Rapid seismic surveys for non-intrusive fault location, basin structure, critical-zone characterization, and site class for building-code compliance

John N. Louie, Alison Star, & Bill Honjas

Submitted September 7, 2025, SCEC Contribution #14576, 2025 SCEC Annual Meeting Poster #TBD

Seismic surface-wave-arrays offer the opportunity to perform one geophysical survey yielding seismic site class along with a more comprehensive site investigation including assessments of fault location, critical zone characterization, depth to bedrock, and even P-wave velocity and Poisson’s ratio. Building-code-compliant surface-wave surveys, when processed and interpreted with Terēan software, provide this full range of results. The resulting cross sections of shear-wave velocity often locate surface faulting with 3 m accuracy, demonstrated on strands of the Calaveras fault in Pleasanton, Calif. and the Las Vegas Valley faults in Nevada, among others. ASCE/SEI Standard 7-22 has been adopted into building codes by countries, states, and municipalities around the world. Chapter 20 describes new standards for determining seismic site class that encourage geophysical surveying rather than cone penetrometer or standard penetration testing. Invasive methods can fail to achieve compliance because of tool refusal or difficulty for intrusive methods to access sites. For non-intrusive geophysical surveying to achieve code compliance it is important for geotechnical engineers to employ geophysical survey methods effective at determining the time-averaged shear-wave velocity from the surface to 30 m depth, known as Vs30. Without such measurements, taking the default seismic site class may lead to over-design of building structures, inflated construction costs and extended project timelines. Most sites require less than one hour to complete for Vs30 measurement, including narrative report generation. This technology increases the ease of data collection with an untethered, triggerless hammer and the ability for the same array of 24, 4.5 Hz geophones to collect S- and P-wave data simultaneously. Many case histories at scales from 5 m to 1000 m serve to demonstrate these rapid and comprehensive results, including assessments of basin structure to kilometer depths. Simpler geophysical surveys with more comprehensive results allow engineers and geologists to more efficiently complete safety and environmental assessments.

Key Words
faulting, critical zone, bedrock depth, Vs30

Citation
Louie, J. N., Star, A., & Honjas, B. (2025, 09). Rapid seismic surveys for non-intrusive fault location, basin structure, critical-zone characterization, and site class for building-code compliance. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Community Earth Models (CEM)