MASW / VS30 Testing in San Bernardino — Shear Wave Velocity for Seismic Site Classification

The post-war expansion of San Bernardino reshaped how we look at basin soils. Old alluvial fans from the San Bernardino Mountains spread coarse debris across the valley, but the rapid subdivision boom starting in the 1950s placed thousands of structures on materials whose dynamic stiffness was never characterized. With the San Jacinto and San Andreas faults nearby, the ASCE 7 requirement for site class based on VS30 is not a formality here — it directly controls the seismic design category and foundation costs. Our team runs MASW surveys across commercial lots, school sites, and industrial parcels so the geotechnical report includes a measured shear wave velocity profile, not just a conservative assumption from blow counts. We complement this with seismic refraction when bedrock depth is in question and liquefaction analysis where shallow groundwater in the Santa Ana River corridor triggers additional evaluation under the CBC.

A measured VS30 under 600 ft/s moves a Site Class D to E and can double the base shear in design — that single number changes the entire structural budget.

How we work

What we see repeatedly in San Bernardino is that N-value correlations misclassify sites sitting on partially cemented alluvium. The classic Lytle Creek fan deposits can carry surprisingly high blow counts while their low-strain stiffness remains modest — a blind spot if you rely solely on SPT data. Our field setup uses a 24-channel seismograph with 4.5 Hz geophones spaced at 5 or 10 ft, depending on target depth, and a 10 lb sledge source for most urban lots. Dispersion curves are picked manually, and inversion runs through an iterative least-squares routine until the misfit between measured and theoretical dispersion stays under 5%. The output is a 1D VS profile down to 100 ft or more, yielding VS30 directly for IBC Chapter 16 classification. When the upper 5 ft is heavily desiccated crust, we often run a parallel test pit log to calibrate the near-surface velocity with actual material description, tying the geophysical model to visual ground truth.

Site-specific factors

San Bernardino sits approximately 1,050 ft above sea level, but elevation alone doesn't capture the seismic hazard. The city is within 15 miles of the San Jacinto fault and roughly 30 miles from the southern San Andreas — both capable of producing strong shaking with significant basin amplification. The 2021 CalOES hazard maps and the updated CBC 2022 reinforce that Site Class D versus E determination directly affects the mapped spectral acceleration parameters. Guessing VS30 from blow counts in interbedded gravels and cemented siltstone can lead to a one-class error, which means under-designed lateral systems or over-designed foundations. We measure, we don't guess. The MASW method lets us cover multiple locations in a single day, delivering results while the drill rig is still mobilizing, so the design team doesn't wait on seismic data to lock the structural system.

Technical parameters

Parameter	Typical value
Seismograph channels	24 channels, 24-bit A/D
Geophone frequency	4.5 Hz vertical component
Receiver spacing (typical)	5 ft or 10 ft, array length 115-230 ft
Source type	10-12 lb sledge hammer on aluminum plate
Depth of investigation	50 to 100+ ft depending on array geometry
Key output parameter	VS30 (ft/s) per IBC / ASCE 7-22
Data format delivered	1D VS profile, dispersion curve, site class letter

Associated technical services

VS30 Site Classification Surveys

MASW arrays positioned to achieve 100 ft depth of investigation; dispersion processing and inversion yielding VS30 for IBC/ASCE 7 site class A through F.

Combined Geophysical and Geotechnical Campaigns

MASW paired with SPT borings or CPT soundings at the same coordinate so the design team gets both dynamic stiffness and static strength in one report section.

Cross-Hole and Downhole Alternatives

For sites where surface access is limited, we deploy downhole seismic in cased boreholes to measure interval velocities directly, correlating with the MASW profile.

2D Shear Wave Velocity Sections

Multiple parallel MASW lines processed into a continuous cross-section showing lateral velocity variations — useful for large-footprint buildings and cut-and-cover trench analysis.

Questions and answers

What does a MASW / VS30 test in San Bernardino typically cost?

A single MASW line processed to VS30 in San Bernardino generally runs between US$1,880 and US$3,310 depending on the array length, number of shot points, and site access conditions. Surveys requiring multiple arrays for 2D coverage or night work near active roadways fall at the higher end.

How long does the field acquisition take?

For one array with 24 geophones, field setup and shooting takes about 45 to 75 minutes. Most commercial lots in San Bernardino are finished in half a day including layout, testing, and demobilization.

Can you run MASW on asphalt or concrete pavement?

Yes, but coupling changes. We either drill small pilot holes through the pavement to plant geophones in native soil, or use base plates with gypsum paste on clean concrete. Paved surfaces reduce the surface wave energy slightly, so we increase stacking and adjust the source offset.

Does the VS30 result change if groundwater is shallow?

VS30 is computed from the travel-time average of shear wave velocity in the upper 100 ft, so shallow groundwater does not directly alter the velocity measurement. However, saturated fine sands can exhibit slightly lower VS, and if liquefaction is a concern, we recommend pairing the MASW with a site-specific liquefaction triggering analysis.