Investigation of the high frequency attenuation parameter kappa (κ) beneath the volcanic region of Kyushu using surface and borehole dataset

Abstract

This study focuses on a detailed analysis of the high-frequency attenuation parameter, kappa (κ), to better understand seismic wave propagation in the volcanic region of Kyushu. In this analysis, κ values are examined beneath the volcanic area of the Kyushu region using strong motion data from the 2016 Kumamoto earthquake. The Surface and borehole data are utilized to evaluate the effects of site conditions and regional attenuation characteristics, respectively. The site attenuation parameter (κ₀) ranges from 0.022s to 0.068s, as estimated from 21 surface stations. The κ₀ values correlate with average shear-wave velocity in the top 30 m ($V_{S30})$, showing a decrease as $V_{S30}$ increases. Additionally, using borehole data, the region-specific S-wave quality factor (Q_s) and κ₀ are estimated in this region, resulting in values of 846 ± 75 and 0.050 ± 0.002s, respectively. The relatively lower Q_s values and higher κ₀ values observed in this study may be due to the extensive volcanic activities in the Kyushu region. The findings closely match previous studies, highlighting significant attenuation in the volcanic region. The average κ values for borehole data are 0.043s–0.053s for horizontal components (κ_H) and 0.038s–0.051s for vertical components (κ_v). Surface data shows κ_H values from 0.061s to 0.070s and κ_V from 0.039s to 0.046s. A relative comparison shows κ_H and κ_v are roughly equal in borehole conditions, while surface conditions reveal κ_H exceeds κ_v due to site effects on horizontal components. The estimated κ values are crucial for future site-specific seismic hazard analysis in Kyushu's volcanic regions.