MASW & VS30 Shear Wave Velocity Testing in Los Angeles

The 24-channel Geode seismograph deploys across Los Angeles sites with a 4.5 Hz geophone spread, capturing Rayleigh wave dispersion from a 10-pound sledgehammer source. In the LA Basin, where Quaternary alluvium and artificial fill conceal abrupt velocity contrasts, the surface-wave method resolves shear-wave velocity profiles to 30 meters depth without the drilling logistics that congested urban corridors demand. The field crew lays out 115-foot receiver lines along parking lots, graded pads, and narrow right-of-ways, stacking three to five shots per spread to suppress ambient noise from the 405 freeway or Metro rail vibrations that propagate through the sedimentary column. Data processing runs through SurfSeis software, inverting dispersion curves with a ten-layer model that captures the impedance boundaries where Holocene sands transition into Pleistocene gravels. For projects requiring deeper bedrock mapping, we pair the seismic refraction survey with MASW on the same spread, extracting P-wave and S-wave velocities from a single mobilization.

Velocity reversals in the top 30 meters can shift a site from Class C to Class D, doubling the seismic base shear in the structural design.

Methodology and scope

The Los Angeles Basin sits atop a deep sedimentary trough where alluvial fans from the Santa Monica Mountains and San Gabriel range interlace with marine terrace deposits, creating VS30 variations that swing from 180 m/s in soft San Pedro Formation silts to over 760 m/s on weathered Mesozoic schist outcrops in the Hollywood Hills. These contrasts matter because the 2022 California Building Code (CBC 1613.2) maps the entire city within Seismic Design Categories D and E, and the ASCE 7-22 site coefficients amplify spectral accelerations by factors of 1.6 to 2.5 at Site Class E locations. A typical survey on a half-acre parcel in the Mid-Wilshire district runs twelve geophone spreads in orthogonal directions, delivering a spatially averaged VS30 value that the geotechnical engineer plugs directly into the ground motion calculation. When near-surface basalt flows of the Miocene Conejo Volcanics appear at 12 to 18 feet depth, the dispersion curve shows a characteristic velocity inversion that demands careful higher-mode analysis rather than a fundamental-mode-only inversion. The resulting one-dimensional shear-wave velocity column feeds site-specific response spectra and liquefaction triggering analyses per the Idriss and Boulanger (2014) procedure.

Local geotechnical context

ASCE 7-22 Section 20.1 requires Site Class determination for every new building over one story in Los Angeles, and the default assumption of Site Class D — unless substantiated by measured shear-wave velocity data — can misclassify soft clay basins and stiff hillside rock alike. The consequence of relying on proxy correlations from SPT blow counts alone is a potential underestimation of short-period spectral acceleration at soft-soil sites in the San Fernando Valley, where basin-edge effects amplify ground motion beyond the code-prescribed coefficients. The 1994 Northridge earthquake demonstrated that sedimentary basins trap and focus seismic energy, producing peak ground accelerations of 1.8 g at Tarzana on soils that conventional borings had not flagged as exceptional. A direct MASW measurement eliminates the regression uncertainty inherent in N-value-to-VS conversions, and for liquefaction assessments the measured VS profile feeds the Andrus and Stokoe (2000) triggering curve, which the California Geological Survey accepts for Seismic Hazard Zone review submissions. Skipping the direct velocity measurement on a Site Class E or F parcel invites structural overdesign in the wrong places and underdesign where it matters most.

Reference parameters

Parameter	Typical value
Target depth of investigation	30 meters (100 ft) standard; extendable to 45 m
Geophone natural frequency	4.5 Hz vertical-component, 24-channel array
Source type	10–16 lb sledgehammer on aluminum strike plate
Receiver spread length	115–230 ft (35–70 m), site-dependent
Sampling interval	0.25–0.50 ms; record length 2.0 seconds
Dispersion inversion method	SurfSeis 6, fundamental + higher-mode joint inversion
Vs30 reporting	Time-averaged shear-wave velocity per ASCE 7-22 Section 20.4

Other technical services

Seismic Refraction Tomography

P-wave refraction on the same geophone spread maps rippability and depth to bedrock, distinguishing saturated alluvium from weathered shale where velocity overlaps occur.

Downhole & Crosshole Seismic

Borehole-based VS measurements in deep urban excavations provide layer-by-layer velocity calibration that ties the MASW surface-wave model to logged stratigraphy.

Electrical Resistivity Imaging

ERT lines across fault-zone parcels in the Hollywood and Santa Monica fault traces map low-resistivity gouge zones and groundwater perched on bedrock benches.

Reference standards

ASCE/SEI 7-22 Minimum Design Loads and Associated Criteria for Buildings, 2022 California Building Code (CBC), Title 24, Part 2, Chapter 16, ASTM D7400-19 Standard Test Methods for Downhole Seismic Testing (cross-referenced for MASW), IBC 2021 Section 1613 Earthquake Loads, California Geological Survey Special Publication 117A – Guidelines for Evaluating Liquefaction Hazards

Common questions

How long does a MASW survey take on a typical Los Angeles lot?

A standard 12-spread setup with 24-channel acquisition on a half-acre parcel requires approximately four to five hours of field time, including layout, shooting, and equipment breakdown. The data processing and dispersion inversion add another two to three days in the office before the signed VS30 report is delivered.

What does MASW testing cost for a single-family residential project in LA?

For a standard residential parcel requiring twelve to sixteen geophone spreads and a single mobilization within the LA metro area, the MASW survey ranges from $1,430 to $2,980 depending on site accessibility, traffic control requirements, and whether the City of Los Angeles Department of Building and Safety requests additional documentation for hillside or liquefaction zone parcels.

Does the City of Los Angeles accept MASW for Site Class determination instead of SPT-based correlations?

Yes. The LADBS Structural Plan Check division accepts direct VS30 measurements from MASW, downhole, or crosshole methods as the preferred approach for Site Class determination per CBC 1613.3.2. A measured shear-wave velocity profile eliminates the need for the default correlation tables and often results in a more favorable (and code-compliant) site coefficient, particularly on stiff soils where SPT-based methods overestimate Site Class conservatism.