Re: looking for a clear explanation of the earthquake

[Dora Smith <villandra@austin.rr.com>]

Jerry:

Well, it does help some.  I actually did find much of
what you cite in here last night using what Meor told me.

What is actually important about the Burma and Sunda plates is how whatever
they are doing that is different from what you'd expect the Eurasian plate
by itself to do.

If they are doing nothing differently from what would happen if we were
talking about the Eurasian plate, then they should have just said the
Eurasian plate for the sake of simplicitity and ability of the public to
understand what they are talking about.   Obviously the Burma plate is the
leading edge of one of the world's most violent subduction zones, so it
doesn't take rocket science to explain for instance why it might be full of
faults.

I learned that both the Sunda and the Burma plates are former parts of
Gondwana that broke away and ended up stuck to the Eurasian plate.   The
Sunda plate moves eastward at a slightly greater rate of speed than north
and south China, which is moving completely differently from the rest of the
Eurasian plate.   Alternatively, the Sunda plate is rotating and revolving
around some pole south of Australia.

I learned that the Sunda plate is somehow moving southward along the
boundary with the Indian/ Australian plate (s) - not even these technical
geological articles could agree on whether it's the India or Australian
point at that point, which is located between northern Sumatra and the
Nicobar and Andoman Islands - eeven though the articles consistently state
and diagrams consistently show that the entire Sunda plate moves eastward.

The only two things that I could learn about the Burma plate are that it is
composed of a chunk of Gonwandaland, and that the INdian/ Australian
plate(s) subduct underneath it.

Now right there this doesn't make sense.   The Burma plate sits between the
Indian/ Australian plate(s) and the Sunda plate in the entire region where
Sundaland allegedly moves straight south while moving straight east.   What
does the Indian/ Australian plate(s) do when it gets to the Sunda plate on
the other side - slide back ABOVE GROUND so that the Sunda plate can slide
south alongside it?    That would sure make for some spectacular seismic
antics, LOL!

What I want is a simple plain language explation of what these plates are,
wehre they are, and how tehy contributed to the earthquake, if they did.

The way all my sources describe a strike/ slip fault, they are two
dimensional, and if something changes altitude, it drops.    So something
wouldn't thrust suddenly upward.  What is more, all of the reports
specifically say that the earthquake was a megathrust fault even though it
involved a strike/ slip fault.   And moreover, ALL earthquakes that violent
result from megathrust faults.   So now it's telling us the earthquake was
caused by subduction.

You're right, it might be true that there was more than one kind of motion
at once - though atleast analytically that is not logically possible.   Two
plates cannot slide smoothly past each other and under each other.    I can
think of several possible ways that the facts presented in the NEIS web
pages on the earthquake could be accurate as far as they go.   The problem
is they don't specifically explain their contradictory statements.

If anyone gets a clearer explanation of what happened I sure would
appreciate learning of it.

I won't be able to get my hands on that article Meor pointed me to before UT
Austin returns from the dead in three weeks.    The map and geology
libraries are both closed for all of Intersession, and it looks like the
members of the geology factulty who specialize in Southeast Asian tectonics
have flown south or something.

Yours,
Dora Smith
Austin, Texas
villandra@austin.rr.com
----- Original Message -----
From: "Jerry D. Harris" <jharris@dixie.edu>
To: "DINOSAUR Mailing List" <dinosaur@usc.edu>
Sent: Thursday, December 30, 2004 12:21 PM
Subject: Re: looking for a clear explanation of the earthquake


