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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