San Francisco's unique geology, from the soft Bay Mud of the Marina to the Franciscan Complex bedrock on Nob Hill, creates sharp contrasts in seismic site response over very short distances. The 1906 rupture wasn't just a history lesson, it exposed how fill and young alluvium can amplify shaking by factors of three or more compared to nearby rock. We run MASW surveys across the city, on tight urban lots in the Mission and on steep slopes near Twin Peaks, to measure Vs30 directly and assign a proper Site Class per ASCE 7 Chapter 20. A seismic microzonation study often starts from a handful of these profiles, but for individual projects the raw Vs30 number is what triggers the design ground motions. Our field crew uses 24-channel arrays with active and passive sources to reach the 100-foot depth that Chapter 20 demands, even when traffic noise on Geary Boulevard tries to eat the signal.
A Vs30 number without a dispersion curve is just an opinion. We deliver the full inversion so you can see where the velocity breaks actually sit.
Scope of work in San Francisco
Critical ground factors in San Francisco
The 1989 Loma Prieta earthquake registered a magnitude 6.9 with an epicenter 60 miles south, yet it collapsed the Marina District's soft-story buildings and cracked the Bay Bridge. The lesson for geotechnical work in San Francisco is that distance doesn't guarantee safety when the soil column is thick and soft. Relying on generic Vs30 maps from the USGS can miss thin high-velocity caps that artificially raise the average, a dangerous blind spot on sites with interbedded sands and clays. A single MASW line costs far less than the structural overdesign that follows an incorrect Site Class assumption. We've seen projects where a proper Vs measurement downgraded the site from C to D and added 15 percent to the base shear, a number no structural engineer wants to discover during peer review. For sites near the waterfront or on artificial fill mapped by the California Geological Survey, skipping the field measurement is a liability decision, not an engineering one.
Our services
We run MASW in two modes depending on the target depth and lot access. Both follow the same inversion workflow, but the field setup changes.
Active MASW for shallow Vs30
A 24- or 48-channel spread with a sledgehammer source, ideal for tight urban lots in the Richmond or Sunset districts where the depth to rock is often less than 50 feet. We stack multiple shots per spread to clean up cable car and bus noise, and deliver a 1D Vs profile with uncertainty bounds within three business days.
Combined active-passive MASW
Adds ambient-noise recording with circular or L-shaped arrays to extend the dispersion curve below 10 Hz. Required when the site sits on deep Bay Mud or Colma Formation, where the shear wave velocity stays below 200 m/s well past 80 feet. This is the configuration we recommend for Site Class E and F determinations under ASCE 7.
Frequently asked questions
What does a MASW test cost in San Francisco?
Active MASW surveys in the Bay Area run between US$1,480 and US$3,210 depending on array length, number of spreads, and whether passive data collection is included. Sites with limited access or nighttime work requirements in downtown corridors add mobilization complexity. We send a fixed-price scope after reviewing the lot geometry and target depth, so you won't see a change order for extra geophones.
How deep do you need to measure for a proper Vs30?
ASCE 7 requires a shear wave velocity profile to at least 100 feet, or 30 meters. If the array can't see that deep, you're extrapolating, and the code only allows that under strict conditions. Our passive arrays pick up ambient energy below 5 Hz, which on Bay Mud sites gives us a reliable dispersion curve down to 30 meters without any guesswork.
Can MASW replace a downhole seismic test?
It can in many cases, especially for site classification. MASW measures the fundamental-mode Rayleigh wave dispersion, which is sensitive to the same Vs layering as a downhole test. The advantage is that you don't need a borehole, which saves time and avoids drilling through contaminated fill. We do cross-check with downhole data when the client already has a boring, particularly for critical infrastructure where the structural engineer wants both datasets.
How does San Francisco's geology affect the Vs30 measurement?
The city sits on a mix of Franciscan Complex bedrock, Colma Formation sands, and Holocene Bay Mud, often all in the same neighborhood. Soft clays below the water table around Mission Bay can show Vs values under 100 m/s, while the dense sands of the Colma hit 400 m/s. The weighted average can mask a soft layer at depth, so we always report the full profile, not just the number, and flag any velocity inversions that could affect the site response.