Application of Methods to Characterize VS30 in Alabama

Abstract

One of the most important characteristics of a soil that is a basis for seismic analysis, is shear wave velocity (VS). VS has proved to be a valuable tool for site response analysis for seismic design since it has the potential to significantly affect the amplitude and frequency content of expected ground motion. Understanding how the site characteristics will affect the seismic response is necessary for designing robust structures for seismic hazards. Several codes and standards have considered the impacts of site characteristics on anticipated ground vibrations through the site classification. Current design codes have incorporated the VS profile of the soil in the upper 30 m, known as VS30, as a proxy for the stiffness of the soil and its response under dynamic loading, taking into account the potential amplification and changes in the frequency content of the ground motion. Direct methods to measure VS30 have been developed worldwide through invasive and non-invasive testing. Invasive methods require excavations, in most cases, to be conducted and specific equipment to develop VS30 measurements. Non-invasive testing has become increasingly popular due to its quick implementation and relatively low cost compared to invasive testing methods. Researchers have developed different methods to estimate VS in cases where direct methods are not available or cannot be applied. The indirect methods rely on a property or characteristic of the soil, such as fundamental frequency, geology, and topographic slope, which is correlated with VS30 measurements available in the area of study. However, VS testing is less common in regions with relatively low seismicity, like Alabama, which challenges validating any indirect approaches developed for these areas. The absence of measurements also prevents the generation of local correlations and the evaluation of the validity of alternative methodologies. Direct and indirect methods were applied to several locations in Alabama to compute and estimate VS30. The direct methods applied in this study include multichannel analysis of surface waves (MASW) and seismic refraction, while the indirect methods used are the P-wave seismogram method, HVSR correlations, joint inversion with R-wave ellipticity, and proxy methods that were applied to sites and seismic stations. The different approaches were applied to different sites and seismic stations located in Alabama and surrounding states and compared to provide recommendations. Direct methods were applied to a total of 11 sites in this study, and another four VS30 values were collected from the publicly available profiles. Indirect methods were applied to the seismic stations available in the state. The measurements and estimates presented in this work are preliminary approaches to providing VS30 values for Alabama. From the results generated by this study, the use of direct methods such as MASW is recommended because they are more reliable. MASW combined with R-wave ellipticity can provide additional constraints on the inversion. The P-wave seismogram method is very useful in sites where permanent or longer-term seismometer installations are available. When using the P-wave seismogram method in Alabama, it should be considered using the original amplitude ratio correlation and the proposed lower boundary of the 95% confidence interval. However, several recorded earthquakes are required, which limits the application of the method. Proxy methods can be employed at sites without any measurements but tend to underestimate VS30 at stiffer sites. All three proxy methods should be considered in the Gulf Coast region, while the hybrid geology-slope and topographic methods should be considered in Southern Appalachian and Central Tennessee regions. Furthermore, incorporating new measurements of VS30, especially in the north part of the state where the seismic hazard is higher, would allow an accurate assessment of the reliability of the proxy methods and potential modifications to the amplitude ratio correlation for the P-wave method for use in Alabama.