Southeast Fremont, near the Coyote Hills, sits on deep alluvial deposits that can amplify ground motion during an earthquake, while the northwestern neighborhoods around the Bay edge are underlain by younger, softer bay mud. This contrast means a building on Mission Boulevard may experience very different seismic response than one near the Newark Slough. We use MASW (Multichannel Analysis of Surface Waves) to measure shear wave velocity profiles across these soil units, providing the VS30 values required by ASCE 7 for site class assignment. Before finalizing the foundation design, we often combine this survey with a georradar GPR scan to map shallow utilities or anomalies, ensuring the seismic array layout is clean and reliable.
A VS30 value below 360 m/s places the site in ASCE 7 Site Class D, triggering more stringent seismic design requirements for most structures in Fremont.
Methodology and scope
Fremont's Mediterranean climate with dry summers and wet winters creates seasonal moisture changes that affect near-surface shear wave velocities. During winter rains, the upper 3 to 5 meters of the alluvial soils can become softer, lowering VS30 values temporarily. Our MASW surveys account for this by scheduling fieldwork in stable dry periods and recording soil moisture content at the time of testing. We deploy a 24-channel landstreamer with 4.5 Hz geophones spaced 2 meters apart, generating a 1D shear wave velocity profile down to 30 meters depth. The data is inverted using a genetic algorithm that fits the Rayleigh wave dispersion curve. For deeper profiles or complex stratigraphy, we integrate the results with a tomografia sismica refraction survey, which helps resolve velocity reversals that MASW alone might miss.
Technical reference image — Fremont
Local considerations
We reviewed a condominium project near the intersection of Mowry Avenue and I-880 where the original geotechnical report used a generic Site Class D assumption based on regional geology. The developer wanted to save on foundation costs, so we performed a MASW survey on the property. The measured VS30 was 410 m/s, placing the site right at the boundary between Site Class C and D. That 50 m/s difference directly impacted the seismic base shear calculation, allowing a more economical concrete shear wall design instead of a more expensive moment frame. Without the site-specific VS30, the structural engineer would have overdesigned the lateral system by about 15%, adding unnecessary cost.
Typically 30 m; extendable to 50 m with longer arrays
Frequency range of surface waves
5 Hz to 50 Hz
Inversion method
Genetic algorithm with 200 generations, RMS error < 5%
Associated technical services
01
Standard MASW Profile for VS30
A single 46-meter linear array with 24 geophones providing a 1D shear wave velocity profile to 30 meters depth. Includes on-site data acquisition, inversion analysis, and a report with VS30 value and ASCE 7 site class assignment. Suitable for low- to mid-rise buildings, townhomes, and commercial pads on relatively uniform ground.
02
2D MASW Tomography for Complex Sites
Multiple overlapping MASW lines processed as a 2D pseudo-section, revealing lateral variations in shear wave velocity across the site. Ideal for large subdivisions, school campuses, or sites with suspected paleochannels or buried stream deposits. Includes a contour map of VS30 values and recommendations on zone-specific foundation strategies.
Applicable standards
ASTM D4428/D4428M (Standard Test Methods for Crosshole Seismic Testing), ASCE 7-22 Section 20.3 (Site Class Determination Using VS30), IBC 2021 Chapter 16 (Seismic Design Categories)
Frequently asked questions
What is the difference between MASW and a standard SPT boring for seismic site classification?
MASW measures shear wave velocity continuously along the array, covering a larger area than a single SPT boring. SPT borings provide discrete blow counts at 1.5-meter intervals, which are then correlated to shear wave velocity using empirical formulas. MASW gives direct velocity measurements, reduces the need for multiple borings, and is non-invasive, making it ideal for sensitive sites in Fremont where drilling access is limited.
How much does a MASW survey cost for a typical residential lot in Fremont?
For a standard single-family lot (around 5,000 to 8,000 sq ft), the cost typically ranges between US$1,760 and US$2,870. This includes mobilization within Fremont, one linear array, data processing, and a report with VS30 and site class. Larger lots or multi-line 2D surveys increase the cost proportionally.
Do I need a MASW survey if my site already has SPT borings?
Not always, but we strongly recommend it for sites on alluvial fans or bay mud. SPT blow counts are localized; a MASW profile captures the average shear wave velocity over a 46-meter line, which is more representative for VS30 calculation. In Fremont, we have seen cases where two adjacent SPT borings differed by 20 m/s in VS30 estimates, while MASW provided a consistent value that resolved the discrepancy.
How long does a MASW survey take from field work to final report?
Field mobilization and data collection for a single array take about half a day in Fremont. Data processing and inversion require another 2 to 3 business days. The full report with VS30, site class, and dispersion curve plots is typically delivered within 5 working days after the field work is completed.
