MAPPING THE JUAN DE FUCA SLAB BENEATH THE CASCADIA MARGIN

An accurate map of the Cascadia plate boundary is essential for studies of active margin processes such as deep fault creep, crustal strain accumulation, and metamorphism of the Juan de Fuca (JdF) slab. We have updated our model of slab geometry beneath the Cascadia subduction margin with depth control from recent studies of double-differenced (DD) hypocenter locations, and 2D and 3D velocity models based on active and passive source experiments. Our model attempts to integrate depth control from several data types. However, mismatches both within and between data types illuminate remaining uncertainties on the location and geometry of the slab.

For example, does Wadati-Benioff (WB) seismicity beneath Washington (WA) and Vancouver Island (VI) occur within JdF crust, JdF mantle or both? If shallow WB seismicity occurs mainly in JdF mantle, our slab model could shift upwards as much as 5 km beneath Puget Sound. Where DD hypocenters resolve double seismic zones in California (CA), we can identify the JdF Moho, thus infer the location of the slab surface. These data allow us to delineate a more complex geometry beneath northern CA that suggests buckling of the JdF plate. DD hypocenter locations have the potential to better map the arched slab geometry beneath Puget Sound and VI.

Interpretations of seismic velocity models based on s-wave data depict JdF crust up to 10 km shallower beneath WA and VI than p-wave models. Does this reflect model uncertainties or are the two data types mapping different structures? Resolving the source of this mismatch and determining the most accurate slab location are critical to seismic hazard assessments of subduction earthquakes.

Differing interpretations of subsurface structure are prompting re-evaluation of the geologic architecture beneath the Pacific Northwest margin, in particular, oceanic terranes accreted during previous subduction regimes. Could some of these terranes mimic the JdF plate? Such tectonic complexities hamper efforts to directly compare Cascadia velocity models with those developed in subduction settings such as Japan or Alaska.

A more accurate and detailed slab model will also provide a crucial tool for investigating the relationship between complexities in slab geometry and seismic moment release. Development of a community seismic velocity model is a key first step.