As Qshake said the vertical component of earthquake generally would be of order 1/3 of horizontal coponent. But it is not a standard value since the earthquakes I have studied, for example Kobe, Northridge, Chalfant, whitter field and many more, the vertical componant is in comparision with that of horizontal motion, few cases it exceeded the horizontal motion.
The vertical component is not at danger, b'coz the stuructures designed to the respective building codes have adeaquate factor of safety against vertical loads and the additional axial load arising from vertical motion will be easily taken care off by this factor of safety.
The vertical motion produced cosiderable effect on thin-walled structures susceptible to local buckling. In this case the local buckling coupled with the additional axial load due to vertical ground motion produces considerable damage during an event of severe earthquake.