Many parts of the USA and the world are located in areas that experience a significant level of seismic activity, a level that engineers recognize requires them to be factored into tower structures.

The Uniform Building Code (UBC) and related similar codes have a simple method of factoring seismic forces into a tower (or other structure) overall capacity.  It can be said that our tower design is governed by wind forces rather than seismic forces.  It is considered unlikely, even in areas of high seismicity, that the forces generated by eathquakes would exceed the wind loads calculated on the tower.

The map below shows different seismic zones (UBC '97):

Your engineer may want to factor these ratings into the antenna load capability of our towers. 

However, as implied above, the UBC states that:

Where wind forces prescribed (by Chap. 16, Part III) produce greater deformation of stresses, such loads shall be used for design in lieu of the deformations and stresses resulting from earthquake forces. 
(UBC '97, Sect. 1654; pg. 2-405). 

So, unless your tower site is attached or a part of an essential facility, like a police or fire station or located at a hazardous facility, like a toxic or explosive chemical processing housing, both of which could have a Seismic Importance factor of 1.25 or 1.5, then you likely will not have to be concerned with rating our towers for seismic forces.  

In the event that your tower will be used or located at one of the above important facilities, you would simply derate the load capacity of the tower with the 1.5 factor.  Thus, if you intend to put up 10 sq. ft. (sq. mtr.) of antenna, select a tower that has a 10 x 1.5 or 15 sq. ft. (sq. mtr.) wind-load capacity. 

SEISMIC

The following table (tab. 16-I) lists the five basic seismic zone categories and assigns a Z-factor to each (UBC Apndx. Chap. 16):