> Hi Dora et al. -
>
>       OK, I'm no geophysicist, but I'll give this a go:
>
> >First of all, what are teh Burma and Sunda plates - besides part of the
> >Eurasian Plate?
>
>      They are microplates -- small(ish) plates bounded by unique tectonic
> boundaries that have sort of accreted onto the current southeast Asian
> "plate" (actually, they constitute a volcanic arc that ultimately will
> completely accrete onto the Asian "plate").  This is not an unusual
> phenomenon at all; there are literally hundreds of microplates of varying
> sizes on the planet today; most of them are loci for serious seismic
> events -- for example, the huge earthquakes in Turkey of a few years ago
> were the result of the Anatolian microplate being jammed into Europe by
> Africa's north(ish) movement.  Other than brief mention, microplates
aren't
> usually discussed except in advanced geophysics classes simply because
it's
> easier to understand the global system with a smaller number of
components,
> so usually only the "big boys" get star billing.  However, in the
technical
> sense (the devil is ALWAYS in the details...), it is probably best not to
> think of these plates as "part" of the Eurasian "plate" until they become
> firmly sutured, which, to they extent of my knowledge, hasn't yet
happened.
> An excellent and well-written primer for anyone interested in tectonics
(and
> that has some geological background) is:
>
> Cox, A. and Hart, R.B. 1986. Plate Tectonics: How It Works. Palo Alto:
> Blackwell Scientific Publications, 392 pp. ISBN 0-86542-313-X
>
> ...though even they onle touch on microplates in a single paragraph, with
> respect to paleomagnetic reconstructions (my own comments added in
> brackets):
>
> "Although the term microplate is sometimes used to describe crustal blocks
> that have rotated, many of these are clearly not microplates in the sense
of
> being bounded by ridges, transforms [transform, or strike-slip faults],
and
> trenches that cut through the entire lithosphere.  In some cases the
> tectonic domains that have rotated as a unit appear to be only 10 km long
or
> less, suggesting that structural blocks in the upper part of the crust are
> being rotated in response to a broad shear zone at greater depth."
>
> I am not familiar with the nature of the eastern (Asia-side) boundaries
> between the Sunda and Burma plates and Asia, so I can't comment on how
> "correctly" they are being referred to as "microplates."
>
> >Is the Burma "microplate" actually just a zone of loose rock and faults?
> >It appears to constitute the entire region immediately east of the java/
> >sunda trench, which is the point where the Australian and Indian plates
> >(which of those plates is actually there and whether they are two plates
is
> >clearly a matter of controversy) slides under the Eurasian plate.   Why
is
> >the Burma plate or microplate called a plate at all?    Logically it is a
> >fault zone, probably uplifted above the trench.
>
>     It is being considered a microplate because it has its own system of
> movement independent of the remainder of the Eurasian "plate" (see
> http://www.geologie.ens.fr/ ~vigny/giac2-e.html), to whit:
>
> "However, there are also significant differences, which form the basis for
> the identification of a rigid "Sunda block" exhibiting distinct relative
> motion with respect to Eurasia. The rotation of this block is such as to
> accomodate relative motions with respect to India (no more than 3.5 cm/yr
of
> NS opening in the Andaman sea) and with China (no more than 1.5 cm/yr of
> right lateral slip along the Red River fault). In an Indian reference
frame,
> Sundaland is moving due South so that the motion is purely dextral
> strike-slip north of Sumatra. The predicted velocity of India at the
> latitude of Myanmar is close to the rate of opening of the Andaman sea,
> suggesting a low subduction rate along the Andaman front. Further North,
the
> Sagaing fault in Myanmar can be regarded as the continuation of the
Andaman
> sea opening."
>
> Remember that the Earth isn't, in the short term, headed toward becoming a
> small number of fixed plates: larger plates are being fragmented into
> smaller ones all the time (e.g., eastern Asia is being severed from the
> larger Eurasian mass along the Baikal zone, the result of what is called
> "tectonic escape" driven by India's crashing into Eurasia; also, east
Africa
> is being severed along the Rift Valley zone and will eventually be its own
> plate. Baja California is actually on a microplate, and most of western
> North America, particularly Canada and Alaska, are made up of microplate
> terranes accreted over the last 100 million years or so.  You can see some
