seismic activity, particularly in the central region with respect to relatively high seismicity zones to the north and south. Some researchers attribute this behavior to subducted seamounts, although their impact on seismicity remains poorly understood and highly debated in the literature. Examining subducted seamounts is challenging due to complexity of the overburden strata. That is why this study focuses on an isolated not-yet-subducted intraplate seamount known as the Diebold Knoll, located approximately 60 km west of the deformation front. Seismic reflection, gravity, and magnetic data were acquired over that seamount during the cruise RR1718.
The first objective of this research was to determine heterogeneities in physical properties of oceanic crust resulted from addition of the seamount by integrated geophysical analysis along two intersecting profiles. The second objective was to reveal the tectonic history of the Diebold Knoll by combining magnetic polarity reversals with cross-cutting relationship analysis of seismic structures and constrained by the nearby ocean drilling sites.
These models revealed that Diebold Knoll does not require complete isostatic compensation. Instead, a flat Moho or a less than 1 km thick root is sufficient to satisfy gravity data; both of these Moho geometries are observed in the study area. To achieve a better fit with observed gravity and magnetic anomalies, lower density and magnetic susceptibility values within the top portion of the seamount are required, which are attributed to faulting and hydration. This finding is also in agreement with a recent study that analyzed seismic velocity variations within seamounts. Furthermore, the negative magnetic anomaly of the seamount and the cross-cutting relationships between sedimentary layers and the seamount body indicated that Diebold Knoll is a very young feature formed approximately 0.8-1.8 Ma.
Advisor: Irina Filina