Step 3: The December 26, 2004 earthquake near coast of Sumatra.

Now determine the focal mechanism of the December 26, 2004 earthquake near the coast of Sumatra (epicenter 3.31N 95.85E). Use the P-wave polarities of the following 11 seismograms.

Station Vertical seismogram
COCO View vertical component seismogram
PALK View vertical component seismogram
QIZ View vertical component seismogram
DGAR View vertical component seismogram
LSA View vertical component seismogram
CTAO View vertical component seismogram
KMBO View vertical component seismogram
EIL View vertical component seismogram
SBA View vertical component seismogram
SJG View vertical component seismogram
OTAV View vertical component seismogram

Table 2. Input data of the December 26, 2004, Sumatra earthquake.
D = epicentral distance, f = azimuth of the station, ih = take-off angle, Up/Down designates the polarity of the P arrival.
>
Station Lat (N) Lon(E) D(deg) f(deg) ih(deg) Up/Down?
COCO-12.19 96.83 15 17647
PALK 7.27 80.70 16 28544
QIZ 19.03 109.84 21 4036
DGAR -7.41 72.45 26 24529
LSA 29.70 91.15 27 35029
CTAO -20.09 146.25 54 11723
KMBO -1.13 37.25 59 26622
EIL 29.67 34.95 63 30121
SBA -77.85 166.76 89 16815
SJG 18.11 -6.62 152 3205
OTAV 0.24 -7.85 173 3011

8. Plot the polarities in a lower hemisphere stereonet, and determine the strike, dip and rake of the two possible fault planes. Also plot the P- and T-axes.

9. Can you determine which of the two planes is the most likely fault plane based upon the historical seismicity ?

10. Does this focal mechanism fit in the tectonic setting of the area? Explain.

11. Now check if your focal mechanism based on first motions corresponds to the Global Centroid Moment Tensor Solution , which is obtained from amplitude modeling of entire seismograms (NP1 and NP2 specify the nodal planes, or possible fault planes, of the best double couple solution, and slip is the same as rake).



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