[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!






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 8:34 AM, Inchin, Pavel <INCHINP at my.erau.edu> 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> 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>
>

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