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

[Inchin, Pavel]

Good day!

Backing to my question, there are a couple of simulation in the attachments: NEX_XI=96 and NEX_XI=336. (x,y - 1000x1000 km grid). Both videos are velocities.
There are definitely hour-glass looking like effect for low resolution simulation in the vicinity of the epicenter, but no in NEX_XI=336. Thanks!

NEX_XI=96:

NEX_XI=336:

However, I backed to my initial question. You can find in NEX_XI=336 simulation long decaying pattern very close to the epicenter showing constant velocities. It doesn’t look like hour-glass effect, so I am not sure whether this is of real nature or not.

Thank you in advance.

Yours faithfully,
Paul



On Apr 9, 2018, at 10:02 AM, Dimitri Komatitsch <komatitsch at lma.cnrs-mrs.fr<mailto:komatitsch at lma.cnrs-mrs.fr>> wrote:


Dear Pavel,

You are welcome!

Best regards,
Dimitri.

On 04/09/2018 02:34 PM, Inchin, Pavel wrote:
Dr. Komatitsch
Thank you so much for help! I think this is what I need.
Yours faithfully,
Paul
On Apr 9, 2018, at 6:01 AM, Dimitri Komatitsch <komatitsch at lma.cnrs-mrs.fr<mailto:komatitsch at lma.cnrs-mrs.fr>> wrote:


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
<Schmicker_et_al_SEM_spurious_Hourglass_oscillations_spurious_modes_and_explanation_2014_with_analysis_in_yellow_by_Dimitri.pdf>

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

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