8.9 quake in Sumatra...

[Jon P. Inghram]

Looking at the focal mechanism beachball, it was either almost entirely vertical, or, much more likely given the geology of the faults there and the apparent lack of tsunami, almost completely horizontal.

               -------                
          --##############-           
        -####################         
      -########################       
    #---########################-     
   #------#############   ########    
   ---------########### T ########    
  ------------#########   #########   
  --------------###################   
  ----------------#################   
  ------------------###############   
  --------------------#############   
   --------   ----------##########    
   -------- P ------------########    
    -------   ---------------####     
      -------------------------       
        ---------------------         
          -----------------           
               -------

[GuchiGuh]

Jon P. Inghram, can you explain your diagram please?

[Jon P. Inghram]

http://quake.wr.usgs.gov/recenteqs/beachball.html

[mmp121]

I think what he's saying is that any TSUNAMI's that were created as a result of this quake were in a direction that was away from any major land masses. I think that most of the energy was focused towards the south-west.

[KvHagedorn]

No, what he's saying is that if two plates move sideways against each other it doesn't cause as much disturbance in the ocean as if one plate suddenly drops (gets subducted) 25-30 feet.

[Jon P. Inghram]

Those diagrams show the plane that the fault slipped on. In this case, it was either a horizontal fault with one side sliding under the other to the northeast, or a vertical one sliding nearly vertically up on the SW side and down on the NE. The December '04 one dipped down at a little steeper angle, 11 degrees vs. 4 degrees, perhaps that was enough of a difference as to not disrupt the sea floor to the point of causing a tsunami.

[cjolley]

No, what he's saying is that if two plates move sideways against each other it doesn't cause as much disturbance in the ocean as if one plate suddenly drops (gets subducted) 25-30 feet.

However this does not seem to be relevant to this pair of quakes.
One producing a Tsunami and one not.
Apperently it is more complicated than that:

http://www.cnn.com/2005/TECH/science...ami/index.html

[Jon P. Inghram]

However this does not seem to be relevant to this pair of quakes.
One producing a Tsunami and one not.
Apperently it is more complicated than that:

http://www.cnn.com/2005/TECH/science...ami/index.html

From what I've read, most of the quakes 9.0 and larger have been from similar situations, one plate thrusting over/under another at a subduction zone, and virtually all of them have produced big tsunamis, so it's definately a tad unusual.