A 15-story tower on the River Walk sits on alluvium that amplifies motion differently than the limestone hills a mile north. That contrast defines seismic microzonation in San Antonio. We run the boreholes, log the shear-wave velocities, and send intact samples to our lab. The result is a grid of site coefficients and response spectra tied to ASCE 7-22 Chapter 11. The Balcones Fault Zone runs through the city’s west side. We’ve mapped it in multiple projects. Each report includes PGA maps, spectral acceleration at 0.2s and 1.0s, and liquefaction potential where the water table is high. For deep soil profiles near the Medina River we combine downhole seismic with MASW to resolve Vs30 without assumptions.
Vs30 is not a number you look up. We measure it at every boring and map the variation across the site.
Methodology and scope
Local ground factors
The transition from Edwards Limestone to quaternary alluvium creates sharp impedance contrasts across short distances. A site at IH-10 and Loop 410 might be Class C while half a mile south it drops to Class E. Missing that shift means under-designing the lateral system. The high water table along Salado Creek introduces liquefaction risk that standard USGS mapping does not resolve at the project scale. We run SPT-based triggering analysis with fines content correction. Where cyclic stress ratio exceeds cyclic resistance ratio, we map the liquefiable layer and provide residual strength. Another risk is the fault trace. The Balcones Fault Zone is considered non-capable for surface rupture per USGS, but differential settlement across the fault gouge can crack slabs and utilities. Our maps flag these zones with a 50-foot buffer.
Regulatory framework
ASCE 7-22 Minimum Design Loads, IBC 2021 Chapter 16, ASTM D7400 Downhole Seismic, ASTM D4428 Crosshole Seismic, ASTM D1586 Standard Penetration Test, ASTM D2487 Soil Classification
Associated technical services
Site-Specific Response Analysis
We install triaxial geophones at 5-foot intervals in mud-rotary boreholes. Shear-wave arrivals are picked manually on stacked records. The Vs profile feeds into 1D equivalent-linear analysis using DEEPSOIL or SHAKE. Output includes surface spectra, amplification factors, and time histories matched to the USGS uniform hazard spectrum.
Liquefaction Potential Mapping
SPT blow counts, fines content from wash sieves, and groundwater monitoring wells. We plot factor of safety against liquefaction on plan-view maps at 1-foot contour intervals. Lateral spreading displacement is estimated using the Youd et al. empirical method where free-face conditions exist near creek banks.
Typical parameters
Frequently asked questions
When does San Antonio require a site-specific seismic study instead of the IBC default values?
When the site is Class D with S1 greater than 0.04g, or Class E with Ss greater than 0.2g. Also when the structure is Risk Category III or IV and the site is within 3 miles of a mapped fault trace. The building official may also require it for structures over 160 feet tall.
How much does a seismic microzonation study cost for a typical San Antonio commercial lot?
The range is US$3,670 to US$19,140. The final number depends on the number of boreholes, the depth to refusal, whether we run downhole or crosshole seismic, and the lab testing suite required for dynamic soil properties.
How long does it take to get the final microzonation report?
Field work takes 3 to 7 days depending on the number of boreholes. Lab testing runs 2 to 3 weeks because resonant column and cyclic triaxial tests require specimen saturation and consolidation. The analysis and report writing add another week. Total turnaround is typically 4 to 6 weeks.