MASW & VS30 Testing in Seattle — Shear Wave Velocity Profiles for Seismic Site Class

The field setup starts with a 24-channel seismograph. We lay out geophones at two-meter spacing along a straight line. A ten-kilo sledgehammer strikes an aluminum plate. That impact sends surface waves through the glacial drift that underlies most of Seattle. The geophones record the entire wave train. We process the dispersion curve back at the lab. The goal is a one-dimensional shear wave velocity profile. Seattle sits in a basin with abrupt soil transitions. Outwash sands near Green Lake behave very differently from the compact till on Capitol Hill. A seismic refraction survey sometimes pairs with MASW when we need both P-wave and S-wave data for deeper bedrock mapping. Our seismograph runs at 0.25 ms sampling intervals. That resolution matters in Seattle. You might hit stiff lodgement till at six meters on one lot. Two blocks away, you find thirty meters of compressible fill over old tide flats. The VS30 value dictates your IBC site class.

VS30 in Seattle can jump from 200 m/s to 600 m/s within a single block because of glacial topography — site class is a lot-line decision here.

Methodology and scope

Seattle's subsurface is shaped by Vashon glaciation. The last advance left a cap of over-consolidated till across much of the city. Beneath that till, you often find advance outwash or lacustrine clays. The water table sits shallow in the Duwamish valley and Interbay area. We have measured VS30 values ranging from 180 m/s in soft alluvium near the industrial district to over 760 m/s on the till uplands of Queen Anne. The IBC classifies those as Site Class E and Site Class B respectively. Our crew uses a 4.5 Hz geophone spread. That frequency captures the low-velocity surface layers well. For deeper profiles we run passive MASW with ambient noise. A single line can resolve velocity down to thirty meters. The processing software applies f-k transformation. We pick the fundamental mode manually. No auto-picking. Every dispersion curve gets a quality check against the coherence spectrum. The result is a VS30 value that feeds directly into the seismic design parameters of ASCE 7-22. Seattle's building department expects that number on every permit application for structures over two stories.

Local ground factors

Seattle sits in a seismic hotspot. The 2001 Nisqually earthquake registered magnitude 6.8 at a depth of 52 kilometers. It damaged the Alaskan Way Viaduct and dozens of masonry buildings in Pioneer Square. A bad VS30 value has consequences. Classify a site as C when it is really E, and you underestimate spectral accelerations by a factor of two. We see this risk in the Duwamish waterway corridor. The young Holocene deposits there amplify ground motion significantly. The IBC site coefficients for Site Class E push the design base shear up sharply. Some projects trigger a site-specific ground motion hazard analysis under ASCE 7-22 Section 21.2. We deliver the shear wave velocity data in a format that the geotechnical engineer of record can plug directly into those calculations. No conversion steps. No reprocessing delays. The Seattle DPD reviews these reports carefully. They know the local geology. They will ask about your VS30 measurement if the number looks generic.

Technical data

Parameter	Typical value
Geophone spread length	46 meters (24 channels at 2 m spacing)
Source type	10 kg sledgehammer on aluminum strike plate
Geophone frequency	4.5 Hz vertical component
Sampling interval	0.25 milliseconds
Recording duration	2 seconds per shot
Maximum investigation depth	30 meters (active source)
Passive MASW method	Linear array using ambient microtremors

Related services

IBC Site Classification Package

Active MASW line plus VS30 calculation. We provide the final site class letter (A through F) with the velocity profile signed by the responsible geophysicist.

Active + Passive Combined Survey

For sites needing depth beyond 30 meters. Active sledgehammer source resolves the upper layers. Passive noise recording with a linear array captures deeper velocity structure down to 60 meters.

Cross-Hole Shear Wave Verification

When the building department requires an independent check on the MASW result. We run downhole or cross-hole shear wave measurements in a cased borehole and correlate with the surface wave profile.

Seismic Microzonation Support

Multiple MASW lines gridded across a development site. We produce a VS30 contour map for input to seismic microzonation studies. Used for campus-scale projects and utility corridor planning.

Frequently asked questions

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

For a standard residential lot with one active MASW line processed to VS30, the cost ranges from US$1,540 to US$3,260. The price depends on site access, line length, and whether passive recording is required. Steep slopes or heavy undergrowth on the lot can increase the field time and push the cost toward the upper end of that range.

How does Seattle's glacial geology affect MASW results?

The Vashon till has a high shear wave velocity, often above 500 m/s. That creates a strong impedance contrast with underlying advance outwash or lacustrine deposits. Our processing must separate the fundamental Rayleigh mode from higher modes that appear at frequencies above 30 Hz. We use the full f-k spectrum to isolate the correct dispersion branch. The stiff upper layer can also limit penetration depth with a hammer source, which is why we sometimes switch to passive recording.

Which IBC site class is most common in Seattle?

Most of Seattle's residential uplands fall into Site Class C with VS30 between 360 and 760 m/s. The glacial till dominates there. The downtown core and the Duwamish industrial area often classify as Site Class D or E due to fill, alluvium, and deeper soft soils. We have measured Site Class B on a few Capitol Hill lots where till is exceptionally dense and shallow to bedrock.