Soil Dynamic Response for Seismic Microzonation Purposes: Rancagua-MachalÍ and Rengo Cities, Chile.

Summary Seismic microzonation maps for Rancagua, Machalí, and Rengo cities, located in the Cachapoal basin, Central Chile, were generated based on near-surface geology, well stratigraphy, and soil dynamical properties. In the past, these urban areas have suffered severe damage due to earthquakes occurrence. Cachapoal basin sedimentology predominantly consists of alluvial deposits; at the Cordilleran foothills, in Machalí, volcanic ash deposits (Pudahuel Ignimbrite) crop out. A total of 50 microtremor measurements at Rancagua-Machalí and 22 at Rengo were used to compute the predominant frequency using horizontal to vertical spectral ratio (HVSR); spatial autocorrelation and F-K techniques were used to obtain Vs velocity profiles. Results show a good correlation with near-surface geology; we defined 3 different types of soil, type A, associated with the presence of volcanic ash deposits; type B a transitional zone, and type C that can be linked with fluvial deposits, mainly gravel and dense sand deposits. The Vs map of Rengo shows an interesting pattern which decreases from east to west and could be related to the presence of finer sediments to the west of the area, although no changes of near surface geology are observed. Available maps are key products for seismic risk assessment and urban planning.