Understanding strike_dir output from PyLith

Hi @baagaard and @willic3

I would like to understand better the output strike_dir from PyLith. This output is found on the files fault_info.h5 for example.

In the PyLith manual it says that strike_dir is the “Direction of fault strike in global coordinate system”.

In the example below I show a fault plane that is perpendicular to the x-direction. The field strike_dir that is output from PyLith contains 3 components. I am trying to wrap my mind about this information:

  1. The output from strike_dir are in radians, correct?

  2. In the figure below, why is the x-component of the strike_dir output equal to -0.17? Assuming that this is radians, this would correspond to -9 degrees. Shouldn’t it be 90 degrees ?

  3. The y-components is equal zero, which make sense in this case, right?

  4. Why is the z-component equal to -0.98, which in is ~56 degrees?

Thank you in advance for the help.

The components of strike_dir, dip_dir, and fault_normal are the direction cosines of the vectors pointing in the along-strike direction, up-dip direction, and fault normal direction in the global coordinate system. As discussed in the manual and examples, you can use these to transform the fault output given in the local fault coordinate system to the global coordinate system.