On April 11, 2012, a magnitude 8.6 earthquake struck off the coast of Sumatra, followed two hours later by an 8.2 aftershock. Although no significant tsunami resulted, the after-effects of these quakes are still sending ripples through the seismological community.
The initial quake was the largest strike-slip earthquake ever recorded, and produced a global shaking that "rang the Earth like a bell," according to Ross Stein, a geophysicist at the U.S. Geological Survey in Menlo Park, California. In fact, the seismic event produced 16 moderate quakes (magnitude 5 or larger) around the world during the week following the seismic event, something which has never been seen.
AAASMC asked AAAS member Stein about the significance of the event and what it portends for future world seismic activity.
AAASMC: Could you please explain how the effects of this earthquake are considered greater than the earthquake that caused the Indonesian tsunami in 2004?
Ross Stein, Geophysicist at the U.S. Geological Survey: The magnitude of the 2004 Indonesian earthquake was 9.2, about 15 times larger than the 8.6 quake on April 11th; and yet the 2012 earthquake rang the Earth like a bell more strongly than in 2004, because it was a strike-slip rupture, meaning the material on either side of the fault moves sideways instead of up and down. The San Andreas Fault is a well-known example of a strike-slip fault.
The April 11th quake was the largest strike-slip earthquake we've ever recorded—perhaps by a factor of 10. The 2004 Sumatra earthquake had a large strike-slip component, but we would still call it a subduction quake (where one tectonic plate slides underneath the other).
The 2004 earthquake also took 1000 seconds to rupture, whereas the Indian Ocean event ruptured in about 100 seconds. This much more concentrated burst of seismic energy produced stronger shaking waves then anything we've ever recorded. That was a real surprise to me. While the 2004 earthquake also apparently triggered some global earthquakes, there were many fewer than what we saw in April.
AAASMC: How does the magnitude 9.2 Indonesian earthquake in 2004 relate to the recent earthquake activity, if it is related at all?
Stein: The occurrence of the 2005 magnitude 8.7 "Nias aftershock," if you will, of the Boxing Day 2004 earthquake that was adjacent to it, was probably promoted by that 2004 event; and similarly some of the bigger events along that subduction boundary appear to be the product of what we call "static (or permanent) stress increase." However, there are oddities of the concentration of earthquakes in this region since 2004 which are a little harder to attribute to permanent stress. So maybe there's an element of the shaking that somehow brings the faults closer to failure.
One of the papers in Nature, the (Matthias) Delescluse et al. paper argued that the 2012 site had been brought closer to static failure by the 2004 and 2005 earthquakes. So, in retrospect we can say the site of this earthquake was brought closer to failure by 2004, but that doesn't do us much good if we didn't even know the faults existed.
It's useful to learn that in retrospect, because maybe it's a strategy we can adopt elsewhere; but no one is claiming that the 2012 earthquake could have been predicted, even with hindsight.
AAASMC: These quakes were unusual in that they didn't occur on a fault line between tectonic plates; what kind of process are we seeing here?
Stein: This was another surprise. Nobody would have identified this site for a mega-earthquake. Generally, 95 percent of the energy released in earthquakes is all along the boundaries, and nearly all along the subduction boundaries. Very rarely is deformation seen in oceanic plates. Really almost the only place we see this is in the east Indian Ocean.
If you wanted to finger an oceanic plate that could host a large earthquake, this would have been everybody's bet. But the largest earthquake that had been observed there in the last 100 years was just below magnitude 8; so the April 11th event is much larger than anything seen there.
Also, we don't see evidence for one major fault; we see a lot of ancient, more-or-less extinct faults. This rupture looks more like a car crash than a fault line. It's five or six rather short faults all firing off at once in kind of a "knot" of seismicity; and in that respect it looks very, very different from what we normally see.
AAASMC: There have been three other, smaller strike-slip quakes here, in 2006, 2007, and most recently in January of this year, just a few months before the large quakes in April. Are we looking at an increasingly unstable region that may be prone to an intensification of earthquake activity?
