A common oversight among developers in Sunnyvale is assuming uniform ground behavior across the city. The alluvial deposits underlying the Santa Clara Valley vary significantly in thickness and stiffness, which directly alters how seismic waves propagate. Without a proper seismic microzonation study, structural designs may underestimate site-specific amplification or liquefaction potential. We start by reviewing existing borehole logs and performing a targeted [MASW survey](/masw-vs30/) to map shear-wave velocity profiles down to 30 meters. This initial screening separates the city into response zones, each with distinct design spectra under ASCE 7-22.
A site class D soil in Sunnyvale can amplify peak ground acceleration by 40% or more compared to a class C profile under the same earthquake.
Method and coverage
Summers in Sunnyvale are dry and warm, but the shallow water table from seasonal recharge creates conditions that amplify ground motion. We approach each microzonation by combining passive microtremor arrays (HVSR) with active-source refraction tomography. The goal is to define the predominant period of each soil column. For sites where soft clay layers are present, we also run cyclic triaxial tests to characterize modulus reduction curves. These data feed directly into site-response models using programs like DEEPSOIL or SHAKE. We cross-check results against the NEHRP classification to assign the correct site class — typically C, D, or E depending on the neighborhood.
Technical reference image — Sunnyvale
Regional considerations
Consider two areas: the older alluvial fans near the foothills of the Santa Cruz Mountains versus the bayfill deposits along the Sunnyvale waterfront. The former typically exhibits firm sands and gravels with VS30 above 500 m/s, while the latter contains soft to medium clays with VS30 below 250 m/s. During a magnitude 6.7 earthquake on the San Andreas fault — just 15 km away — the bayfill zone could experience double the ground motion of the foothill zone. Seismic microzonation quantifies this differential risk, allowing engineers to optimize foundation design and avoid over- or under-designing for lateral loads.
Three-component measurements at multiple points to determine the fundamental resonance frequency of the soil column. Data processed using the Nakamura method and validated against borehole stratigraphy.
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Active-Source MASW Survey
Multi-channel analysis of surface waves using a 24-geophone array. We invert Rayleigh-wave dispersion curves to produce 2D VS30 profiles, directly supporting site class assignment per ASCE 7.
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Cyclic Triaxial Testing
Undisturbed samples from thin-wall Shelby tubes are tested under cyclic loading to obtain modulus reduction (G/Gmax) and damping curves. These curves are essential for equivalent-linear site-response analysis.
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Site-Response Analysis Report
We compile all field and lab data into a probabilistic seismic hazard assessment. The final report includes design spectra for each microzone, liquefaction hazard maps, and recommended seismic coefficients for structural design.
Standards that apply
ASCE 7-22 (Minimum Design Loads, Chapter 20 – Site Classification), ASTM D4428/D4428M (Crosshole Seismic Testing), NEHRP Recommended Provisions (FEMA P-1050), IBC 2021 (International Building Code, Chapter 16 – Structural Design)
Frequently asked questions
What is the difference between site class and seismic microzonation?
Site class is a simplified categorization based on VS30 (ASCE 7 defines A through F). Seismic microzonation goes further: it maps the spatial variation of site class, predominant period, and amplification factor across a city or district, producing a continuous hazard map rather than a single label.
How long does a seismic microzonation study in Sunnyvale typically take?
For a typical residential subdivision (5–10 acres), field measurements take 3–5 days, lab testing adds 2–3 weeks, and data analysis plus reporting require another 2–3 weeks. Total turnaround is usually 4–6 weeks from mobilization.
Does Sunnyvale have specific seismic design requirements beyond the IBC?
Yes. The City of Sunnyvale enforces the California Building Code (CBC), which adopts IBC with amendments. Additionally, the Alquist-Priolo Earthquake Fault Zoning Act applies to areas near the San Andreas fault — any structure within a fault zone requires a detailed fault-rupture hazard investigation.
What is the typical cost range for a seismic microzonation study in this area?
The cost varies with site size and complexity but generally falls between US$4,100 and US$14,800. Small residential lots on the lower end; large commercial parcels requiring multiple MASW lines and cyclic testing on the higher end.
How do you account for liquefaction in the microzonation?
We evaluate liquefaction potential using the simplified procedure from Youd and Idriss (2001), based on SPT blow counts, fines content, and cyclic stress ratio. The resulting LPI values are mapped across the site, and zones with LPI > 5 are flagged for mitigation such as Improvement or deep foundations.
