Many construction teams in Austin skip the site response analysis and rely on generic seismic coefficients from the code. That is a mistake. The Balcones Fault zone runs straight through the city, creating a sharp contrast between hard limestone on the west side and deep alluvial clays on the east. A one-size-fits-all design does not work here. Without a proper site-specific study, buildings on soft soils can amplify ground motion by a factor of two or more. The result is an under-designed structure that may not perform during a moderate earthquake. A thorough site response analysis captures the actual soil column behavior, not a table value from Appendix C of ASCE 7.
On the east side of Austin, soft soils can amplify seismic waves 2.5 times more than the limestone ridges west of MoPac.