Straightforward methods to detect non-linear response of the soil. Application to the recordings of the Kobe earthquake (Japan, 1995)

摘要

We propose the use of simple straightforward methods to detect the non-linear responseof the soil, using recordings of the main shock and aftershocks of the 1995, Hyogo-kenNanbu (Kobe) earthquake. We investigate the non-linear effects using two different techniques, ona collection of data, for 12 sites, situated on different geological structures, in the Kobe and Osakaareas. The first method used is the so-called receiver functions technique (Langston, 1979), whichconsists in competing the spectral ratio between horizontal and vertical components of motion. Thisratio has been shown to reveal the fundamental frequency of a site (Lachet and Bard, 1994; Lachetet al., 1996; Theodulidis et al., 1995, 1996). For each site, we consider the main shock and a set ofaftershocks recordings. We then show the variation of this ratio for different values of the maximumacceleration recorded at a site, for the main-shock and different size aftershocks. We lookboth at variations of the amplitude of the H/V ratio (due to non-linear behavior, on the horizontalcomponents in particular) and variations of the frequency position of the amplified band-width. Thesecond technique is related with the variation of the high frequency content of the recordings duringthe main-shock and its aftershocks. The high frequency spectral decay of the motion, characterizedby k parameter, is assumed to be related mainly with the near-surface attenuation. It shouldthen increase with increasing peak velocity, in case of non-linearity. We calculate the value ofkappa, for the 12 sites considered in the Kobe area, for different types of soil conditions, and againdifferent values of peak ground acceleration. We then show the variations of kappa, related to nonlinearbehavior of the soil during the Kobe earthquake.