Monday, August 29, 2011

The essence of the induced earthquake

Look up 'Induced earthquake', and you get the Wikipedia version.  I didn't write it, but you can use all the references, which I always find tedious to do.

Here is my version.

Induced Earthquake

Seismicity starting in an area with no know earthquake history, once a fluid disruption has commenced.  This can be in the form of deep water injection, or a man-made lake.  The definitive proof is that the earthquakes stop after the injection has stopped.  For a dam, which cannot be stopped, detailed seismic monitoring can show a correlation between water levels and micro-seismicity.

Since the physics of earthquakes has not been adequately explored, except by a certain individual :), the argument of 'induced vs. natural' can be quite heated.  At stake is usually the 'worth' of a dam, or an entire industry.  Some would argue that even if a given earthquake were induced, the injection might only have advanced it a few years.

With a dam, which cannot be turned off, the earthquakes must continue.  As with the growth of the Arkansas injection swarm, a dam may produce larger and larger earthquakes, at a rate which exceeds the 'norm' for the area.  Some have stated the M8 in China was induced by a dam, but the detailed seismological records for this, are not available.  In the US, piss-poor recording condemns us to ignorance.

For Virginia, we can only make assumptions, since the presence of a nuclear plant has created a 'Science Shadow' over the area.

 The local geologists are all gaga about the Appalachian Cruft, but the earthquakes aren't there.  They are down in the Precambrian, of which little is known.  Since this earthquake had an identical, distorted felt area like every other earthquake in the east, we will assume a standard cause.  That is, it was located on a gently dipping megathrust, which have been demonstrated under Toronto.

The Dam Lake provides the water to activate this zone, since it appears to be well off the old central seismic zone.  Aftershocks define the fault plane. The hanging wall is active, the footwall is not.

A new set of aftershocks is jiggling the rock just at the tip of the hanging wall.  This is unusual and may indicate new fluids entering.  The fault is dipping down to the SE, but current seismic recording will never be able to define it that well.

The fault rupture has opened the rock and will allow new fluids to enter.  The aftershocks should start creeping out of the rupture plane, and set up a situation for the next stage.  An M6.5 or greater should rupture from the lake to the old seismic zone.

1 comment:

Anonymous said...

I think your theory is interesting and I hope someone picks up on it and runs with it.

For what it's worth I dug up one of the most recent applications for Lake Anna Nuclear Power Plant--its permit is up for renewal sometime soon, and the utility wants to build another reactor nearby. There is apparently a fault close to the surface that runs right under the plant. Whether it's been trenched or not I don't know. What little paleoseismological studies that have been done have concluded there've been no large earthquakes (well, bigger than an M5.8) in the last 10,000 years.