hi gang

here's a weird one for you to ponder ....
there's just been 3 events offshore NW  Sumatera  (aftershocks of the 8.6 
and 8.2)

6.1   2012/04/20 23:14:33    2.218    93.395  34.3   OFF THE WEST COAST OF 
NORTHERN SUMATRA
5.5   2012/04/20 22:29:00    3.280    93.858  28.3   OFF THE WEST COAST OF 
NORTHERN SUMATRA
5.6   2012/04/20 22:19:46    3.261    93.850  16.9   OFF THE WEST COAST OF 
NORTHERN SUMATRA

the 5.6  followed by the 5.5 and then the 6.1, as you can see they were all 
relatively shallow and similar
location.

The P and S of the 5.6 was visible with a bit of imagination, nothing of 
the 5.5 and nothing of the 6.1
The surface waves of the 5.6 obvious, those of the 5.5 significantly larger 
than the 5.6 and nothing from
the 6.1

Even the ASL station in northern Queensland, Australia backs this up where 
its vertical sensor shows the
surface waves of the 6.1 being similar amplitude to the 5.5
http://earthquake.usgs.gov/monitoring/operations/data/Seismic_Data/telemetry_data/CTAO_24hr.html

My sensor is N-S, the CTAO sensor is as said Z.   the epic's are roughly NW 
of me and WNW of CTAO

just an interesting observation. It really must have something to do with 
the orientation of the fault slip
and the direction of propagation of the waves. Interestingly on any of the 
ASL  WWSSN, the 6.1 surface waves were only a little bigger than those of 
the 5.5

cheers
Dave

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