rlotfi --
I wrote a response on reddit to a similar question after having taken and passed all (3) Civil Engineering CA exams this past April. Please note attached link for my detailed response: http://morganmwright.com/reddit-comrcivilengineering/
In short for seismic I would recommend the...
ONENGINEER,
It would be most appropriate to use the sum of counts from the 2nd and 3rd 6-in drives (from 150mm to 450mm) for your SPT N-Value. Additional counts should be recorded and reported as they provide additional information which could prove useful in your analysis.
- MorganMWright.com