Seismic Analysis - Why use 2/3 Horizontal value for the Vertical Value

[readethis]

I was once told by a Structural Engineer that when evaluating for Seismic loading, the load combination for Seismic should include 2/3 Horizontal value for the Vertical Shear and 100% Horizontal Shear. However, I have not been able to justify the reasoning behind this rule of thumb. Can anyone tell me why this applies?

 

[ATSE]

Load combinations are defined by the code.
Most structural engineers in the US use ASCE 7-05 for bldgs, non-structural components, and MEP anchorage.
Seismic ground shaking produces accelerations in the 3 principal directions.
Peak vertical acceleration can be estimated as 2/3 of peak horiz acceleration. However, structural engineers typically work with pseudo-accelerations (code defined "spectra"), and use SRSS.
For a typical seismic load combo, ASCE 7 requires 20% of vert pseudo-accel with 100% of max horiz seismic forces in principal direction and 30% horiz seismic forces in the perpendicular direction (in addition to dead and scaled live loads).