[CIG-SEISMO] SPECFEM3D-Globe, near epicenter velocities

[Dimitri Komatitsch]

Hi,

Yes, SPECFEM solves for the full wave equation and thus includes the 
near-field terms, computed accurately (see e.g. 
http://komatitsch.free.fr/preprints/GJI1_2002.pdf figure 20).

However, if you are in the mesh element of the source, or very close to 
it, you may get spurious Hourglass-like modes, see Schmicker et al. 2014 
(attached). When they are present, getting rid of them requires changing 
the mesh (for instance randomizing it) near the source.

Best regards,
Dimitri.

On 04/05/2018 07:39 PM, Inchin, Pavel wrote:
> Good day,
> 
> I have a problem with simulation of velocities at close to the epicenter 
> distances.
> I need to obtain surface vertical velocities with 2 km step to the North 
> (+3000 km) and South (-3000 km) from the epicenter along longitude of 
> epicenter, so I just get synthetic seismograms for every such point from 
> SPECFEM3D-Globe.
> 
> Here is a result of global simulation SPECFEM3D-Globe for NEX_XI=96 at 
> some time of simulation (20mHz-example figure). The simulation looks good.
> 
> 
> Now, I do the same simulation for NEX_XI=240 and I get a very slowly 
> decreasing rise/drop near the epicenter. Here are several figures for 
> different times (50mhz-1.32, 50mhz-2.23, 50mhz-4.02, 50mhz-6.22 files; 
> time is shown on figures - minutes from event):
> 
> 
> 
> 
> 
> So it doesn’t look like sinusoid, but slowly decreasing rise/drop (to 
> the left and right from epicenter respectfully). It takes about 10-12 
> minutes to fully suppress this.
> 
> I do not have it for NEX_XI=96 simulation, but for shorter period 
> simulation it starts to appear. I tried different Earth models, but 
> seems it doesn’t play any role.
> Does it have physical nature?
> 
> And more general question - how appropriate SPECFEM3D-Globe to simulate 
> velocities at the epicenter and positions very close to it?
> 
> Thank you in advance
> Paul Inchin

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