I noticed that today there are alignments of aftershocks on NE-SW trends between the Elsinor and San Jacinto faults (in USA). There were papers published on these in the mid and late 80s by Nicholson, Seeber, Hudnut, etc. The NE-SW faults are left-lateral. Activity on these together with right-lateral on the main faults means clockwise vertical axis block rotation has been activated. It tends to mean that depending on where you are along a fault like the San Jacinto (various strands) or Elsinore, the fault is being made stronger in some places and weaker in others. If you make it weaker in an area where it was within months or a couple of years of failing anyway, it may go.

I called Craig Nicholson and he had noticed the same pattern, and also thought aftershocks were expanding to the NW. He said that Ken Hudnut (USGS) had said in an interview that the foreshocks to the M7.2 were also on cross faults.
Craig suggested that the cross faults are being activated in the extensional quadrant of the main earthquake; they are being given a little space to move (well, not really...maybe normal stress is reduced is more accurate). Craig said I could post this.

So, one question is, what do you do about this? What do you monitor?

Chris

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