Missoula sits at 3,209 feet, tucked into a mountain valley where glacial Lake Missoula once shaped everything beneath our feet. The 2017 Lincoln earthquake—a 5.8 magnitude event with an epicenter just 90 miles away—rattled homes across the valley and reminded everyone that western Montana is earthquake country. Seismic microzonation takes that reality and maps it block by block, distinguishing stiff gravelly terraces near the Clark Fork River from soft alluvial deposits in the valley center. Our lab, accredited to ISO/IEC 17025, runs the full suite of field and lab tests that feed these maps—because a uniform seismic hazard number for the whole city doesn't help an engineer designing on 30 feet of silty floodplain sediment. We pair downhole shear wave velocity profiling with MASW surface surveys to capture lateral variability, and when the stratigraphy gets tricky we pull in CPT soundings for continuous soil behavior type logs without disturbing the sample.
A site class boundary mapped through downtown Missoula can shift design spectral accelerations by 40 percent—microzonation makes that line visible.
How we work
ASCE 7-22 requires Site Class determination based on the upper 100 feet of stratigraphy, and Missoula's glacial history means that profile can change drastically within half a mile. You might hit dense, overconsolidated till under the South Hills—Site Class C—then drop into interbedded lacustrine silts and clays near Reserve Street that classify as Site Class D or even E. Seismic microzonation work here leans heavily on ASTM D1586 SPT data and ASTM D2487 soil classification to build reliable 3D geotechnical models. We run resonant column and cyclic triaxial tests on undisturbed samples to pin down modulus reduction and damping curves specific to Missoula Valley soils, rather than relying on generic textbook curves that can overestimate or underestimate amplification by 20–30%. The output isn't just a map—it's a defensible geotechnical dataset that zoning boards, structural engineers, and emergency planners can actually use when updating building codes or evaluating critical infrastructure like the bridges crossing I-90 and the Bitterroot River.
Questions and answers
Why does Missoula need seismic microzonation when big earthquakes are rare here?
Rare doesn't mean impossible. The Intermountain Seismic Belt runs right through western Montana, and paleoseismic studies show magnitude 7+ events have occurred on faults within 50 miles of Missoula in the Holocene. The valley's deep glacial lake sediments amplify ground motion significantly—so a moderate earthquake can produce shaking intensities here that a rock site wouldn't feel. Microzonation quantifies that amplification so buildings aren't under-designed.
What's the difference between a USGS hazard map and a microzonation study?
The USGS National Seismic Hazard Model gives you regional probabilistic ground motion at a reference rock condition (Vs30 = 760 m/s). A microzonation study takes that rock motion and propagates it through local soil columns—measured by our borings and geophysical surveys—to produce surface-level hazard maps that account for site amplification, basin edge effects, and liquefaction potential specific to Missoula's geology.
How long does a microzonation project take from start to finish?
For a typical neighborhood-scale study covering a few square miles, plan on 8 to 14 weeks. Fieldwork—drilling, CPT soundings, MASW lines—takes 2 to 4 weeks depending on access and weather. Lab testing on the samples runs concurrently but the dynamic tests (resonant column, cyclic triaxial) need 3 to 5 weeks because of consolidation and staged loading. The analysis and mapping phase takes another 2 to 3 weeks.
What's the typical cost range for seismic microzonation in the Missoula area?
Cost depends heavily on the study area size and drilling depth requirements. A targeted site-specific microzonation for a single development runs between US$3,890 and US$7,500. A broader district-scale study covering multiple city blocks with 8–15 investigation points typically falls in the US$9,800 to US$14,870 range. The biggest cost drivers are the number of deep borings (100 ft+), MASW line coverage, and how many dynamic lab tests are needed.
Can you use existing geotechnical reports from our site, or do you need new field data?
We can incorporate existing SPT logs and lab data if they meet current ASTM standards and include the full upper 100 feet of stratigraphy. But most older reports stop at 30 or 40 feet, and they rarely include shear wave velocity measurements or undisturbed sampling for dynamic testing. In practice, we typically need to drill at least one new deep boring per zone and run MASW lines to calibrate Vs profiles—existing data fills gaps but doesn't replace the primary investigation.
