The oddity of the Banda Aceh earthquake

Earlier I posted an info bulletin about this morning’s Banda Aceh earthquake.  Rather than muddle it with more and more stuff, I thought it might be better to include this update as a separate post, as it is more of a discussion than a news piece in any case.

The truth is that this earthquake is properly strange.

The part of the Indian Ocean in which this earthquake occurred has two very different types of geologic structure very close to each other; there is the Ninetyeast ridge – a volcanically produced range,  and a destructive margin subducting the Indian plate eastwards under the Pacific margin. It is important to note (in the context of this earthquake at least) that the sea floor under the Ninetyeast ridge was originally produced by standard constructive margin seafloor spreading.

These two structures are shown quite nicely in this image taken from GeoMapApp, with the earthquake location shown in red.  The Ninetyeast rise is in the left of the image, with the subduction zone and Indonesia to the right.

What is interesting is that these types of feature generate very diffierent kinds of earthquakes.  Subduction zones form thrust faults, where the two plates are moving together, while constructive margins and volcanic structures are dominated by normal faults, where extension is accomodated by fault slipping.

Because the Earth is a spheroid, a constructive plate margin cannot extend at the same speed at all latitudes, therefore constructive margins also have a whole load of transform strike-slip faults which allow the ridge to offset, and accommodate variable spreading rates.  There’s an example of these from the Mid Atlantic Ridge shown below. These things, once formed, remain as weaknesses in the oceanic crust throughout its lifecycle.

Bathymetry showing transform faults offsetting a spreading ridge (Mid Atlantic fast spreading section). From GeoMapApp.

The way the fault moved should therefore tell us what the source of the earthquake was.  And it was strike-slip. Which suggests it should be related to one of those offset transform faults.

The weird thing is, that as you can see, the bathymetry aropund this earthquake does not show any transform faults at that location.  Indeed, that volcanic ridge is inactive.  In fact, the Encyclopedia Britannica even use it as an example of an aseismic ridge (i.e. a ridge which does not have earthquakes). While it is possible that an old fault formed when teh seafloor was first produced at a constructive margin has reactivated, such a large earthquake is unusual.  It’s worth noting that this movement has historical precedent, with other large earthquakes occurring in a tight cluster around this area in the past.

This map is perhaps most interesting of all.  It shows two other strike-slip motions related, one in 2006, one in 2007.  You can see the thrust faults associated with the subduction zone all clustered up in the North East of the map.  This relationship with other strike-slip activity, in what is a fairly linear pattern might well suggest reactivation of an old transform fault.  Seeing as there’s no real expression in the bathymetry, it must be a pretty old one though, perhaps being deformed as it passes into the subducting region and the plate is forced to begin bending.

The fact it’s strike slip is very good news indeed for the populations around the Indian Ocean – because the movement is sideways rather than vertical, it means the water doesn’t get moved around anywhere near as much, so the tsunami risk is much lower.  It also seems to have occurred in deep water.

That is little comfort to many geologists, however, who I suspect are going to be scratching their heads over the precise geometry of this one for some time.

UPDATE: a realtime map screengrab from the USGS site showing the quake and its aftershocks (ignore the yellow ones near the mainland, they’re unrelated).  The pattern is very weird – not what you’d expect from activation along a single linear and subvertical strike-slip fault. I possibly suggests that the change in the stress regime is activating multiple faults. Whether those are NE/SW or NW/SE trending is something of an open question at this point.  What is certainly worth highlighting is the range of depths.  Remembering that oceanic crust is only 6-8km thick in most places (with perhaps a few km of sediment on top of that), these faults are generating either at the very base of the crust, or within the rigid lithospheric mantle.

There’s been a series of emails firing back and forth through our department this morning discussing the possibilities here, and what has been pointed out by Graeme Eagles – something of an expert in plate motions – is that the India-Australia plate is under a really acute stress regime as it gets split into 3 and twisted.  That leads to these large build ups, and eventual release under strike-slip.

For those of you who want the real technical low-down on it, here’s Graeme’s take on it:

This happened in the middle of the boundary between the Australian and Capricorn component plates of the Indo-Australian plate. The Indian-Australian plate is twisting apart into three pieces about rotation poles contained within very broad boundary zones. The relative motion is slight and very slow, allowing large stress build ups without the possibility of localisation into a narrow zone where familiar plate boundary processes might lead to periods of aseismic slip between smaller events.

This slow motion can be determined from misfitting magnetic anomalies on the Indian, Capricorn and Australian parts of the Indian-Australian plate – you can’t close a circuit with the African and Australian plates without moving these parts relative to each other. As well as the broken circuit and the intraplate seismicity, gravity and seismic data show the boundary zones are characterized by moderate wavelength folding (and possibly elastic buckling too) affecting the entire crust.

