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Model Surface Wave Dispersion Analysis Across a Basin Boundary
AuthorGraham, Jenna Ruth
AdvisorLouie, John N
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A common occurrence through the geotechnical and engineering communities is that assessments for ground and building response to earthquakes are completed using one-dimensional modeling in multidimensional situations. This simplification has complex implications in areas such as the Basin and Range, where the 3D basin structure seen in shallow shear wave modeling suggests striking amplification effects on seismic shaking. Deep refraction microtremor data previously taken in western Reno, Nevada revealed a vertical displacement of the floor of the Reno-area basin of approximately 350 m. This research explores the effects of this lateral boundary on dispersion analysis of seismic surface waves from 0.2-7 Hz. To determine the effects of surface waves as they cross a lateral boundary, three synthetic models were developed using the 3D computational software, SW4. Each model employs a virtual linear, north-to-south oriented array consisting of 500 geophones with 10 m spacing. This array is surrounded by an omnidirectional arrangement of eight virtual sources. Two synthetic 1D models were created utilizing the maximum and minimum velocity profiles from the prior refraction microtremor dataset. A third, 2D model combines the 1D models across a lateral boundary running east-to-west. The analysis of resulting synthetic subarray records demonstrated that the arrays crossing the lateral boundary are biased toward low-velocity results. Overall, Rayleigh-wave dispersion picks from the resulting 1D model arrays took velocities up to 10% lower than 1D theory suggests for the original input velocity profiles. The 2D models yielded picked dispersion accuracy better than 20%, also below 1D theory. Synthetic dispersion curves from arrays crossing the boundary revealed the potential to interpret false velocity inversions, as well as the presence of low phase-velocity zones and basin edge effects.