Monday, April 16, 2012

The growth of the conjugate set fault system

We have always had the 'New Madrid Paradox'.  When you look at it in a static manner, you can't see how it can exist.  There are no mountain ranges, no huge slips like the San Andreas.  It simply cannot exist, especially not continually produce monster earthquakes.  Thus you conclude that it doesn't exist, it was a one-time deal, like a big meteor strike.  And this is the view of many earthquake scientists.  Most of the others just stay away.  :)

But what if you are only person in the world who believes it does exist.  What can you do?  The first thing is to not view it as a static thing in a static stress field, it is a dynamic thing.  Then you've got something to work with.

Basically, the new developments in Arkansas have led to my latest inspiration.

Base Case:

Generally unstable Precambrian crust under high horizontal stress, NW direction more than twice as much as the NE direction.  The Precambrian is riddled with many faults and large through-going fault systems.  Where it has proven to be a soft and yielding base, it has caused extensive fracturing in the upper Paleozoics (limestones, shales).  The PC is composed of silicates (granites, etc).   It can only be called 'stable' by some in that nothing hardly ever happens.  The water in this crust has been sitting there for millions of years and has corroded everything it can.  Thus, it is saturated and can have no further effect on the rock.

The new fault grows

Along comes a disturbance.  This is mainly in the form of a new source of water, probably from glacial epochs, or shifts in rivers.  A disturbance must start small.  It is not conceivable that a large fault system suddenly appears, like that meteor strike.

A conjugate set starts exactly as Arkansas did.  You have a disturbance, in this case injection, and you have earthquakes along a NW trending structure.  These are thrust earthquakes, reacting to the main horizontal stress.  You can only go so far before you have compressed a zone, and are causing shear strain along a NE trending vertical plane.  At this point the shear (strike-slip) earthquakes start.  This was on a very long line from the original thrust zone.  The whole thing is driven by water infiltration and silicate stress corrosion.

So, how can this grow?  The thrust zone could get larger, and maybe a second small shear wing, but this configuration must die.  My suspicion is that one of the shear wings takes over (the upper one in this case) and the whole initial system becomes one shear.  The combinations of the shears and thrusts become one big oblique shear up the slope of the megathrust.

Thus the original complex is swallowed by the new system.  If the new thrust is successful, it will again throw off new shear wings, as with New Madrid.

The confirmation comes in the way the pattern will fill in time.  I have no idea when the thrust coughs up its first large earthquake.  :)

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