every code usually describes how they formulate the Response spectrum (can be based on ductility or damping). Try to read through the commentary, hopefully you get the formula.
If you know the standard earthquake records used in seismic design for you area, you may use SPECTRA ( part of ruaumoko).
All you do is take your 5% damped acceleration and multiply it by the ratio of 1.2/1.39 (per the table on the website) to get the 10% damped acceleration.
If you click on the next arrow at the bottom of the page "six" times, there is a good example of what I'm talking about.