Assessment of Kappa Values in the Chilean Subduction Zone for Interface and In-Slab Events

Abstract

Abstract One way to study the physical process that occurs behind earthquakes and how they affect different sites depending on the source type and the geological structure of the site is the shape of the Fourier spectrum. A parameter related to the shape of the spectrum is the spectral decay factor—kappa (κ), which characterizes how the amplitude of the Fourier acceleration spectrum decays at high frequencies. The parameter κ can be important in the characterization and estimation of the surface seismic demand, being useful in, for example, the adjustment of ground-motion prediction equations. We calculate the values of κ and its site component κ​0 from 36 seismic stations of the National Seismological Network of Chile to determine the dependence that this parameter has to the site conditions as well as to the properties of the path in which greater values of κ are observed for subduction earthquakes that occur in the interface between the Nazca and the South American plates, compared with the values obtained from events occurring inside the subducting plate, known as in-slab earthquakes. We find that κ​0, calculated using the hypocentral distance correlates more closely with the site fundamental frequency f​0, rather than the commonly used V​S30 (time-averaged shear-wave velocity in the top 30 m). Our kappa value results are field estimates of near-surface attenuation, which can be used to calculate the minimum site-specific damping or crustal attenuation in seismological models that have a strong impact on seismic site characterization, particularly, in subduction settings.