The mobile laboratory unit parked near the Boise River carries a three-component seismometer and a 24-channel MASW acquisition system. Field technicians deploy the geophone array along a 100-meter line while the hammer source generates shear waves that travel through the alluvial sediment and basalt layers beneath the city. This is the core field operation for a seismic microzonation study in Boise, where the topographic relief and the presence of the Boise River alluvial plain create sharp contrasts in subsurface stiffness. The recorded data is processed to produce a Vs30 profile, which directly feeds into the ASCE 7 site classification system used by structural engineers for dynamic analysis.
The Vs30 threshold between Site Class C and D is 366 m/s; Boise's floodplain often falls below that value, triggering amplification factors of 1.2 to 1.5.
Methodology and scope
The methodology follows a tiered approach: (1) ambient noise measurements using microtremores hvsr to identify resonance frequencies, (2) active-source MASW for Vs30 determination, and (3) geotechnical boreholes with SPT for validation. We also run resistividad electrica sev to map the depth to basalt, which directly influences the seismic site class according to ASCE 7-16 Table 20.3-1.
Local considerations
A six-story apartment complex planned on West State Street encountered a hidden risk. The geotechnical report assumed Site Class C based on nearby basalt outcrops, but the seismic microzonation study in Boise revealed a 12-meter-thick layer of soft alluvial silt overlying the basalt at that specific lot. The measured Vs30 was 245 m/s, placing the site in Class D with an amplification factor of 1.5. Without this refined zonation, the design would have underestimated the base shear by nearly 20 percent. The structural engineer had to redesign the lateral system to accommodate the higher seismic demand, adding two shear walls and increasing the foundation tie-beam dimensions.
Applicable standards
ASCE 7-16 Section 20.3 (Site Classification), ASTM D4428/D4428M-14 (MASW), NEHRP Recommended Seismic Provisions (FEMA P-1050), IBC 2021 Chapter 16 (Seismic Design)
Associated technical services
Ambient Noise HVSR Survey
Single-station microtremor measurements at 100 m grid spacing to map the fundamental frequency of soil deposits across the Boise River corridor.
MASW Vs30 Profiling
24-channel active-source MASW along linear arrays of 70-100 m, processed to produce 1D shear-wave velocity profiles and site class assignment per ASCE 7.
Geotechnical Boreholes with SPT
Boreholes to 30 m depth with Standard Penetration Test at 1.5 m intervals, including sample recovery for laboratory testing of cyclic resistance.
Seismic Hazard and Liquefaction Mapping
Integration of Vs30 data, PGA from USGS hazard curves, and SPT-based liquefaction assessment to produce zonation maps for the urban area.
Typical parameters
Frequently asked questions
What is the difference between a seismic microzonation and a standard geotechnical site class?
A standard site class assigns a single category (A through F) to the entire property based on average Vs30. Seismic microzonation in Boise produces a spatial map showing how the site class varies across a parcel or district, because the alluvial depth and basalt topography change within short distances. This allows engineers to place buildings on the most favorable subzone and design foundations accordingly.
How much does a seismic microzonation study in Boise cost?
The typical range for a residential-to-medium commercial project in Boise falls between US$4.780 and US$18.910, depending on the number of HVSR stations, MASW lines, and boreholes required. Larger parcels or multiple building footprints push the upper end. Contact us for a project-specific quote.
Which Boise neighborhoods are most likely to require microzonation?
Areas along the Boise River floodplain, including the Central Bench and parts of the North End near the river, often have soft alluvial soils that produce Site Class D or E. The South Bench and Foothills tend to be underlain by basalt (Class B or C), but local pockets of colluvium can still amplify ground motion. A microzonation study is recommended for any project with a seismic importance factor greater than 1.0 in these transition zones.
What field data is needed for the microzonation to be valid?
Minimum requirements are at least three HVSR stations per building footprint, one MASW profile for every 4,000 m² of site area, and one SPT borehole to 30 m depth per 10,000 m². The data is correlated with the USGS Seismic Design Maps and the ASCE 7 site classification procedure. If liquefaction is a concern, we also run cyclic triaxial tests on undisturbed samples.