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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