> of these at http://www.dpc.ucar.edu/VoyagerJr/JVV_Jr/help/helpvel.html,
and
> for SE Asia, there's a "simplified" picture in
> http://www.gl.rhul.ac.uk/seasia/pubs/Hall_1997.pdf that will show you how
> insanely complex the area is!). So while north of Sumatra, the motion is
> nicely parallel and strike-slip, moving south, the boundary bends and
> relative movement becomes oblique, so I think there must be some
strike-slip
> motion and some subduction going on.
>
> >On one of the faults in the area I found evidence that some rock tries to
> >move out of the way, along the fault zone.   Is the Burma plate an area
of
> >crust that is behaving in this way?    That would logically account for
> >NEIS's insistence that the India and Australia plates move northward as
> >well
> >as eastward with respect to it ( though it would require that the Burma
> >plate in fact be moving south) - and that there are slip/ slide faults
> >along
> >the boundary.
>
>     The problem really is that while plates are easy enough to comprehend
in
> 2 dimensions (like sliding pieces of cardboard around on a flat surface),
> things aren't nearly so nice and neat on a sphere.  Remember that all
plates
> are curved, like pieces of a shattered light bulb, trying to ride around
on
> a gooey spherical undersurface.  That makes for some really ugly geometry.
> Tectonic boundaries, whether subduction zones, spreading centers, or
> transforms, aren't always moving in nice, 90-degree angles, so there are
> movements that are part subduction, part transform where two plate motions
> aren't either neatly parallel or perpendicular to each other. This is also
> why relative movement on different parts of the same plate can have
> different velocities (meaning different speeds and/or different
directions).
> Incedentally, the Indo-Australian plate appears to be splitting into two
> separate plates just because of this difference in relative motions.
>
> >WHAT exactly is the Sunda plate, and why is it called a separate plate?
>
>     See above.  This analogy (far from perfect, but may be helpful as a
> first-order approximation) may help: imagine breaking a cookie in half,
then
> moving each half in opposite directions (strike-slip motion).  Small
crumbs
> in between are going to move, but their movements will not be the same as
> either main cookie half -- they will rotate, lift, drop, etc. depending on
> their unique geometries and the geometries of the edges of the cookie
> "plates" on either side (not to mention other crumbs that abut them).  The
> Sunda plate is analogous to one of the crumbs; the cookie halves represent
> the Indo-Australian and Eurasian "plates" on either side.
>
> >The Sunda plate is part of the Eurasian plate and not usually depicted as
a
> >separate plate.   In fact before last night I could find next to nothing
> >about it in Google, except that two countries sit on it.    I have been
> >unable to find a map that shows the entire Sunda plate, but it appears to
> >constitute Burma, Thailand, and Vietnam, and the Indonesian islands to
the
> >north and east of the java trench/ island arc formed where the Australian
> >and Indian plates meet the Eurasian Plate.    On the western border, it
> >meets with the Pacific plate, and its rigidity relative to plates around
it
> >purportedly causes trouble for Bali and another nation that sits on its
> >boundaries.
>
>      See above for why it's probably so hard to find any info on the
> microplate.
>
> >I didn't find anything that says it moves differently with respect to the
> >rest of the Eurasian plate or something - which is the rationale for
> >thinking there is an Indian plate separate from the Australian plate.
I
> >found a whole article showing that those two paltes move differently with
> >respect to each other though thye have a poorly defined border and noone
> >can
> >decide where it is.
>
>      Again, I'm no geophysicist, so I can't comment on the current
zeitgeist
> of that community, but what little perspective I have indicates that there
> is sort of a split (ba-dum chik!) between those who want to view the
> Indo-Australian plate as one plate and those who perceive it as two.  I
have
> no idea the state of development of any sort of boundary (probably a
> spreading center) between them.  Yet:
>
> "At abount the time of anomlay 19 (43 Ma), there was a major reoganization
> in the plate motions. Spreading on the Carlsberg-Central Indian and
> Southeast Indian ridges changed from approximately north-south to the
> present northeast-southwest. This was the time when the Indian and Asian
> continents collided...The fate of the ridge between Australia and India is
