A damping scaling factor model for vertical elastic response spectra from subduction earthquakes in offshore and onshore regions of Japan

Abstract

This study aims to develop a damping scaling factor (DSF) model for scaling 5%-damped vertical elastic response spectra to other damping ratios for subduction earthquakes in both offshore (S-net) and onshore (K-NET and KiK-net) regions of Japan. To achieve this, more than 36,000 onshore records and 3400 offshore records from subduction earthquakes were used to develop the DSF model. The proposed DSF model is a function of moment magnitude, rupture distance, damping ratio, and station type, employing a linear function of ln(β) to differentiate between onshore, offshore buried, and offshore unburied ground motions. The coefficients of the DSF model were determined using Bayesian inference with Integrated Nested Laplace Approximation (INLA). The DSF model is applicable to subduction earthquake scenarios with moment magnitudes ranging from 5.5 to 9.1, rupture distances up to 1000 km, and damping ratios between 0.5 % and 30 %. The results show significant differences among onshore, offshore buried, and offshore unburied ground motions across most spectral periods at various damping ratios. An intersection point occurs near a spectral period of 1 s at the same damping ratios, where opposing variation trends are observed for magnitude and distance scaling before and after this point. Additionally, a standard deviation model for the proposed DSF model was developed using a cubic function of ln(β/5) for onshore, buried, and unburied ground motions. Compared to the observed standard deviation, the proposed model effectively captures the observed trends. The proposed DSF model can be effectively used to scale 5%-damped onshore and offshore GMMs to other GMMs with damping ratios ranging from 0.5 % to 30 % for subduction earthquakes. This is important for implementing probabilistic seismic hazard assessments, seismic design, and seismic zoning in both offshore and onshore regions of Japan.