An Empirical Correlation between the Residual Gravity Anomaly and the H/V Predominant Period in Urban Areas and Its Dependence on Geology in Andean Forearc Basins

Abstract

The study of site amplification effects is crucial to assess earthquake hazards that can produce great damage in urban structures. In this context, the gravity and the ambient noise horizontal-to-vertical spectral ratio (H/V) are two of the most used geophysical methods to study the properties of the subsoil, which are essential to estimate seismic amplification. Even though these methods have been used complementarily, a correlation between them has not been thoroughly studied. Understanding this correlation and how it depends on geology could be important to use one method as an estimator of the other and to make a distinction between the seismic and gravimetric basement. In this research, a comparison between the residual gravity anomaly and the H/V predominant period is performed using a long dataset from different projects on sedimentary basins in a group of the most important cities in Chile. To simplify the geological information, a seismic classification is used for soils, which considers the Vs30 and the predominant period of vibration (T0). The results of this comparison show a direct correlation between both parameters, the higher the negative residual gravity anomaly the higher the H/V predominant period. This correlation improves when only soft soils are considered, increasing the R2 value in more than a 50% in all the individual cities with respect to the overall correlation. When all the cities are considered, the R2 value for soft soils increases up to 0.87. These results suggest that the ideal geological background for this correlation is when a soft soil layer overlies a homogeneous bedrock. Heterogeneities in the bedrock and in the soil column add dispersion to the correlation. Additionally, the comparison between the depth to basement inferred by both methods show differences of less than 15% in soft sites; in denser sites, the difference increases up to 30% and the definition of a clear H/V peak is more difficult. In general, the gravimetric basement is deeper than the seismic one. However, gravimetric depths to basement can be under/over-estimated in zones with a heterogeneous soil column.