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Re: More questions on the Burma microplate (allegedly where the quake occurred)



John Milson, in a chapter for a forthcoming book on Sumatra,
http://www.es.ucl.ac.uk/people/milsom/smtrntct.htm, doesn't mention the
Burma plate either - or the Sunda plate, atleast by name.

He says that the fault-ridden zone between the Java trench and the Sumatra
Fault, which he calls the forearc region, must be separated from both the
"Indian Ocean" and "Eurasia".    He says it is often called a sliver plate
but to call it a plate suggests that it has a degree of "strength and
rigidity" that the long and narrow strip does not have.   He also says that
any analysis of subduction beneath Sumatra must take into account the
probability of independent movement of fragments in this zone.

We have another academic geologist who appears not to have heard of a "Burma
plate".

I wonder what these two would say if I contact them, tell them to look at
those NEIS web pages,  and ask what's the Burma plate?   I think I'll try
it.    (Grin)

So is the forearm strip the same thing as the "Burma plate"?


Yours,
Dora Smith
Austin, Texas
villandra@austin.rr.com
----- Original Message -----
From: "Dora Smith" <villandra@austin.rr.com>
To: "DML" <dinosaur@usc.edu>
Sent: Friday, December 31, 2004 8:50 PM
Subject: More questions on the Burma microplate (allegedly where the quake
occurred)


> Most ot what I'm finding on the Burma microplate is confusion.   Someone
> sent me a bunch of links he thought would help; one link doesn't work, one
> is to that ol' NEIS chart, and the others are to small textual
descriptions
> of the plate.
>
> I have  Danny Hilman Natawidjaja's 2002 dissertation, "Neotectonics of the
> Sumatran Fault And
> Paleogeodesy of the Sumatran Subduction Zone" , at
>
http://etd.caltech.edu/etd/available/etd-05222003-155554/unrestricted/DHN_Th
> esis.pdf.
>
> Natawidjaja identified and  mapped every fault on the island of Sumatra
and
> between the Sumatra fault and the subduction trench.   He discusses all of
> the geologic features.   He never mentions a Burma plate.   He doesn't say
> it doesn't exist; he literally never mentions it, though his work is
> extremely recent.   I'd call that a good argument to think that many
> geologists would have as little idea as I do what the NEIS descriptions of
> the earthquake are  talking about.
>
> Natawidjaja does mention an outer arc sliver, and describes it as a
separate
> piece of plate - is that the same thing as this Burma plate?
>
> Apparently the answer to the question about what is sliding in relation to
> what, is that, actually, the plates aren't sliding in a direction parallel
> to the subduction fault, at the trench, nor along one fault between two
> plates; nor along two faults between three plates.    The two plates
> converge in a very oblique angle.   Like apparently most oblique
subduction
> faults, the subduction trench takes perpendicular motion and a complex
> lacework of strike/slip faults between the trench and the central ridge
take
> motion parallel to the subduction trench.    In the case of the Sunda/
Java
> subduction faults, a lacework of faults large and small between the trench
> and the Sumatra fault in the central mountain range take the motion
parallel
> to the trench.
>
> Natawidjaja appears to suggest that more of the motion is absorbed off of
> the coast of Sumatra than on Sumatra, but I'm not very clear on that
because
> I've only read two chapters of his long thesis so far.   He explains that
> one reason to believe that only so much displacement has occurred on the
> island of Sumatra since it began only a few million years ago, is that the
> rest of it occurs offshore.
>
> The Burma plate is supposed to be separating from the Sunda plate.
> Natawidjaja's version of that is that there is spreading occurring at
> various places on the island of Sumatra and between the island and the
> subduction trench, and that the spreading increases in the Andaman Sea to
> the north.
>
> North of Sumatra, the north-south aspect of the oblique convergence of the
> plates is handled by a network of faults in the Andoman Sea and a major
> fault in Burma.
>
> Natawidjaja does not mention a Sunda plate, either.
>
> I am also partway through the Hall article - so far he mentions the Burma
> plate but does not properly discuss it.    He appears to be far less
> interested in teh details of what is taking place than Natawidjaja is.
>
> Does anyone have any ideas?
>
> Is there anything about the boundaries of the Burma plate that would
enable
> me to identify its boundaries on a detailed chart of the faults of the
area?
>
> By the way, since all of the faults between Sumatra and the subduction
> trench have been mapped within the past few years, it should not be
> difficult for the NEIS to identify the fault that was affected.
>
> Their explanation of the quake makes less sense than ever;  there are no
> faults in the area where sections of crust move longitudinally along a
fault
> and thrust upward.    The thrust upward part and the move longitudinally
> part are the work of different faults.   Conceivably a piece of land could
> have moved upward if a new fault were created and a piece of land that had
> been held down bounced up, but again NEIS's explanation does not say that
> happened.
>
> Certainly if they could tell us so much as they did they ought to be able
to
> identify the fault - particularly if it is a preexisting fault 600 miles
> long.    Only one fault on Natawidjaja's chart appears to be a candidate.
> That would be the West Andaman fault.   The chart does not show whether
the
> West Andaman fault is 600 miles or more long; but it is the only fault
that
> exists in the area wehre the epicenter of the quake was, that extends off
of
> the northern edge of the chart, and its name suggests that it must extend
> for some distance in the Andaman Sea.   There is also something funny
about
> how Natawidjaja has it labelled.  What is a nomocline fault?   He also ahs
> syncline faults and anticline faults, those terms refer to two sides of a
> hill in a folded area; does a nomocline fault also have something to do
with
> moving up and down?
>
> Yours,
> Dora Smith
> Austin, Texas
> villandra@austin.rr.com
>