[CIG-SEISMO] How to determine moment tensor in SPECFEM2D

[Dimitri Komatitsch]

Dear Xin,

To go from strike / dip / slip to CMTSOLUTION format using the classical 
formulas (of e.g. Aki and Richards 1980) you can use these small C 
programs from SPECFEM3D_GLOBE:

./utils/strike_dip_rake_to_CMTSOLUTION.c

./utils/CMTSOLUTION_to_AkiRichards.c

but then as mentioned by Carl it is another story to make a good 2D 
approximation of that, because in plain-strain P-SV what you get is the 
equivalent of a line source in the third direction (orthogonal to the 
plane) rather than a 3D point source (for more details on this see e.g. 
Section 7.3 "Two-dimensional point sources" of the book of Pilant (1979) 
below, or more recent textbooks; that book being hard to find, if you do 
not find it let me know and I can scan you the 5 or 6 pages of Section 7.3)

Pilant, W. L. Elastic waves in the Earth Elsevier Scientific Publishing 
Company, 1979, vol. 11 of "Developments in Solid Earth Geophysics" Series.

Best wishes,
Dimitri.

On 04/02/2014 07:03, Carl Tape wrote:
> Dear Xin,
>
> I have not used SPECFEM2D in quite awhile, but my recollection is that
> the moment tensor is 2D, so 2 x 2. I'm not aware of any tool in the code
> to reduce a 3D moment tensor into a 2D version, or even whether this has
> been done in any other 2D codes.
>
> Carl
>
> ---------------------------------------------------------------
> Carl Tape
> Assistant Professor
> Geophysical Institute
> University of Alaska Fairbanks
> Phone: 907-474-5456
> Email: carltape at gi.alaska.edu <mailto:carltape at gi.alaska.edu>
> Web: http://www.giseis.alaska.edu/input/carl/
> ---------------------------------------------------------------
>
>
> On Mon, Feb 3, 2014 at 2:51 PM, 汪昕 <peterxin8023sara at gmail.com
> <mailto:peterxin8023sara at gmail.com>> wrote:
>
>
>     Hi, i want set moment tensor as source type, but i want to know the
>     way how to determine these three components, Mxx, Mzz, and Mxz?  For
>     example, i assume the fault source is dip45/strike0/slip45 and the
>     moment magnitude is Mw=3.0, then calculate the seismic moment M0
>     (about 3.5E13 N.M). Can i use the equation to calculate the 9
>     components and set these value in 2D modeling? if not, what should
>     be the right way in SPECFEM2D.
>
>     Thanks a lot!
>
>     --
>     Cheers,
>     Xin
>
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-- 
Dimitri Komatitsch
CNRS Research Director (DR CNRS), Laboratory of Mechanics and Acoustics,
UPR 7051, Marseille, France    http://komatitsch.free.fr