> not clear. It is possible that India and Asutralia then lay on the same
> plate. There has been slight motion between India and Australia since
about
> the time of anomaly 13."  (from: Fowler, C.M.R. 1990. The Solid Earth: An
> Introduction to Global Geophysics. Cambridge: Cambridge University Press,
> 472 pp. ISBN 0-521-38590-3 -- a much more technical and math-oriented book
> than Cox & Hart.)
>
> >Speaking of noone knowing where the boundary is between the India and
> >Australia plates, where is it?   Before yesterday, the eastern border of
> >teh
> >India plate was I think the mid-ocean Ninety-east ridge.    Now suddenly
> >the
> >India and Australian plates have an east-west boundary that ends at the
> >java
> >/ sunda trench off of northern Sumatra, near where the quake occurred -
and
> >that helped cause the earthquake.
>
>     I couldn't say what its contribution to the quake was, but yes, the
> current (though tentative -- it's always a dashed line) reconstructions
I've
> seen seem to show that the "boundary" between the "Indian" and
"Australian"
> plates does seem to terminate right around where the quake occurred.
>
> >Now the part I REALLY don't understand.   NEIS's explanation of how the
> >quake happened - which I found more or less paraphrased elsewhere.    The
> >India and Australia plates move northeast with respect to the Eurasia
plate
> >and subducts underneath it where the plates meet.   Logical so far.
> >Where
> >this takes place there are thrust faults.   Also logical.   It more or
less
> >requires that parts of the Eurasia plate are thrust upward at the
boundary.
> >That in fact created the entire island arc.
>
> >Now for not logical.    According to NEIS, the India and Australian
plates
> >also move northeast with respect to the "Burma microplate", which makes
> >slip/ slide faults like the San Andreas fault in California, where two
> >plates glide sort of smoothly past each other.   Occasionally they hang
up,
> >and when the rock breaks there is a quake.  Then the two plates continue
> >their seperate northward and southward journeys.   Quakes in California
do
> >not result in land near the fault thrusting upward.
>
>      That may be because there is no oblique (subductive) component to San
> Andreas movement, and thus no relative thrusting.
>
> >According to NEIS, as paraphrased in various places with no other
detailed
> >explanation of the quake available, this quake resulted from a rock
> >breaking
> >loose along a SLIP/ SLIDE fault, between the India/ Australia and Burma /
> >Eurasia plates.    When that happened, a nine- mile wide strip along the
> >entire near boundary of the Burma/ Eurasian plate, abruptly SLIPPED, and
> >THRUST a hundred feet UPWARD.    That just doesn't sound like something a
> >slip/slide fault could do.
>
>      Again, possibly the function of the combined strike-slip and
subductive
> motion of the plates in the vicinity of Indonesia in question here that
> aren't typical of better studied plate boundaries elsewhere.
>
> >How is this even possible?   Is what really happened that the Australia/
> >India plate got stuck while sliding UNDER the Burma plate, and when it
> >broke
> >free the edge of the Burma plate abruptly thrust upward?     That would
be
> >far more logical.
>
>     Yes, but again, a combined strike-slip and subducting motion would
> explain both.
>
> >By the way, is there ANY justification for thinking that New Zealand sits
> >on
> >its own plate?  Two plates meet and twist in a bizarre way there - hard
to
> >see how there could be a separate plate, nor how there would be just ONE
> >separate plate, logic says there would have to be two;  but maybe it's a
> >Burma microplate sort of thing?
>
>     Well, New Zealand _is_ on two plates, but neither is a microplate:
part
> is on the Pacific plate and part is on the Australian plate.  But it is a
> complex system -- see
> http://members.tripod.com/NZPhoto/volcano/atectonic.htm.
>
>      Dunno if this was helpful or not, but there it is.
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Jerry D. Harris
> Director of Paleontology
> Dixie State College
> Science Building
> 225 South 700 East
> St. George, UT  84770
> Phone: (435) 652-7758
> Fax: (435) 656-4022
> E-mail: jharris@dixie.edu
>  and     dinogami@hotmail.com
> http://cactus.dixie.edu/jharris/
>
> "The subject which I have chosen for the customary
> Address this year lays no claim to authoritativeness; it is
> not a wide synthesis of the state of knowledge reached
> in any particular field; nor does it pretend to any
> particular intrinsic importance." -- Sir Gavin de Beer,
> opening sentence of his 1947 Presidential Address to
> the Linnean Society of London
>
>