Wednesday, October 29, 2008

Earthquake in Pakistan

BBC: Scores dead after Pakistan earthquake
Scientific American 60-second-science post

There were two moderately large (M6.4, M6.1) earthquakes within twelve hours:

The earlier one had an epicenter right beside a populated valley - Khanozai, Pakistan. (Unfortunately.)

The second one had an epicenter beneath some spectacular plunging folds (WOGE-worthy, except that I'm posting the picture out of turn):

Both earthquakes had similar focal mechanisms. North-south compression (along the India/Asia collision), earth-west extension, along a strike-slip fault (again, common in Asia - see Tapponier et al., 1982, for example). Based on the aftershock locations, I would guess the NW-striking solution is the real fault. I'm curious about the relationship of the seismogenic faults to the plunging folds, though. I don't have a quick explanation... and I'm just starting to talk about faults in class this week. I would love to show the Google Earth images, but the folds show up so well on the satellite images that I should be prepared to answer questions about them in a semi-coherent way.

The earthquakes are also right at the place where the folds near the plate boundary make a sharp bend:

(By the way, does the apparent strike of the fault line up with the boundary of that basin by Kandahar, Afghanistan? It's the brown area west of the earthquakes; I don't have the cities layer turned on in my image.)

I'm not sure how that affects the faulting, either.

Reference: Tapponier, P., Peltzer, G., Le Dain, A.Y., Armigo, R., and Cobbold, P., 1982, Propagating extrusion tectonics in Asia: new insights from simple experiments with plasticine: Geology, v. 10, p. 611-616.

Edit: There's discussion of landslide potential from these two earthquakes at Dave's Landslide Blog: post #1; post #2.


Miguel Vera said...

Could that 5.2 eq only half-hour before the first strong one be considered as a foreshock? What do you think?

Kim said...

You know, I didn't actually look at the time of all the other earthquakes. I'm not a seismologist, but, well, if a little earthquake comes before a bigger earthquake (and it's on the same fault, or clearly related in some way), then I guess it would be a foreshock.

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Hawkman said...

they are occured in the thrust-fold belt,so there is a plunging fold is not a question,the struct of this area must be complex,for it has normal faults thrust faults and folds.