For those who don't want to read through all that stuff on Canie's link, yes, there is an oblique subduction zone north of P.R...capable of large (probably "great") quakes. Given that it is oblique, there are probably strike-slip faults involved. I noticed that there was a large swarm of quakes in the P.R. region a couple weeks ago but did not check where they were and what type of slip...
Chris