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About Pete Rowley

Earth Science researcher. Enjoys a good rant, beer, and watching films with Angelina in them. Dislikes reality TV, crowds, and unreasonable people.
This entry was posted in Earthquakes, General, Geology, Geophysics, Hazard Assessment, News, Science and tagged , , , , , , , , , , . Bookmark the permalink.

17 Responses to The oddity of the Banda Aceh earthquake

  1. Pingback: M8.9 Banda Aceh | lithics

  2. Nate says:

    Reblogged this on Adventures in Geology and commented:
    Seeing that I’m in the middle of some extensive lab tests, and am preparing to present my research (thus far) next week, I thought this would be the perfect opportunity to test the “reblog” feature on WordPress which allows hosted blogs to easily share another blogger’s work. I’m not sure if I like the feature, yet, but it serves as a great way to showcase some fine work by others…especially when I don’t have the time to put in to writing, myself!

    Pete Rowley, on his blog “Lithics”, wrote a couple great articles on today’s Magnitude 8.6 earthquake in Indonesia. Read below for his second article on the quake and, please, visit his site to see the first article as well as plenty of other good reads!

  3. Reblogged this on The Trembling Earth and commented:
    I’ve got tons of stuff I should be doing, but I can’t resist the allure of discussion of the fascinating pair of earthquakes that happened last night in the Indian Ocean. I don’t have much of anything to add to the existing discussion, and I need to, like, get to work, so this is a “reblog” of the most thorough description I’ve seen of the setting of these quakes, by Pete Rowley on his “Lithics” blog. Note that there were two, both were massive (8.6 and 8.2), and they had a “foreshock” on January 10–sameish place, similar focal mechanism. Both of these are larger than any strike-slip earthquake we’ve recorded before, and the larger one makes it into the top 10 list of all time in any setting. It’s additionally fascinating (read: perplexing, at the moment) that they happened in oceanic crust, which is generally considered too thin to support massive strike-slip earthquakes. This will be an absolutely fascinating earthquake sequence to learn from in the coming hours, days, and weeks.

  4. Bob Myhill says:

    Could they have happened on some of those whacking great NNE-SSW striking transform faults (90E – 93E) formed ~30-50 Ma? That would seem the most sensible answer to me. I agree with Graeme’s general argument, but the stress doesn’t really need to be that high if we’re reactivating old faults.

  5. volcanocafe says:

    Really good article.
    I just had to try out the reblog function, and since EQs are not my field really…
    Put it on http://www.volcanocafe.wordpress.com

    Regards!
    Carl

  6. volcanocafe says:

    Reblogged this on volcanocafe and commented:
    This is the best article so far on the Banda Aceh quakes. So I decided to Reblog it.

  7. Ursh says:

    these EQ happenend around the Equator, with the axis changing and the bulge of the EQ changing, could the plates seperate? there are now 40 EQ between 10-30km dept, listed in two distinct lines, like a zip

  8. Pete Rowley says:

    You have to remember that the equator is relatively meaningless geologically – it is simply a great circle, no different to a line of longitude, or any other circle we draw around the Earth which crosses at the opposite side of the planet.

    These earthquakes are simply releasing stress around an area, likely along pre-existing fractures (hence the parallel lines). This happens after pretty much every big earthquake; after the first event has released stress from one region, that stress is transferred further along the fault and often triggers these aftershocks. This is best demonstrated by some of the animations of the Honshu aftershocks; the dipping subduction zone fault sheared in one place, then triggered stress release at other places along the fault. If you map them by depth you get a really nice idea of the dip of the subducting plate, which tallies with the regional seismicity map, e.g. http://www.emsc-csem.org/Images/EVID/21/217/217156/217156.regional.seismicity.jpg

  9. Pieter says:

    Is the Ninety East Ridge really a former spreading center? I always thought it was the ‘tail’ of the Kerguelen hotspot. An extinct hotspot trail does not have any ‘inactive’ faults, which makes this theory pretty implausible. I’d rather say it has something to do with the supposed (transform?) fault between the Carlsberg ridge and the subduction zone near Aceh which has already produced some impressive earthquakes (M7.6) near the Chagos Archipel. Correct me if I’m wrong, I’m not an expert in this field.

  10. Renato Rio says:

    Great discussion!!!!

  11. Renato Rio says:

    How about THIS earthquake? Located in the Southern tip of the ridge, same longitude…Related?
    4.8 2012/04/12 23:15:48 -44.509 94.263 10.0 SOUTHEAST INDIAN RIDGE

  12. Pingback: 1200 anak sekolah nonton film inspirasi | inspirasi.me

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