# The issue of how to output the surface velocity field in 3D simulations of strike-slip faults

Please provide a diagram of the boundary problem you are trying to solve and describe what you are trying to do. Also the title should be a few work description and then put your description in the body of the text along with diagrams, screenshots, etc.

Hello, members of the Pylith team. I am a beginner with Pylith 4.0.0, aiming to simulate the surface deformation velocity field of a left-lateral strike-slip fault and output the stress-strain field. I initially experimented with a simple 3D example of a buried strike-slip fault. My model size is 150km×150km×80km, and the materials are divided into three layers, including upper crust (IsotropicLinearElasticity), lower crust (IsotropicLinearMaxwell), and upper mantle (IsotropicLinearMaxwell). DirichletTimeDependent boundary conditions are applied at x_pos, x_neg, y_pos, and y_neg, while the surface (z_pos) is free and z_neg is fixed only in the z-direction. I used observers.observer.data_fields = [displacement, cauchy_stress, cauchy_strain] and obtained displacement, stress, and strain information, but encountered difficulties in outputting the surface velocity field. While reading the Pylith 4.0.0 user manual, I thought using pylith.problems.SolnDispVelLagrange instead of pylith.problems.SolnDispLagrange would output velocity information, but in reality, it threw the error as described above. So, how should I modify it to address this error? The above is the parameter file I used. I would greatly appreciate your assistance.

See my reply in the other thread How to model the rate of interseismic deformation of the surface in 3D fault zones? - #4 by zhaxj