Monday, June 16, 2014

Oklahoma earthquakes start to go weird

We've had a bunch of earthquakes on the big thrust fault, including an M4.3 with foreshock and aftershocks.  If we get another M4+ in that area, my next headline with read "Thrust fault starts to unzipper".

We have here an intensely locked high-angle reverse fault.  All those other shear earthquakes have piled on the stress, so it's probably twice ambient, which is saying something, since we know the predominant horizontal stress is NE and quite high.

Normally, we would expect a thrust fault to react to this stress, but what if the fault is very rough and just doesn't want to go?  Then it cracks on another fault plane.  With a high horizontal stress, we can go around the stereonet and find planes with all types of stresses, from shear to tension.  You can see this if you put a rock specimen in a uniaxial testing machine.  Vertical cracks appear before all the shear cracks line up and it fails in shear.

That is why we have this weird extension, as a preliminary stage to a major thrust.  It's on a plane perpendicular to the main fault.  This failures are good for the mechanism, as the tension cracks allow more fluid to flow.  I'll bet the pumpers right now are thanking their lucky stars.  Much more fluid is going in.

I am 6 months out from my initial predictions due to the tremendous fractal roughness of the fault.  We might get more of these extension earthquakes.

On a seismological note, an extension earthquake has the hammer driving down into the ground.  On all earthquakes, there is always one side more rigid than the other, usually the footwall.  The other side has its main slip in some direction, and is called directivity with strong ground motion studies.  I call it the 'hammer'.  That little M3.5 foreshock was a thrust with the hammer upwards on the hanging wall.  Thus, people in certain areas had very strong shaking.  The M4.3 was hammer down, and the distant SOSN seismometers picked it up strongly, and the only MM - V was a 100 km away.  That's due to 'Mantle Bounce', and it happens for a lot of earthquakes.

Update:  M4.1 tension earthquake. Probably very low ground motion.

Uppity. OK goes nuts!  Lots of earthquakes and I'm stuck in No with my N7.

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