MASW and VS30 Shear Wave Velocity Testing in Pomona

A common mistake we see in Pomona is running a full drilling program without first understanding the site’s seismic class. The city sits on a mix of young alluvium from the San Jose Creek and older terrace deposits pushed up against the San Jose Hills. With average annual temperatures hitting the mid-70s and occasional Santa Ana winds, the near-surface soils dry out and crack, which throws off simplistic velocity assumptions. Our team uses MASW to map the shear wave velocity profile directly, giving you a defensible VS30 value that satisfies both the IBC and ASCE 7. Instead of guessing whether you are on Site Class C or D, you get a measured profile that can move the project toward a more efficient foundation design. For deeper verification when soil conditions look suspicious, we often pair the survey with SPT drilling to correlate N-values with the velocity data.

A measured VS30 of 270 m/s versus an assumed 180 m/s can shift a site from Class D to C, reducing the base shear demand by a full seismic design category.

Our service areas

Our approach and scope

One thing we have noticed across Pomona is that the stiffness contrast between the top few feet of desiccated crust and the softer silty layers underneath can fool a standard refraction survey. The Rayleigh wave dispersion curve from MASW picks up that hidden low-velocity zone at depth, which dominates the site classification. We deploy a 24-channel seismograph with 4.5 Hz geophones, typically using a 46-meter spread, though we adjust the array depending on whether the lot is a tight infill parcel near Garey Avenue or an open industrial lot closer to the 60 Freeway. Data processing includes forward modeling to handle higher-mode contributions, and we report Vs profiles down to at least 30 meters. The final deliverables include a 2D cross-section and a site-specific VS30 value, ready for the structural engineer to use in the seismic design category determination.

MASW and VS30 Shear Wave Velocity Testing in Pomona — Technical reference — Pomona

Local geotechnical context

We set up a 24-channel land streamer with 4.5 Hz geophones spaced at 2 meters. The sledgehammer hits an aluminum striker plate, generating Rayleigh waves that propagate through the upper 30 to 40 meters of the subsurface. In Pomona, the biggest operational headache is cultural noise. The constant rumble of freight trains on the Union Pacific tracks and heavy truck traffic on Holt Boulevard can saturate the low-frequency channels if we do not time the shots carefully. We typically run surveys early morning to beat the urban noise floor, and we stack a minimum of five shots per position. If the site is a small lot squeezed between older commercial buildings, we switch to a shorter array and use active-source modeling to extract the fundamental mode cleanly, ensuring the VS30 calculation is not contaminated by near-field effects.

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Relevant standards

IBC 2021 Section 1613 (Earthquake Loads), ASCE 7-22 Chapter 20 (Site Classification Procedure), ASTM D7400-19 (Standard Test Methods for Downhole Seismic Testing, referenced for MASW principles)

Typical values

Parameter	Typical value
Test Standard	ASTM D7400 / IBC Section 1613
Geophone Frequency	4.5 Hz vertical component
Typical Array Length	46 m (24-channel spread)
Maximum Investigation Depth	30-40 m (depending on site conditions)
Reported Value	VS30 (time-averaged shear wave velocity)
Seismic Source	10 kg sledgehammer on aluminum plate
Data Format	Dispersion curve, 1D Vs profile, 2D cross-section

Common questions

How much does a MASW survey cost for a standard commercial lot in Pomona?

For a typical single-family or small commercial parcel, the cost ranges between US$1,740 and US$2,680. The variation depends on the number of spreads required, the survey line length, and how much ambient noise mitigation we need to apply.

Can MASW replace a boring and downhole seismic test for getting my VS30?

In many cases, yes. MASW is accepted by the IBC as a non-invasive method for determining VS30 and assigning Site Class. If the subsurface has a clear velocity increase with depth, MASW alone is fully sufficient. We only recommend supplemental downhole testing if the site has a strong velocity reversal that surface-wave inversion struggles to resolve uniquely.

How long does it take to get the final VS30 report?

Fieldwork on a single spread usually wraps up in two to three hours. We deliver the processed dispersion curves, the inverted velocity profile, and the final signed report within three to five business days after data collection.

What Site Classes do you typically encounter in Pomona?

We mostly encounter Site Class D, with VS30 values between 200 and 300 m/s, especially in the alluvial areas south of the 10 Freeway. Closer to the San Jose Hills where weathered bedrock is shallower, we sometimes see Site Class C with VS30 between 360 and 500 m/s. The exact boundary shifts block by block, which is why a measured value matters.

Location and service area

We serve projects in Pomona and surrounding areas.

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