A split subduction zone beneath Vancouver Island?

The subducting Juan de Fuca (JDF) Plate dips at a relatively shallow angle beneath Vancouver Island (4 to 15 degrees) and then dips more steeply beneath Georgia Strait and the mainland parts of the North America Plate.  The southern Vancouver Island region is characterized by twin belts of seismic activity, one along the west coast of the island, and one on along the east coast, beneath Georgia Strait (see map below).  The area has also been characterized by large megathrust earthquakes, most recently in January of 1700.   (see: http://www.mala.bc.ca/~earles/1700quake/)

Andrew Calvert from the Earth Sciences Department at Simon Fraser University has recently compiled marine seismic data from the area in order to understand the nature of the rocks and the subduction process beneath Vancouver Island.  He has delineated two main reflector systems, labeled E and F on the diagram below.
Seismic section line in red.  Line of schematic diagram (below) in dashed green.  Black dots are in-slab earthquakes.

 
Composite seismic section across Vancouver Island.  The double red line is the interpreted base of the Juan de Fuca Plate.   The area of imbricated faulting is outlined in yellow.

 
Calvert’s interpretation is that the subduction zone is not a simple thin interface between oceanic and continental crust.  He defines a thick zone of inbricated faulting (up to 16 km thick) extending from west of Vancouver Island to Georgia Strait, and possibly involving sedimentary rocks from the oceanic plate.  As shown on the section below, he suggests that the main subduction interface, (top of the subducting JDF plate) is characterized by significant changes in dip at the edges of the imbricated zone.  These interpreted dip changes coincide generally with the regions of enhanced seismicity noted above.

 
Schematic section across the Caascadia Subduction zone on southern Vancouver Island.  The North American Plate is in pink and the Juan de Fuca plate is in yellow.  Mantle rock is in green.
 

Calvert also speculates that the episodic tremor and slip described by Dragert et al. (2001) and  Rogers and Dragert (2003) may be taking place within the imbricated zone, which otherwise appears to be aseismic.

 
References

Calvert, A, 2004, Sesimic reflection imaging of two megathrust shear zones in the northern Cascadia subduction zone, Nature, V. 428, p. 163-167 (March 2004)

Dragert H, Wang K, and James T, 2001, A silent slip event on the deeper Cascadia subduction interface, Science, V. 292, p. 1525-1528.

Rogers G and Dragert H, 2003, Episodic tremor and slip on the Cascadia subduction zone: the chatter of silent slip, Science, V. 300, p. 1942-43.

Steven Earle, 2004. Return to Earth Science News