Long Beach poses a distinct geotechnical challenge that separates it from other Southern California cities: the deep sedimentary fill of the Los Angeles Basin meets the active Wilmington Anticline and the notoriously seismogenic Newport-Inglewood Fault, which ruptured catastrophically in 1933. ASCE 7-22 Chapter 20 requires a defensible site class determination, and when surface geology is complicated by artificial fill over Holocene alluvium—common across the port and downtown districts—borehole data alone cannot resolve lateral velocity contrasts that control site amplification. Our laboratory deploys seismic tomography using both refraction and reflection acquisition geometries to deliver P-wave and S-wave velocity cross-sections that feed directly into the site-specific ground motion analysis required by IBC 2024. The signal penetration routinely reaches 30 to 50 meters in the compacted sands beneath the Terminal Island Freeway corridor, which is critical because the impedance boundary between Quaternary deposits and the underlying Pico Formation varies sharply across less than 200 meters in some locations. We couple the tomographic inversion with a MASW survey when the client needs a continuous Vs profile for site classification, and we integrate seismic refraction data when the project demands a higher-resolution bedrock depth map under proposed heavy structures.
A 24-channel refraction tomogram across the Wilmington Anticline captured a 40-percent velocity jump over 30 meters—a lateral gradient that a standard borehole grid would have missed entirely.
Local geotechnical context
The contrast between the Belmont Shore spit and the inland Signal Hill uplift tells the whole story of seismic risk in Long Beach. Belmont Shore sits on loose beach and dune sands with a water table often above 8 feet depth, where Vs30 values can fall below 200 m/s and liquefaction potential extends through the upper 12 meters—a Site Class F scenario under ASCE 7 if not mitigated. Just three miles northeast, Signal Hill exposes the Repetto and Pico Formations at the surface, with Vs30 exceeding 600 m/s and a Site Class C classification that produces fundamentally different spectral accelerations. Between these two extremes, the downtown core and the Port of Long Beach lie on artificial fill placed during the 1940s harbor expansion, where undocumented thickness variations create a velocity structure that cannot be reliably categorized through site class alone. Seismic tomography bridges this gap by mapping the shear-wave velocity field in two dimensions, allowing the geotechnical engineer to identify pockets of low-velocity material that would amplify ground motion at periods matching mid-rise structures. The 1933 Long Beach earthquake demonstrated that surface geology controls damage patterns more than epicentral distance, and a liquefaction assessment integrated with a tomographic Vs profile gives the structural designer a defensible basis for ground improvement decisions.
Common questions
What is the typical cost range for a seismic refraction tomography survey in Long Beach?
For a standard 48-meter spread with 24 geophones and a sledgehammer source, the cost ranges from US$2,960 to US$5,120 depending on the number of shot points, the required depth of investigation, and whether S-wave data acquisition is included. Projects requiring a buffalo gun source for deeper penetration or reflection processing add to the upper end of the range. All proposals include the full inversion report, Vs30 calculation, and raw data files.
How does seismic tomography compare to a standard MASW survey for site classification?
MASW yields a 1D shear-wave velocity profile beneath the center of the array, which is adequate for a uniform site. Seismic tomography provides a 2D velocity cross-section that reveals lateral velocity gradients, buried channels, and pinch-outs that a 1D method averages out. In Long Beach, where the fill thickness can change sharply across an old pier line or a buried stream channel, the 2D result prevents misclassification that could lead to an unconservative design spectrum.
Can the survey be performed on a congested urban site with limited access?
Yes, we routinely work in downtown Long Beach and the port area with constrained access. We use a land streamer configuration when pavement is present—a towed array of geophones mounted on base plates that couple through asphalt—which eliminates the need to plant individual geophones. For sites with underground utilities, we coordinate with DigAlert and can adjust the spread geometry to avoid known utility corridors while maintaining adequate ray coverage in the zone of interest.
What deliverables does the structural engineer receive for a seismic design package?
The engineering package includes the processed 2D P-wave and S-wave velocity cross-sections with annotated lithologic interpretation, the calculated Vs30 value and corresponding ASCE 7-22 Site Class, a technical report describing the acquisition parameters and inversion methodology, and the digital velocity model as a CSV grid for direct input into site response software. If reflection data are acquired, we also provide the depth-converted seismic section with interpreted horizons tied to the regional stratigraphy of the Los Angeles Basin.
