2D Refraction Microtremor Analysis of SW4 Synthetics

Chelsey E. Assor, Jenna Graham, Lauren M. Lewright, Aditya Prathap, & John N. Louie

Submitted August 15, 2021, SCEC Contribution #11461, 2021 SCEC Annual Meeting Poster #040

Geotechnical measurements of shear-wave velocity currently rely on one-dimensional surface-wave dispersion analyses. Such analyses do not account for 3D wave propagation across lateral velocity heterogeneities. Pancha and others, as an example, used successive overlapping 1D dispersion array analyses to effectively characterize lateral-velocity variability in West Reno. Previous refraction microtremor analyses have been made on simulation results from 1D simulation velocity models, such as by Naphan and others. To get a start at examining potential challenges and opportunities of 1D dispersion analysis in areas of lateral velocity variations, this study runs a 1D refraction microtremor analysis of array seismograms computed over a 3D grid representing 1D and 2D velocity models. We are using the SW4 code from LLNL/geodynamics.org to simulate seismic wave propagation across a 400⨉400⨉100-meter grid, to a maximum frequency of 45 Hz. Grid spacing is 1.0 m, and the source central frequency is 15 Hz. After compiling SW4 for the first time under MacOS Big Sur for Apple’s new M1 chip, we ran SW4 on personal laptops for about two days to complete each model run. In assembling the synthetic arrays, both the 2D and 1D model data were outputted from SW4 and organized into records using a Jupyter notebook to efficiently organize the data into SEG-Y files. Refraction microtremor software created subarray slowness-frequency for dispersion picking. The dispersion picks allow estimation of Vs(z) profiles. Comparison of 1D dispersion-modeling results to the input 3D velocity models will suggest where the 1D analysis is succeeding or failing in the 3D context. The streamlined modeling and processing procedures will allow running of a wide variety of test velocity models.

Key Words
Shear-wave velocity, ground motion, side effects

Citation
Assor, C. E., Graham, J., Lewright, L. M., Prathap, A., & Louie, J. N. (2021, 08). 2D Refraction Microtremor Analysis of SW4 Synthetics. Poster Presentation at 2021 SCEC Annual Meeting.


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