We ran a MASW line behind an old warehouse off Pacific Avenue last month. The owner needed a Site Class for a four-story addition. Surface gravel hid a deeper layer of soft alluvium that would have dropped the site to Class E if we had only looked at SPT blow counts. The shear wave velocity profile told a different story. Tacoma sits on a mix of Vashon glacial drift, recessional outwash, and lahar deposits from Mount Rainier. The upper 30 meters can change velocity by 300 m/s within a single block. That variability makes the VS30 number critical for any structural design over two stories. We pair the MASW survey with seismic refraction when the water table sits high and the velocity contrast is subtle.
A 50 m/s difference in VS30 can shift a Tacoma site from Site Class D to E, doubling the seismic base shear in the structural design.
Service characteristics in Tacoma
Critical ground factors in Tacoma
ASCE 7-22 Section 20.4 requires VS30 as a primary metric for Site Class. In Tacoma, the risk is misclassifying a site as Class D when the soil column below 15 meters is actually soft enough for Class E. That mistake produces an unconservative design for the long-period response, which is exactly what controls mid-rise buildings in the Puget Sound basin. The Nisqually earthquake in 2001 showed amplified shaking in the tideflat areas and parts of the Port of Tacoma. We have seen VS30 values as low as 165 m/s on fill over estuarine mud, and over 500 m/s on the gravelly uplands near the Proctor District. No two sites read the same. The cost of getting it wrong is a structure that underperforms during the next deep slab event on the Cascadia subduction zone.
Our services
Our Tacoma MASW work is part of a broader geophysical and geotechnical testing program. Every project gets a field report with the dispersion image, the inverted VS profile, and the site classification letter ready for the structural engineer.
MASW & VS30 Mapping
Active-source surface wave survey with 24-channel seismograph for IBC Site Class determination. Includes Rayleigh wave dispersion analysis and 1D VS profile inversion.
Combined Geophysical Package
MASW paired with seismic refraction tomography or downhole seismic for sites with complex stratigraphy, velocity reversals, or where a deeper bedrock profile is required.
Quick answers
How long does a MASW survey take on a Tacoma site?
A single array with setup and breakdown takes about two hours. We can run three or four lines in a full day if access is clear. You get the VS30 number and Site Class the same week.
What does VS30 mean in the IBC context?
VS30 is the time-averaged shear wave velocity for the upper 30 meters. IBC uses it to assign Site Class A through F, which determines the seismic design coefficients for the structure per ASCE 7.
What does a MASW survey cost in Tacoma?
A single-line MASW survey with VS30 reporting typically runs between US$1,480 and US$3,510 depending on access, array length, and whether we combine it with refraction or a CPT tie-in.