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Z factor definition

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I would like clarity in defining terms related to Z in the UBC seismic
formula.  As I understand it the Z is the estimated peak horizontal ground
acceleration (.4g for zone 4) on rock with a 10% probability of being
exceeded in 50 years. I am looking at a site 1.5 km from a type B fault in
the LA basin on natural alluviam.In the soil report the statement using
identical terms states the acceleration as .55g. The soil report basis is
the National Seismic Hazard Mapping Project by the USGS. I recognize that
site specific soils play into this and my question is should I use a Z value
of .55? Are the C, N & S  factors for near source going to ratchet the .55
beyond intent?
Thanks in advance,
Bob Hanson