MASW & VS30 Shear Wave Velocity in Oxnard, CA

We roll out the 48-channel seismograph and a spread of 4.5 Hz geophones across the site, usually starting early when the Oxnard Plain is quiet and the wind hasn't kicked up yet. The active source is a 16-pound sledgehammer striking an aluminum plate; on softer ground we drop a weight from the truck hitch for better low-frequency penetration. Each line takes about 45 minutes to lay out, shoot, and stack. We process the dispersion curves back at the office, inverting the fundamental mode to get a 1D shear wave velocity profile. For a standard Oxnard lot between the Santa Clara River and the 101, two lines give us enough coverage to nail the VS30 without ambiguity. If the site is over 2 acres we add a third line along the diagonal. The whole field campaign wraps up in a morning, and the report goes out within three business days.

A 30 m/s difference in VS30 can push a project from Site Class D into E on the Oxnard Plain, triggering a 20% increase in base shear.

Our approach and scope

Most of Oxnard sits on Quaternary alluvium from the Santa Clara River: interbedded silts, clays, and loose sands that can vary 30 feet apart. The water table is high—often less than 6 feet below grade near the Channel Islands Harbor area—which slows down S-wave propagation and drops VS30 into the D or E site class. We've seen profiles where the top 15 meters average 180 m/s, then jump to 350 m/s once you hit the stiffer Pleistocene deposits underneath. That gap changes everything for the structural engineer. We supplement the MASW lines with a CPT test when we need continuous soil behavior type logs without disturbing the sample, especially in saturated silts where SPT recovery is poor. For projects requiring a full site class determination across larger parcels we overlay the VS30 map with seismic refraction to cross-check bedrock depth and identify hidden velocity inversions that a dispersion curve alone might miss.

Local geotechnical context

Oxnard grew fast after the 1940s when the military bases expanded and the port started shipping citrus and lima beans. A lot of the housing stock near La Colonia and South Oxnard was built before modern seismic codes, on fill that nobody characterized properly. Those neighborhoods sit on 3 to 8 feet of undocumented fill over soft alluvium, and when you run MASW lines through them you routinely find VS30 below 180 m/s. The combination of a shallow water table and loose silty sand creates a real liquefaction risk during a Ventura-Pitas Point thrust event—the same fault system that ruptured in 1973. An owner who skips the shear wave velocity survey is gambling with resonance amplification that could turn a moderate shake into a structural failure. We've seen it in post-earthquake reconnaissance: same magnitude, same distance, and the building on Site Class E gets condemned while the one on stiff soil just needs drywall repair.

Typical values

Parameter	Typical value
Vs30 range (Oxnard alluvium)	160 – 360 m/s
Typical site class	D or E per ASCE 7-22 Table 20.3-1
Geophone frequency	4.5 Hz vertical component
Receiver spread	46 m (24 geophones at 2 m spacing)
Source type	Sledgehammer / accelerated weight drop
Depth of investigation	30 m (extends to 45 m with passive microtremor)
Reporting standard	IBC 2021 / ASCE 7-22 Section 20.4

Related services

MASW & Vs30 Classification

Active and passive surface wave acquisition with 24-48 channel seismograph, dispersion curve inversion, and signed report assigning site class per IBC/ASCE 7-22. Includes 1D Vs profiles at each line and a site plan with survey coordinates.

Combined Geophysics + CPT Package

When the structural engineer wants both shear wave velocity and bearing capacity in one submittal, we pair the MASW with a CPT sounding to 60-80 ft depth. The CPT gives tip resistance and sleeve friction in the same silty soils where the Vs profile flags the site class boundary.

Common questions

What does a MASW survey cost for a single-family lot in Oxnard?

For a standard residential lot under 5,000 sq ft, budget between US$1,620 and US$2,200 for two MASW lines with Vs30 classification and a signed report. Sites over half an acre or with poor access run US$2,800 to US$3,450 because we need a third line and possibly passive microtremor recording to reach 100 ft depth.

How long does the field work take?

A two-line survey on a clear lot takes about 2.5 hours on site, including setup, shooting, and break-down. We process the data off-site and deliver the draft report in three business days. If the city needs the Vs30 for a same-week permit submittal, we can expedite to 48 hours.

Do you need to drill or excavate to run the test?

No. MASW is entirely non-invasive. The geophones are planted at the surface with spikes, and the energy source is a hammer blow or weight drop at ground level. There is no drilling, no trenching, and no soil disturbance—which matters a lot in Oxnard where shallow groundwater turns a borehole into a mud pit.

Which site class is typical for Oxnard?

Most of the Oxnard Plain falls into Site Class D (stiff soil, Vs30 between 180 and 360 m/s) or Site Class E (soft clay soil, Vs30 below 180 m/s). The boundary between D and E is very local—we have mapped it shifting from one side of a street to the other depending on how close the Santa Clara River paleochannel runs.