Cathyrn, they are all the glop scraped off the subducting plate (the crust that is sinking into the earth). OK, its not quite that simple....In subductions zones, which this was, the ocean crust (basalt on top, gabbro deeper)...which is on top of some uppermost part of the next layer down, the mantle, is sinking into the earth. On top of the basalt (lava) layer, which was at the seafloor when first formed, would be a bunch of sediment that rained down from little critters, and in this area close to large islands, eroded off of the islands. This is liable to be scraped off and not subducted.....it would be "accreted" to the upper plate. In addition, the subduction zone in this case is almost flat...it dips NE about 10 deg. While most of the motion is on this mega-thrust, the crust above, being stuck to the subducting slab between quakes, is being stresed and strained.....so it will be full of active and inactive thrust (and other) faults. So, the accretionary prism is a big mess.

Ask sometime later what a critical wedge is and what that has to do with southern California and tsumamis there. I don't feel like getting into that today and it may make your head hurt.
Chris