# How to obtain the gridpoint acceleration in 3DEC

**URL:** <https://forum.itascainternational.com/t/how-to-obtain-the-gridpoint-acceleration-in-3dec/2180>\
**Category:** 3DEC\
**Created:** [July 28, 2025, 12:00am UTC](https://forum.itascainternational.com/t/how-to-obtain-the-gridpoint-acceleration-in-3dec/2180 "2025-07-28T00:00:56Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![JLUJYP](https://avatars.discourse-cdn.com/v4/letter/j/9f8e36/32.png) [@JLUJYP](https://forum.itascainternational.com/u/JLUJYP)\
**Post date:** [July 28, 2025, 12:00am UTC](https://forum.itascainternational.com/t/how-to-obtain-the-gridpoint-acceleration-in-3dec/2180/1 "2025-07-28T00:00:56Z")

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Hi,  
How can I obtain the gridpoint acceleration in 3DEC, similar to what is shown in the FLAC below?  
Thanks in advance!

 ![image](https://canada1.discourse-cdn.com/flex031/uploads/itasca/original/2X/3/35606d429c9bb1910eeffb533a02b456fdbcc244.png)

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<div class="post-metadata">

**Author:** ![JLUJYP](https://avatars.discourse-cdn.com/v4/letter/j/9f8e36/32.png) [@JLUJYP](https://forum.itascainternational.com/u/JLUJYP)\
**Post date:** [July 28, 2025, 12:12am UTC](https://forum.itascainternational.com/t/how-to-obtain-the-gridpoint-acceleration-in-3dec/2180/2 "2025-07-28T00:12:44Z")

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I want to obtain the peak (maximum) acceleration of each gridpoint in the model during the earthquake process and visualize it using `gp.extra`.

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**Author:** ![jhazzard](https://avatars.discourse-cdn.com/v4/letter/j/4af34b/32.png) [@jhazzard](https://forum.itascainternational.com/u/jhazzard)\
**Post date:** [July 28, 2025, 2:28am UTC](https://forum.itascainternational.com/t/how-to-obtain-the-gridpoint-acceleration-in-3dec/2180/3 "2025-07-28T02:28:43Z")

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It seems that the FISH function for block.gp.accel is missing. We will add it ASAP. (what version are you using?).

In the short term, you can get it the old fashioned way:

```
fish def get_acc
  $kxacc=41 ; fortran offset for x acceleration.   
  loop foreach gp block.gp.list
    gpi = block.gp.index(gp)
    ax = memory.fortran.float(gpi+$kxacc)
    ay = memory.fortran.float(gpi+$kxacc + 1)
    az = memory.fortran.float(gpi+$kxacc + 2)
    
    ; this is a vector. Use math.mag if you want just the magnitude
    block.gp.extra(gp) = vector(ax,ay,az)
  end_loop
end
[get_acc]

```

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<div class="post-metadata">

**Author:** ![JLUJYP](https://avatars.discourse-cdn.com/v4/letter/j/9f8e36/32.png) [@JLUJYP](https://forum.itascainternational.com/u/JLUJYP)\
**Post date:** [July 28, 2025, 2:57am UTC](https://forum.itascainternational.com/t/how-to-obtain-the-gridpoint-acceleration-in-3dec/2180/5 "2025-07-28T02:57:39Z")

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Hi, Jhazzard  
I’m using 3DEC 9.0.177.  
Thank you very much for your help! I’ll try the FISH code you provided first.  
Have a good day!  
JLUJYP