A few years back we worked on a six-story medical office building near the Fullerton Transportation Center. The client had the typical geotechnical report with blow counts and soil classifications, but the structural engineer needed dynamic input for the base shear. That is when we stepped in with a full site response analysis. We deployed a two-station array of broadband seismometers and recorded ambient vibrations for 72 hours. From that data we derived the fundamental period of the soil column and computed the response spectra at the ground surface. It turned out the site fell into ASCE 7 Site Class D with a Vs30 of 290 m/s, which bumped the seismic coefficients up compared to the default assumptions. That kind of refinement matters when you are designing a steel moment frame in a seismically active region like Fullerton, where the Whittier fault and the Puente Hills thrust system both influence ground motion.
A 2.8 Hz resonance peak with 3.2 amplification in Fullerton alluvium can double the seismic demand on a three-story building.
Methodology and scope
Local considerations
Fullerton has a population of about 140,000 and sits less than 10 kilometers from the Whittier fault, which produced a M5.9 earthquake in 1987. That event caused significant damage to unreinforced masonry buildings in the historic downtown district. The risk is not just from the fault rupture itself but from the basin-edge effects that amplify long-period motions in the deep alluvium. A site response analysis identifies these amplification zones before you pour a single cubic yard of concrete. Without it, you are designing for a uniform hazard spectrum that may under-predict the actual shaking by a factor of two or more in the 0.3 to 0.8 second period range.
Explanatory video
Applicable standards
ASCE 7-22 (Minimum Design Loads, Site Class definitions and response spectra), ASTM D4428/D4428M-14 (Standard Test Methods for Crosshole Seismic Testing), NEHRP Recommended Seismic Provisions (FEMA P-1050)
Associated technical services
Ambient Vibration Array (HVSR + MASW)
We deploy 2 to 4 broadband seismometers in a circular or linear array around your site. The HVSR method gives the fundamental resonance frequency, while MASW provides the shear-wave velocity profile. Both datasets are inverted jointly to produce a reliable Vs profile down to 30 meters or deeper.
1D Equivalent Linear Analysis
Using the Vs profile and input motion from the USGS NSHMP, we run DEEPSOIL or STRATA to compute the surface response spectra. Output includes PGA, spectral accelerations at all periods, and peak ground displacement. Results are delivered as tables ready for ETABS or SAP2000.
Site-Specific Response Spectra Report
A full report documenting the field methodology, data processing steps, inversion results, and final design spectra per ASCE 7 Chapter 21. We include the probabilistic and deterministic hazard deaggregation, plus a comparison with the code-prescribed spectrum.
Typical parameters
Frequently asked questions
What is a site response analysis and when is it required in Fullerton?
A site response analysis computes how local soil conditions modify seismic waves from the bedrock to the ground surface. In Fullerton it is required under ASCE 7-22 for buildings in Seismic Design Category D, E, or F when the site class is F, or when the structure has a period longer than 0.5 seconds on a deep soft soil deposit. We also recommend it voluntarily for any project with irregular geometry or high occupancy.
How much does a site response analysis cost in Fullerton?
For a typical commercial project the cost ranges between US$1.180 and US$3.840 depending on the number of test locations, the depth of profiling required, and whether you need a full probabilistic hazard deaggregation. That includes field deployment, data processing, inversion, and the final report with design spectra.
How long does a site response study take from start to finish?
Field recording takes 48 to 72 hours for ambient vibrations. Data processing and inversion add another 5 to 7 business days. For urgent projects we can expedite the field component to 24 hours and deliver preliminary results within 3 days, but the full report with all documentation takes about 2 weeks.
What is the difference between a site response analysis and a standard geotechnical report?
A standard geotechnical report provides soil types, bearing capacity, and settlement estimates. A site response analysis focuses exclusively on dynamic behavior: shear-wave velocity, resonance frequency, amplification factors, and the resulting design spectra. You need both. The geotechnical report tells you how the soil will behave under static loads; the site response analysis tells you how it will shake during an earthquake.