[S-SS11] Strong Ground Motion and Earthquake Disaster

[SSS11-23] New insight into the seismic response characteristics of offshore sites revealed by generalized inversion technique

*Lei Fu¹, Zhinan Xie², Su Chen³, Xiaojun Li³ (1.Institute of Geophysics, China Earthquake Administration, 2.Institute of Engineering Mechanics, China Earthquake Administration, 3.Key Laboratory of Urban Security and Disaster Engineering of the Ministry of Education, Beijing University of Technology, Beijing, China)

Keywords:Offshore ground-motion, Offshore site effects, Generalized inversion, Spectral ratio, Stochastic finite-fault simulation

Seismic response characteristics of offshore site are crucial for assessing seismic hazard in sea areas and for the seismic design of marine engineering structures. However, its understanding is limited due to the insufficient availability of in-situ borehole and offshore ground-motion data. In the past decades, the horizontal-to-vertical spectral ratio method (HVSR) has been widely used to investigate the seismic response characteristics of offshore sites, despite the notable differences between seabed environment and inland conditions, which could potentially result in misunderstandings. Thus, we applied the generalized inversion technique (GIT) to invert the site transfer functions (STFs) of six offshore strong motion stations located in the Sagami Bay, Japan. The results were validated by performing stochastic finite-fault simulations for two well-recorded subduction zone earthquakes, with magnitudes of 6.2 and 5.9, respectively. Comparison of STFs obtained using GIT and HVSR reveals that one of the fundamental assumptions of HVSR – Specifically, that the vertical STF fluctuating around 1 – is violated in five out of six offshore stations. This violation is attribute to the influence of local topography and seawater pressure, which results in misestimate of site fundamental frequencies (f_p) and amplification levels. The only exaptation is station KNG205, which is located in a flat plateau within a sea basin; thus, its site condition can be classified as a free-field offshore site. Consequently, the results strongly indicate that the HVSR should only be applied at free-field offshore site. Furthermore, we also compared the degree of nonlinearity index (DNL) for the six offshore sites. The results reveal two novel observations. Firstly, we observed that DNLs exceed 4 when PGA reaches 20–50 cm/s², which is notably smaller than the previously reported range of 50–100 cm/s². Secondly, as DNL increases, the f_p values of sites situated on flat terrain tend to shift to lower frequencies, whereas those on steep terrain tend to shift to high frequencies. These findings suggest that the STF of offshore sites is significantly impacted by their local topography.