[CIG-SEISMO] more details on moment tensor sources in SPECFEM2D

[Dimitri Komatitsch]

Dear all,

Since that is an important question (that we also got several times in 
the past), I have scanned section 7.3 of Pilant (1979) (see the attached 
PDF) and I am going to add my answer below to the users manual.

Best wishes,
Dimitri.

On 04/02/2014 15:50, Dimitri Komatitsch wrote:
>
> 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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>>
>>
>>
>>
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>

-- 
Dimitri Komatitsch
CNRS Research Director (DR CNRS), Laboratory of Mechanics and Acoustics,
UPR 7051, Marseille, France    http://komatitsch.free.fr
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