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

Lacave-Lachet, C; Bard, PY; Gariel, JC; Irikura, K. JOURNAL OF SEISMOLOGY, 4: (2) 161-173.

Simple straightforward methods are applied to test their ability to detect the non-linear response of the soil. Recordings of the main shock and aftershocks of the 1995, Hyogo-ken Nanbu (Kobe) earthquake are used. Non-linear effects are investigated using two different techniques, on a collection of data for 12 sites situated on different geological structures in the Kobe and Osaka areas. The first method used is the so-called receiver functions technique (Langston, 1979), which consists of computing the spectral ratio between horizontal and vertical components of motion. This ratio has been shown to reveal the fundamental frequency of a site (Lachet and Bard, 1994; Lachet et al., 1996; Theodulidis et al., 1995, 1996). For each site, recordings of the main shock and a set of aftershocks are considered. The variation of this spectral ratio for different values of the maximum acceleration recorded at a site is investigated. Both variations of the amplitude of the H/V ratio (due to non-linear behavior, on the horizontal components in particular) and of the frequency position of the amplified band-width are observed. The second technique used in this study is related to the variation of the high frequency content of the recordings during the main-shock and its aftershocks. The high frequency spectral decay of the motion, characterized by kappa parameter, is assumed to be related mainly to the near-surface attenuation. It should then increase with increasing peak velocity, in case of non-linearity. The value of kappa is calculated for the 12 sites in the Kobe area, for different types of soil conditions, and again different values of peak ground acceleration. Variations of kappa are then related to non-linear behavior of the soil during the Kobe earthquake.