# Static Surface Load **URL:** \ **Category:** PyLith\ **Created:** [April 24, 2019, 2:13am UTC](https://community.geodynamics.org/t/static-surface-load/593 "2019-04-24T02:13:30Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![liangx](https://avatars.discourse-cdn.com/v4/letter/l/e56c9b/32.png) [@liangx](https://community.geodynamics.org/u/liangx)\ **Post date:** [April 24, 2019, 2:13am UTC](https://community.geodynamics.org/t/static-surface-load/593/1 "2019-04-24T02:13:30Z")
Dear CIG team, Hope you are well. I am trying to estimate the stress and displacement caused by changes of hydrological loading of a lake. So I am using Pylith to first calculate the surface displacement, following step 18,19 in the manual 2.2.1. The mesh is 1000 km _1000km_ 100km and the lake is about 100\*100. I use the Neumann bc on +z and constrain +x/-x and +y/-y on [0] and [1], and free -z. Also, I specify the normal stress for the region of lake in a SimpleDB. However, the result is unrealistic, where the z-displacement only occurs under the lake but nearby regions have really low z-displacement (please see the attached figure). This very rapid vertical displacement decay does not agree with other modeled results or GPS observations. I am wondering if I missed some steps or any good methods to estimate the surface load like this. Thank you ahead for your help. Sincerely, Liang ![1D-profile%20of%20loading%20derived%20displacment](https://canada1.discourse-cdn.com/flex036/uploads/geodynamics/original/1X/fa1655da10d6305382c2c68a73eb352a2ab63d8b.jpeg) ---
**Author:** ![willic3](https://yyz2.discourse-cdn.com/flex036/user_avatar/community.geodynamics.org/willic3/32/598_2.png) [@willic3](https://community.geodynamics.org/u/willic3)\ **Post date:** [April 24, 2019, 8:53pm UTC](https://community.geodynamics.org/t/static-surface-load/593/2 "2019-04-24T20:53:23Z")
Dear Liang, I’m not entirely sure how you are constraining your vertical displacements. Is your bottom boundary free or constrained? If it is free, how are you constraining your vertical displacements? Also, to me it appears that your mesh is quite thin in the vertical direction, compared to the dimensions of the lake. Cheers, Charles **Charles Williams I Geodynamic Modeler GNS Science I Te Pῡ Ao** 1 Fairway Drive, Avalon 5010, PO Box 30368, Lower Hutt 5040, New Zealand **Ph** 0064-4-570-4566 I **Mob** 0064-22-350-7326 I **Fax** 0064-4-570-4600 [**http://www.gns.cri.nz/**](http://www.gns.cri.nz/) **I** \*\*Email: \*\***[C.Williams@gns.cri.nz](mailto:C.Williams@gns.cri.nz)** ---
**Author:** ![liangx](https://avatars.discourse-cdn.com/v4/letter/l/e56c9b/32.png) [@liangx](https://community.geodynamics.org/u/liangx)\ **Post date:** [April 25, 2019, 1:12am UTC](https://community.geodynamics.org/t/static-surface-load/593/3 "2019-04-25T01:12:25Z")
Hi Charles, Thank you for the reply! You are right. I just tried the same load on the different thickness, and I found the modeled z-displacement present different magnitudes. You can see them from attached figures, which illustrate the z-displacements for 100 km, 200 km, and 400 km. However, I am not sure the proper thickness I should choose for this case. For the bottom, I simply constrain the vertical displacement with a fixed z [2]. Is there any suggestion for the bottom bc? Sincerely, Liang ![different%20thickness](https://canada1.discourse-cdn.com/flex036/uploads/geodynamics/original/1X/7862342f24b238b3d0a464314691a5f263dd4208.jpeg) ---
**Author:** ![baagaard](https://yyz2.discourse-cdn.com/flex036/user_avatar/community.geodynamics.org/baagaard/32/615_2.png) [@baagaard](https://community.geodynamics.org/u/baagaard)\ **Post date:** [April 25, 2019, 3:17pm UTC](https://community.geodynamics.org/t/static-surface-load/593/4 "2019-04-25T15:17:51Z")
Keep increasing the depth until the solution converges. As a rule of thumb, the boundaries of your domain (sides and bottom) should all be about the same distance from the sources of the deformation. Note that the solution will be quite smooth near the boundaries, so you can use larger cells there. If the displacement and stress field look smooth, then you are likely resolving the solution relatively well. Again, verify convergence of the solution using meshes with different resolutions. ---
**Author:** ![willic3](https://yyz2.discourse-cdn.com/flex036/user_avatar/community.geodynamics.org/willic3/32/598_2.png) [@willic3](https://community.geodynamics.org/u/willic3)\ **Post date:** [April 25, 2019, 8:38pm UTC](https://community.geodynamics.org/t/static-surface-load/593/5 "2019-04-25T20:38:15Z")
Dear Liang, I’m not sure what the proper thickness should be. Is the solution against which you are comparing an elastic half-space solution? I think you will need to consider what you want to measure, and whether the dimensions (and load) of the lake are large enough that you may need to consider the viscoelastic response to loading (either from the upper mantle or lower crust). Cheers, Charles p.s. I think that fixed-z displacements on the bottom boundary should be OK. One option might be to keep increasing the thickness of your mesh until there is no longer a significant difference in the computed vertical displacements. **Charles Williams I Geodynamic Modeler GNS Science I Te Pῡ Ao** 1 Fairway Drive, Avalon 5010, PO Box 30368, Lower Hutt 5040, New Zealand **Ph** 0064-4-570-4566 I **Mob** 0064-22-350-7326 I **Fax** 0064-4-570-4600 [**http://www.gns.cri.nz/**](http://www.gns.cri.nz/) **I** \*\*Email: \*\***[C.Williams@gns.cri.nz](mailto:C.Williams@gns.cri.nz)** ---
**Author:** ![liangx](https://avatars.discourse-cdn.com/v4/letter/l/e56c9b/32.png) [@liangx](https://community.geodynamics.org/u/liangx)\ **Post date:** [April 26, 2019, 1:34pm UTC](https://community.geodynamics.org/t/static-surface-load/593/6 "2019-04-26T13:34:05Z")
Hi Brad and Charles, Thank you for the note. I added it to 550 km and the solution converges well and result is at the same magnitude as elastic half-space observation. Also, the material below is set to be vicoelastic. Thank you again for your help. Sincerely, Liang