The quakes started with a massive 8.9 quake south west of the closest subduction zone. This area is not really part of the subduction fault but is an area of the Indian plate that is bending to allow the subduction to happen. Because of this it is not a mega thrust type of quake but is just either bending or sliding motion and not causing as much of a water displacement so a smaller tsunami is produced. A tsunami was produced but as the quakes was also far away from land the produced tsunami was small when it did make it to shore.
All of the rest of the quakes after this initial quake are aftershocks, even the 8.2 just 2 hours later. It also produced a small tsunami that was not a danger to anyone. My suspicion is that these set of quakes have relieved a lot of pressure on the system and while there will be aftershocks for a while, because they are outside of the subduction zone represent more of a redistribution of the stress left over from the 2004 quake than something new to be worried about. (But having said that it will probably do something new just to prove me wrong.)
People will tell you that quakes in these areas shouldn’t be this large and that is the traditional thinking, but they can be and bigger. In this area the subduction zone is curving and this set of quakes is on the bisection line of that curve. Stress builds up here as the zone subducts (try bending a sheet of paper in a curve and see in the unbent portion where the stress builds.) With such a large quake in 2004 and all of the other activity on this fault zone in the last few years you start to see a pattern of quakes in the record of quakes in this zone. This is more like breaking a keystone and so the release of pressure can be quite large.
The Mexico quakes started with a 6.5 north west of Acapulco. this quake was in a sparsely populated area and so there was no real damage. A few hours later on the same fault line but further north the earth shock again with 2 large quakes, a 6.2 followed closely by a 6.9. These faults are more transform faults than subduction faults in the north but turn into a subduction zone in the south and the mid ocean ridge more or less is down the middle of the Gulf of California. This is the same fault that then proceeds north and becomes the San Andreas fault.
The quakes were all relatively harmless because of location but they make this a region we should keep our eyes on in future. If the quake in the south did set up the two northern quakes then the worry I have is that they may be setting up something else even further north which is getting into more populated and built up areas. Keep tuned I guess.
- April 11..the earth wobbles again (seeker401.wordpress.com)
- Why was the Sumatra quake so large? (newscientist.com)