Hola John!

I see you've cooked up some tantalizing questions today. Let's see how this goes....

Some unanswered questions:

1. Is the nucleation area (area perhaps showing dilatancy) any bigger for an M8 than an M1?

M8 needs more pressure than an M1, no doubt, but some area's don't show any signs of dilatancy prior to, so not necessarily. IE: The Palmdale Bulge. There's uplift and swelling and no earthquake.

2. Does dilatancy start years before an earthquake or millseconds?

Logically it should occur at least a month or more in advance because it is a process, not an event. The rock must be saturated with water which causes a weakening. Even pressure inside the fault cannot make the rock fissures/breaks close and so how does that occur in milliseconds? Sounds impossible and unlikely.

3. Is dilatancy a perceptible effect on a fault with a weak, perhaps over-pressured layer of gouge?

I don't know if there is an answer that encompasses all faults because they all have their own geologic structure. One type of rock might not and the other could.

Dilatancy as summarized by Bolt is so unconfirmed by data that most scientists have moved on to other, more-likely but still long-shot precursors.

Geology says much about the outcome of dilatancy. An easy home exercise would be to take various types of rocks which are dry at late summer for instance, weigh them and then place them in water for a week, take them out and weigh them again and most of them will be heavier; but those that are very solid might only be marginally heavier.

The SAF at Parkfield is abundant with chert nodule which looks like playdough. It's not hard and firm like granite and when you put those rocks in water they get very, very heavy, but they break easily in either case. It could explain why Parkfield has many smaller earthquakes and only occasionally a 6.0, but unlikely to have an M8. The pressure in the fault doesn't have time to accumulate enough strain and just sort of chugs along. I guess it gets stuck (hung up on an asperity) about every 11/22/37 years and when that happens, WOE, here it comes.

I think the bottom line in any case when looking for precursory earthqauke activity has to be in situ. We always have to ask what is different than before, what forces have been placed there that were not there days or years ago. What other events are taking place? Are water wells going up or down, is there uplift, tilt, measurable strain, what gases are present? Were there any far field events that push that fault to the breaking point? What about solar flares making an influence? And in some cases what happened to release that "X" factor that produces what seems like a sound to human beings, makes animals run away before earthquakes, sends birds to the next city, and makes snakes desirous of leaving their underground homes along with moles and other dirt dwellers. Our primary problem is that we don't have instruments everywhere nor people to babysit a pregnant fault.

What to do?

Petra