My guess is that horizontal contact surfaced lying deep are "creepers" owing to the weight of the over lying rock load( constrained along all cardinal points in both yaw and pitch/directions it only has small arch of possible movement). Where as vertical contact faces, have less inertia/burden to overcome and in my musings, would be less constrained favoring a thrust fault type movement upwards as it is a least resistance path (skyward).
The basin which is New Orleans has a deep concave fault which is creeping southward and I haven't looked but don't recall a significant earthquake there in modern times. Overthrust faults as described in the article seem to be slow and constant except at the fault margin where they raise mountains and erupt onto the surface as they are rising along a long incline.
At some point the over lying rock burden's equilibrium tips and makes curling upward the path of least resistance. The horizontal face is pushing up slope without locking so this region probably doesn't have significant quakes at this more mature stage/ age. Were the contact region of this fault (the Alpine fault horizontal margin) more near the surface we could see the types of displacement that caused the 2004(?) Christmas Tsunami which is a start of a curl so to speak.
I think that other than understanding it in a larger way, it is neutral to larger earthquakes but several faults through out the world lie in those inbetween depths and any of them, once equilibrium has tipped, can host massive earthquakes
Eman
From: "Kim Noyes kimnoyes@gmail.com [geology2]" <geology2@yahoogroups.com>
To: Geology2 <geology2@yahoogroups.com>
Sent: Thursday, September 3, 2015 1:32 AM
Subject: Re: [Geology2] Research calls for rethink of Alpine Fault
Wouldn't that also mean that a larger than previously thought quake could occur as a result of the greater fault contact surface area involved?
On Wed, Sep 2, 2015 at 7:50 PM, Lin Kerns linkerns@gmail.com [geology2] <geology2@yahoogroups.com> wrote:
Research calls for rethink of Alpine Fault
The major fault line, which runs almost the entire length of the South Island, has been assumed to be a near vertical crack. However, studies of seismic data have revealed the fault line becomes flatter at depth.
"What we've found is that for approximately 350 kilometres of the length of the South Island, the land mass of the Pacific Plate is actually sitting and sliding right on top of the Australian Plate," says Associate Professor Simon Lamb from the School of Geography, Environment and Earth Sciences.
"So, rather than thinking of the fault line as a vertical crack, we should be thinking of it as a nearly horizontal one that curves up to the surface where the fault line is exposed."
The region where the Pacific Plate is stacked on top of the Australian Plate is believed to be up to 100 kilometres wide in some places.
"As well as vastly increasing the area where the two plates are in contact with each other, the research tells us that the effects of earthquakes may be quite different, and in some big earthquakes, the rupture zone may never break the surface."
According to Dr Lamb, although more research needs to be done to better define the possible rupture zone, this poses a very different geological problem when assessing earthquake risk.
"Someone in the centre of the South Island, for instance, might think they are miles away from the fault line, when, in actual fact, the fault could be right underneath them, making these regions more vulnerable than first thought."
The conclusions were drawn from research into both the thickness of the South Island's crust and the speed of seismic waves.
"The crust is very thick beneath the South Island, which is not what you would expect if the two tectonic plates were just sliding past each other on a near vertical fault. Also, seismic waves generally travel faster the deeper down you go, and yet the wave speeds get slower beneath the Southern Alps," says Dr Lamb.
"The seismic data made complete sense from a recalculation of the physical relationship between the two plates."
The research findings have been published in the American Geophysical Union journal G3.
Note: The above post is reprinted from materials provided by Victoria University.
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Posted by: MEM <mstreman53@yahoo.com>
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