MASW & VS30 Shear Wave Velocity Testing in Sunnyvale

One of the most costly mistakes a project team makes in Sunnyvale is assuming uniform stiff ground across a site that straddles two distinct depositional environments. We have reviewed borings near Lawrence Expressway where the upper 30 meters flipped from dense alluvial fan gravels to compressible Bay Mud within a single city block, shifting the ASCE 7 site class from C to E. A standard boring log misses that lateral contrast; the MASW surface wave method captures it along a continuous linear array, yielding a direct Vs30 profile that feeds into the IBC Chapter 16 ground motion calculation. Our field crews lay out 24- to 48-channel geophone spreads tuned to Sunnyvale's shallow water table, extracting fundamental-mode dispersion curves processed through WinMASW and Geopsy. The result is a defensible shear wave velocity column that structural engineers use to anchor base shear values without resorting to overly conservative default assumptions.

In Sunnyvale's alluvial-to-bay transition zone, a 200-meter lateral shift can change the ASCE 7 site class from C to E—MASW captures that boundary where boreholes alone miss it.

Methodology and scope

The subsurface contrast between the Cherry Orchard district and the Moffett Park area illustrates exactly why Vs30 mapping matters here. Cherry Orchard sits on Pleistocene alluvial fan deposits: gravelly sands and stiff clays that routinely deliver Vs30 above 400 m/s, placing sites in Site Class C with a modest short-period coefficient. Move two miles north toward Moffett Park, and the stratigraphy transitions to Holocene Bay Mud interbedded with loose estuarine sands, where Vs30 drops below 180 m/s—Site Class E territory with a 30–40% increase in design spectral acceleration for the same latitude-longitude pair. Between these end members, the CPT test provides nearly continuous tip resistance and sleeve friction logs that correlate well with shear wave velocity when a site-specific transformation is calibrated against a collocated MASW line. We run both methods together on Sunnyvale projects where the geotechnical baseline report demands a defensible Vs profile for liquefaction triggering per Idriss & Boulanger (2014) procedures.

Site-specific factors

Sunnyvale's rapid transformation from orchard land to high-density tech campuses between the 1960s and 1990s left a patchwork of artificial fill over compressible native soil. Older industrial parcels along Mathilda Avenue often conceal undocumented debris fill 2 to 4 meters thick, which scatters surface-wave energy and degrades the dispersion image if the array is not positioned to avoid buried concrete. The deeper geotechnical concern is the contact between Holocene bay deposits and the underlying Santa Clara Formation; where that contact shallows abruptly, the impedance contrast generates a velocity reversal that can trap seismic energy and amplify short-period motion beyond ASCE 7 mapped values. A site-specific response analysis, constrained by an in-situ permeability profile and shear wave velocity measured below 30 meters, quantifies that amplification and avoids under-predicting the design spectrum for mission-critical structures.

Technical parameters

Parameter	Typical value
Vs30 per ASCE 7-22 Section 20.4	170–520 m/s (typical Sunnyvale range)
Maximum investigation depth, active MASW	35–45 m below grade (48-channel, 2 m spacing)
Maximum depth, passive SPAC array	80–110 m (L-array, 30 min recording)
Geophone frequency, active survey	4.5 Hz vertical-component, 24–48 channels
Source type	10 kg sledgehammer, double-sided aluminum plate, 5–8 stack
Dispersion processing software	WinMASW 8.2 / Geopsy 3.5
Deliverable format	Signed PDF report, SEG-Y, CSV dispersion curves

Complementary services

Active 1D/2D MASW for 30-Meter Vs30

A 48-channel land streamer towed across pavement or compacted subgrade, or a planted 4.5 Hz geophone spread on bare soil. We trigger a 10 kg sledgehammer on an aluminum plate with multiple stacked impacts per shotpoint, processing dispersion images to 30–40 m depth. Deliverable includes Vs30 per ASCE 7-22 Section 20.4, 2D cross-sections, and a site class letter ready for DSA or City of Sunnyvale submittal.

Passive Linear-Array and SPAC Deployments

For deeper basin characterization beyond 50 meters, we record 30–45 minutes of ambient noise using circular and L-shaped arrays. SPAC processing extracts Rayleigh-wave phase velocities down to 80–100 m, capturing the Holocene-to-Pleistocene transition that governs basin amplification in the northern Santa Clara Valley. This data supports site-specific response analysis when the default ASCE 7 site coefficients are demonstrably unconservative.

Applicable standards

ASCE/SEI 7-22 Minimum Design Loads and Associated Criteria for Buildings and Other Structures, 2022 California Building Code (CBC), Chapter 16 (based on IBC 2021), ASTM D7400-17 Standard Test Methods for Downhole Seismic Testing (referenced for cross-check), FEMA P-1050 NEHRP Recommended Seismic Provisions, City of Sunnyvale Standard Details and Submittal Requirements for Geotechnical Reports

Questions and answers

How much does a MASW Vs30 survey cost for a typical Sunnyvale commercial lot?

For a single-family or small commercial parcel in Sunnyvale, an active MASW campaign with one to three array positions typically runs between US$1,800 and US$3,100. The range depends on site access, number of lines, surface conditions, and whether passive SPAC recording is added for deeper basin profiling. This includes field acquisition, dispersion processing, Vs30 calculation, and a signed report ready for City of Sunnyvale plan check.

Does the City of Sunnyvale accept MASW for site class determination under the California Building Code?

Yes. The CBC adopts ASCE 7 Chapter 20, which lists surface-wave methods among the accepted techniques for measuring Vs30. We provide the full dispersion curve, inversion parameters, and a clear narrative justifying the selected site class. Sunnyvale plan reviewers typically expect the MASW line to be co-located with at least one boring or CPT sounding for stratigraphic verification.

How do you handle MASW data quality near Highway 101 or the light rail corridor?

Ambient noise from traffic and rail generates coherent high-frequency energy that can mask the fundamental Rayleigh mode. We mitigate this by stacking 5 to 8 hammer blows per shotpoint, applying an adaptive frequency filter during dispersion processing, and scheduling acquisitions during off-peak hours when possible. If noise remains excessive, we supplement with a short passive array to improve the low-frequency portion of the dispersion curve.

Can MASW detect the depth to the Santa Clara Formation in northern Sunnyvale?

In the Moffett Park area, where the Santa Clara Formation lies within 60 to 80 meters of grade, a passive SPAC array can resolve the velocity contrast at that interface when the impedance ratio is sufficient. For deeper or gradational contacts, we recommend coupling the MASW with a seismic refraction line or a downhole seismic log to reduce inversion non-uniqueness.