I've never seen clusters like these, but that may only mean that we haven't had maps like these. In the old days they may have been buried in journal articles. If ever there was an argument that clusters don't mean much in the short term, it is here. Not enough to get those Italian seismologists off, since I don't think science applies in Italy. :)
Both clusters continue to progress in a very slow fractal manner. That means the systems are 'rough' in a self-similar sense. The Caribbean is finally breaking out of its small area, and the earthquakes are starting to get into the high 3's. The mechanism is that of a very weird strike-slip subduction slab, since many earthquakes are quite deep.
Alaska has a more understandable mechanism - that of a subduction slab being fed by a strike-slip fault along the continent. We must go with slab pull here. As well, these major fault systems are extremely weak, and are only pinned at a few spots. You can think of them as major landslides ready to go. If we fully monitored such a zone, we would see the few pin spots. As the landslide was progressing, these pins would break. The first ones would create small displacement fields, but each succeeding one would ripple displacement farther out. Finally the last ones would break, creating a total displacement ripple that would bring the failure zone to critical displacement. Then, look out!
The fractal roughness is a measure of the pin density. These zones have lots of pins and the cluster may go on 'forever'. I'm expecting the last major pin in Alaska to break by the end of the year, but plus or minus a decade. :)
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