Abstract

In 1960, the city of Valdivia was affected by the strongest earthquake ever recorded in human history (Mw = 9.5). Large surface motion was amplified by the poor quality of the soil, which also have a very complex spatial distribution. Considering the disastrous consequences of these events, the need of evaluating the local seismic hazard in areas of very high seismicity and complex geology are absolutely necessary in order to improve not only the seismological knowledge but also the implementation of local regulations based on a local definition of the seismic demand. In order to quantify the effects of the soil on the seismic response, a microzonation is carried out in Valdivia, Chile, which allowed obtaining dynamic characteristics of soils. The spatial distribution of the predominant periods of vibration of the ground and a quantification of the site‐effects by means of geotechnical and geophysical field surveys is exposed, where 260 exploration points were considered. For the evaluation of the predominant periods and site‐effects, ambient microtremors were used applying the spectral H / V Nakamura’s method. Based on geoprocessing GIS techniques, soil isoperiods and site‐effects were calculated and represented by thematic maps. Subsequently, the dynamic parameters obtained from soil microzonation were compared and calibrated with the available geotechnical and geological data. It is noteworthy that the comparative analysis of the site‐effects is absolutely consistent with the geological data and seismic effects observed in past events in Valdivia, so we can conclude that the estimation of local amplifications obtained from the geophysical surveys is a good indicator of site‐effects. Moreover, the effects of the complexity of the local geology are even more evident when checking variable bedrock depths in different parts of the city.