Thursday, October 15, 2009
Charleston Lines Up With Fishism
I now get my BSSA journals electronically. This saves getting those giant 'Brick Books' that threaten to collapse my bookshelf. The article is from the recent SRL.
Fishism states that the only energy source for these eastern NA earthquakes is the high horizontal compressive stress (strain) of the NA plate. The Niagara Tunnel shows us that this is a very destructive force. The strain energy is released through stress corrosion by fresh water, and each source mechanism can remain active for millions of years. These mechanisms grow from a baby (Hamilton), to a single wing (Charleston), right through to a giant old-sucker double wing that is New Madrid.
Now, when I first started out, there was great confusion as to the source of the Charleston earthquake. Finally, with a lot of seismic monitoring, they have pinned it down to the area in the map. They have increased the monitoring to decent levels, and can relocate events. The pattern takes on the shape of a single wing.
Now, to get these mechanisms started, there has to be a reactivated thrust fault. Most likely, the giant megathrusts that go through ENA. There is not enough deep seismic reflection work to find it in Charleston. Still, to capture compressive strain energy, you need a thrust fault. Lots of water is then required to keep things going, since the most likely outcome is a 'locked fault', where strain energy is relieved, and the fault doesn't grow. The water starts corroding at the edges, and keeps things weakening.
The thrust weakness can't keep growing forever without 'catching'. For a successful mechanism, a strike-slip wing has to break out. You can see this here. More work has to be done to which is the thrust zone, and which is the wing. I still can't imagine the force required to break out a second wing through the footwall, as with New Madrid. Those would be huge earthquakes!
Now, the fundamental conflict with Fishism versus the rest of the world, is that these mechanisms continue to grow, and will always be the centre of great earthquakes in the region. Other people think that these places are 'shot', and some new source zones have to appear. Poo-poo on that!