Higher risk earthquake zones in mid plate?

Question

I hate it when National Geographic Magazine contains a map with interesting information not explained on the map or within the accompanying article. Such in the recent April issue, vis a vis earthquakes. The map indicates larger mid-plate areas of higher occurance and risk for more major earthquakes include much of central and western Australia and surrounding the St Lawrence River Valley and smaller areas in west-central Africa, northeastern Brazil and a few other places, including of course the New Madrid area. Nothing in the article explains the geologic whys for these areas being comparatively more severe earthquake prone. Fortunately a couple of professional geologists' answers to other Yahoo Answers queries detail about the reasons for New Madrid (thank you) but what about the others, especially Austraila's and the St-Lawrence?

Answer 1

I think the answer to your question, that all of these places seem to have in common is: they are failed rift zones.

The New Madrid Fault is a failed rift zone, as is the Amadeus Basin in Australia. West-central Africa is the Great Rift Basin, connected to the Red Sea, which is one arm of that rift that is not a failed rift zone. Northeastern Brazil is another failed rift zone. The St. Lawrence River is another.
http://earthquakescanada.nrcan.gc.ca/zones/eastcan_e.php#LSLSZ

A rift zone is an area of the earth's crust that is being pulled apart by plate tectonic forces. In many instances these rift zones form a three-armed triple junction. As two of these arms dominate, the third arm becomes the failed rift arm. The New Madrid Fault zone is believed to be one of these failed rift arms from the rifting that separated South America from North America during the breakup of Rodinia. Actually the Rio Grande is a failed rift zone as well.
http://home.att.net/~sgeoveatch/rio_grande_rift.htm

The East African Rift Valley (the Great Rift Valley) is the failed arm where Africa is moving away from Asia and Saudi Arabia. Here is a USGS diagram of the Great Rift Basin in Africa:
http://pubs.usgs.gov/gip/dynamic/East_Africa.html
Australia's Amadeus basin is a failed rift that formed duing the breakup of Rodinia, an ancient supercontinent.
http://www.ga.gov.au/oceans/ea_Amadeus.jsp

Each of these rift arms represents a thinned and broken, faulted part of the continental crust that can be reactivated over time and will act as a weak point when either compressive or extensional plate tectonic forces are applied to a continent. Thus the likelihood of earthquakes.

Answer 2

I do not know specifically, but they are usually related to extinct or super-slow tectonic boundaries. For example, the Darwin basin in Australia is a failed rift arm, and I believe the St. Lawerance runs along the 1.0 billion year old Greenville province suture (see link).

The basic explination is these tectonic lines used to be active, but they are no longer. However, there is always force bleeding inboard of a plate (i.e. a very small percentage that is not taken up at the boundary) and over very long periods of time, these can build up strain and cause earthquakes.

It is certain that these areas are understudied and poorly understood. It's easy to consenrtate on the action of the plate boundaries, and only recently have people tried to explain things like the New Madrid fault zone.

This is another example of how much more we have to do still and science and how many unanswered (and unanswerable?) questions there are.

Answer 3

gfl247 has the right information for you.

Most of these mid-plate seismic zones are the result of long-tern accumulated stresses acting on very old, very deep areas of crustal weakness. I'm talking about pre-Cambrian kind of old. Even very old sutures can become seismically active given enough time and enough accumulated stress. In western Africa, much of the crust was implaced during the Pan-African Orogeny (~1000 million years ago) and this left a BIG structural signature over the entire region. There are a number of potential weak zones there including the Damara Mobile Belt--something similar to a suture but more soft deformation than brittle.

Does this help?

(Yeah, I'm another geologist)

Answer 4

Geological attitude is this sort of ordinary gen ed class, i will't trust you've been stupid adequate to placed up the quiz questions about yahoo. heavily that's contained in the e book, no longer that complicated.