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

[Inchin, Pavel]

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>