[CIG-SEISMO] CIG-SEISMO Digest, Vol 99, Issue 4

[Avinash Nayak]

Dear Shahar, Thanks. It occurred to me that Gaussian function was not
zero but it goes to very small values (1e-15) quickly after ~5-7
seconds relative to its value at the peak (>1). I will try out the
GaussianInt function at the first opportunity. I was using a Gaussian
function since I noticed it goes to zero quicker than a filtered Dirac
function, but I may be wrong. I appreciate your help.
Regards,
Avinash

On Mon, Apr 11, 2016 at 2:49 AM, Shahar Shani-Kadmiel
<kadmiel at post.bgu.ac.il> wrote:
> Hi Avinash,
>
> My guess is that you are seeing this because the Gaussian STF only approaches zero asymptotically but never actually reaches it.
>
> If that is important for your calculation consider using the GaussianInt, which I think does not cause this issue or prescribe a Discrete time function based on the Gaussian STF (does not have to be defined over the simulation duration) and subtract the final value of the STF from the entire STF or taper it down to zero.
>
> Good luck,
>
> Shahar
>
>
>> On Apr 11, 2016, at 8:31 AM, cig-seismo-request at geodynamics.org wrote:
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>> Today's Topics:
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>>   1. Help with SW4 (Avinash Nayak)
>>
>>
>> ----------------------------------------------------------------------
>>
>> Message: 1
>> Date: Sun, 10 Apr 2016 22:31:02 -0700
>> From: Avinash Nayak <avinash07guddu at gmail.com>
>> To: cig-seismo at geodynamics.org
>> Subject: [CIG-SEISMO] Help with SW4
>> Message-ID:
>>       <CA+ebHScc3Sa3cYVrdjYE5=1_EiaxVf3qe-AWPQNf9_PG9QNjFQ at mail.gmail.com>
>> Content-Type: text/plain; charset="utf-8"
>>
>> Hi Everyone,
>>
>> I am having an issue with SW4 where I get non-zero values in velocity
>> at source position for a substantial amount of time (after source time
>> function becomes very small) and I am not sure what exactly I am doing
>> wrong.
>>
>> In the attached file (Summary.pdf), Figure 1 shows a screenshot of
>> velocity in YZ plane at x=1.393 km (passing through the source at
>> x=1.393, y=1.925 and z=0.469 km) at t=7.5 seconds. Color is supposed
>> to be scaled as red to blue from negative(peak absolute amplitude) to
>> positive(peak absolute amplitude) with white for zero. As you can see
>> the highest amplitude is a dot at the source position (y=1.925 km,
>> z=0.46 km). If I replace values at 7x7 grid points around the source
>> with zeros, then I can see the wavefield slightly better and the peak
>> amplitude decreases from 1.1e-5 m/s to 2.7e-6 m/s (Figure 2).
>>
>> In figure 3 and 4, I am plotting the velocity and displacement values
>> at the 7x7 grid points around the source, and you can see non-zero
>> very slowly decreasing values of velocity that persist even at 15-20
>> seconds. When I integrate them, I can see a slight increase of
>> displacements with time but I am not sure if they reach steady state
>> value (Figure 4).
>>
>> My grid spacing is 7 m for minimum shear wave velocity 150 m/s. I use
>> a Gaussian source time function with freq 3.1416 (so, f0=0.5 Hz,
>> fmax=f0 x 2.5 = 1.25 Hz, and 150/1.25/7 = 17 grid points per
>> wavelength for fmax). My t0 is 2.0 seconds( which is greater than
>> 6/freq=1.91 s) so satisfies g(t=t0)=3.4e-9 which is less than 1e-8.
>> The source time function goes below 1e-15 after 5 seconds.
>> The source is a moment tensor source  very close to a strong material
>> discontinuity. I use supergrid size 73.
>>
>> I will be very grateful if someone can point out what I have done
>> wrong. Due to computation limitations, I cannot really go to lower
>> grid spacing. I am currently trying another run at higher value of t0
>> (2.5 s).
>>
>> Thanks.
>> Regards,
>> Avinash
>>
>> --
>>
>> Avinash Nayak,
>> PhD Candidate,
>> Berkeley Seismological Laboratory,
>> Department of Earth and Planetary Science,
>> University of California, Berkeley
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>> End of CIG-SEISMO Digest, Vol 99, Issue 4
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-- 

Avinash Nayak,
PhD Candidate,
Berkeley Seismological Laboratory,
Department of Earth and Planetary Science,
University of California, Berkeley