Stein: Well it's been very unstable since 2004; and those of us who look at earthquake interaction have argued that much of that instability is related to the permanent stresses that each earthquake imparts to the surrounding crust. We think of these types of permanent stresses as having the ability to turn on and off and to promote the occurrence of earthquake sequences like the ones we've seen here. If you imagine the process as a line of bricks tethered together by a bungee cord and being pulled, you can foresee the possibility of cascading or progressive sequences of earthquakes which we have kind of seen here, but not completely.
AAASMC: But that assumes you know where that line is, correct?
Stein: That's right; when it comes to the subduction boundary, we really do know where it is. But when we look at some of the ruptures that have occurred since 2004, they're not actually on that subduction boundary but on secondary faults that lie above it. The boundary changes its character from being pure subduction to having this large strike-slip component; and we have no idea if this epoch of seismicity is over.
To the northern end of the rupture lies Myanmar and Bangladesh; they're both also in a heavy seismic zone that was brought closer to failure by the 2004 earthquake. We haven't really seen anything up there since the earthquake, but that doesn't mean that zone couldn't be next.
If we go the other direction toward Java, the southeastern end of this rupture zone, we get a truly frightening situation: that's one of the most densely populated places on Earth and one of the least prepared places for a mega earthquake when it comes to both shaking and tsunamis.
We're in the middle of some kind of slow-motion car crash; we don't quite know why all the pieces are flying off the car, but we have some clues.
AAASMC: What does this portend for the future of earthquake activity around the world?
Stein: That's a tough question, but the 2012 earthquake would appear to be a very rare event. We see nothing else like it—as clear as this or as strong as this—in the record over the past century. We have every reason to believe that though this event occurred, it's not likely to recur. If we go back to all the 8 and larger strike-slip earthquakes that have occurred in the last 15 or 20 years for which we have good records, they produced no obvious global triggering.
If we go back to the next largest strike-slip earthquake ever recorded, which is in Mongolia in 1905, there were a pair of 8.4 quakes two weeks apart. Obviously we can no longer hunt for magnitude 5.5 aftershocks, because the seismic network at that time was very modest; but we can hunt for 7s. We did have a 7 after the 2012 Indian Ocean earthquake, but we see none in the Mongolian quake. So that suggests to us that this global triggering phenomenon is extremely rare, and is not something we're likely to see frequently, if at all.
AAASMC: What about these two quakes has been a surprise to seismologists? What are we learning?
Stein: Almost everything has been a surprise. Before April 11th, we would have said that to have an earthquake over magnitude 8.5, you would have to be in a subduction zone; and we would have been dead wrong. Nobody believed we could have gotten an earthquake this large in this location.
Second, the main quake and its largest magnitude 8.2 aftershock were on distinctly different faults. The faults that ruptured are not parallel to each other; they have at least two different orientations. It's more like a "blob" of aftershocks than a line of aftershocks that you would expect for, say, a San Andreas earthquake.
We also thought only tiny earthquakes could be triggered around the globe from big main shocks; there are very powerful studies that have shown that magnitude 7 and larger earthquakes do trigger little shocks randomly around the globe. Now that we've seen large aftershock triggering in 2012, we need to figure out what clues it gives us about the earthquake preparation process and the rupture process.
All but one of the 16 magnitude 5.5 and larger aftershocks of the 2012 earthquakes around the globe occurred long after the shaking waves had subsided. So it's not as simple as the waves encircle the globe and when they engulf faults that are very close to failure, those faults fire off. There's some delay; it can be hours and up to almost a week after the waves encircle the globe that aftershocks are triggered.
Even though we can't monitor faults to see what's going on in those hours to days, it tells us there's something to look for. It may not be something we can measure at the earth's surface, but it's a target that we should aim for.
I'm in a very humbling field. An awful lot of what we see is a surprise. The key thing is to be willing to throw off your bias when you see something that flies against all of your expectations, and figure out what you can learn from it. That's a process we're just beginning here.
- For more information, see http://www.seismicity.net/