UCLA

A major component of the Next-Generation Attenuation Subduction (NGA-Sub) project is the development of a database that compiles source data, ground motion data, and data from earthquake recording sites in subduction regions around the world. This paper describes work undertaken to assign site parameters for ground motion recording stations in Chile for the NGA-Sub project. The primary site parameter required for NGA-Sub model development is the time averaged shear wave velocity in the upper 30 m, or VS30. This work has two major tasks: (1) develop a database of shear-wave velocity profiles in Chile, whether they are at ground motion recording stations or not; (2) use the available data to either validate the use in Chile of a proxy based VS30 prediction model developed for another region, or to develop a Chile-specific model. For Task 1, we have compiled geotechnical data for locations where shear-wave velocity profiles and associated VS30 values were measured with geophysical methods. The various velocity data sources include a compilation of more than 1,100 measured VS profiles. Task 2 provides a proxy based model for VS30 prediction, needed for the stations without in situ VS30 measurements. Work to date is based on a proxy-based model based on geomorphic terrain classes and follows an approach applied previously in other regions, in which a natural log mean and standard deviation is assigned for up to 16 terrain classes. We find a Chile-specific model is warranted by the data, and demonstrate regional effects based on site location in an arid region in north Chile (Norte Grande and Norte Chico) versus more fertile regions to the south. Using data where available and proxies where needed, we provide estimates of natural log means and standard deviations of VS30 for NGA-Sub recording stations in Chile.