Abstract

Near field earthquakes are those that have their epicenter at less than 20 km with respect to the site of interest. These earthquakes have particular conditions that pose a high seismic threat and are not covered by the regional seismic hazard parameters established from regional studies. Some design codes require a specific assessment of this situation for structure design, but do not specify methods or references for this evaluation. This article presents a description of the characteristic conditions of near field seismic movement based on the analysis of real records. It shows how near field earthquakes have a coherent pattern of long-period, high-intensity movements that originate from the rupture displacement in the source. Because of the long-period movements they significantly affect the coseismic displacements which can become quite large. This effect is attenuated at distances larger than 20 km. This information is useful in characterizing the seismic threat in cases where there are shallow seismic sources near the site of interest.