# External Source Time Functions

**URL:** <https://community.geodynamics.org/t/external-source-time-functions/635>\
**Category:** SPECFEM\
**Tags:** specfem3d\
**Created:** [May 17, 2019, 6:06pm UTC](https://community.geodynamics.org/t/external-source-time-functions/635 "2019-05-17T18:06:33Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![istoneuw](https://avatars.discourse-cdn.com/v4/letter/i/8e7dd6/32.png) [@istoneuw](https://community.geodynamics.org/u/istoneuw)\
**Post date:** [May 17, 2019, 6:06pm UTC](https://community.geodynamics.org/t/external-source-time-functions/635/1 "2019-05-17T18:06:33Z")

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Hello,

I am interested in implementing an external source time function (STF) so I can model a Brune source pulse. I have a couple of questions about the format of the input STF:

1. Is the input function supposed to be the slip rate, or the slip displacement? I ask because when I display the data in “plot\_source\_time\_function.txt” for a default (Gaussian) STF used by specfem, I receive a curve that looks more like the integral of a Gaussian.

2. What is the significance of the amplitude of the STF? Does it matter, or is the shape/duration of the function more significant? For example, would it make any difference in my solution if I scaled this function by a factor of 10?

Thank you,

Ian