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.