Hi Chris - and I sure envy you being able to step out your door and go X-country skiing. One of my favorite activities, 'cept wife doesn't care much for it, and it's a four- to five-hour trip anyway.

You wrote: "Michael...my reading of your post and what I have heard elsewhere both make me think the usage is both spatial and temporal: the hypothesis depends on there being spatially-defined fault segments." So, looks like you, me, and the authorities are in agreement on that, at least.

However - you wrote further: "But, if there is a 200 km spatial gap between, say, 2 Great earthquakes that occurred 10 years ago, but there was one on the intervening gap 30 years ago, and it slipped 5 m on a subduction zone accumulating strain at 50 mm/yr, it would not be "due" and so not identified as a gap on that basis..."

I'm not sure that my excerpt supported that, and I don't think it would be supported, in general by current thinking among seismologists. Perhaps you are not advancing it as a description of reality, but, rather, as your interpretation of the now-discredited seismic-gap model. I think you would be wrong in either case. Seismic gap theory's primary assumption is that _seismic potentioal increases with the absolute time since the last large earthquake_. It takes little account of the stress drop or whether the slip budget has been satisfied.

And, here, as so often with me, I need to provide the caveat that I don't necessarily know what I'm talking about. BUT - my understanding is that seismologists currently would find that your 30-year-old quake's slip having exceeded the total strain build-up within any 30-year period in its last event, is irrelevant. Or, at least, non-predictive. When an earthquake stops, there is, apparently, no current method of determining what portion of built-up strain it has released. It appears to be just another confounding, counter-intuitive result wherein quakes bullheadedly just do their own thing without being reasonable about it.

The "characteristic earthquake" model, or theory, seems to be at odds with the seismic gap model, but does have some credibility when applied to individual fault segments, but not so much for regional quake occurrence. A topic for a later discussion, perhaps.

Michael F. Williams
Arroyo Grande, Ca USA