We present a new approach for broadband ground‐motion simulation that is a combination of high‐frequency (HF, f>0.5 Hz) and low‐frequency (LF, f<0.5 Hz) ground‐motion simulations. In this method, the LF ground motion is first computed by a deterministic approach based on a velocity model and a source model. Then, the HF acceleration envelopes are computed by multiplying the LF envelopes by envelope ratio functions (ERFs), which are defined as the empirical relationships between the characteristics of envelopes of different frequency bands. We apply the method to the 2003 Tokachi‐Oki, Japan, earthquake (Mw 8.3). The LF ground motion is computed by a finite‐difference method with a 3D velocity model and a finite‐fault slip model. Because of the complicated rupture process of the earthquake, the fault is divided into subfaults that can be considered as smaller earthquakes. ERFs are estimated by investigating the ground‐motion data of moderate‐size aftershocks (5.4≤Mw≤6.6) for each station. The HF ground motions are simulated using the LF ground motion and the ERFs. Seismograms of all frequency bands are summed to give a time series of broadband (0.05–16 Hz) ground motion. We find that our method yields realistic broadband ground motion, in terms of the acceleration envelope, velocity waveform, and Fourier amplitude spectrum. Comparing the HF waveforms with those computed by the stochastic Green’s function method (SGFM), we find that the performance of our method is better than that of the SGFM under the condition that LF waveforms are appropriately computed. The proposed method has the potential to be used as a practical method for broadband ground‐motion prediction incorporating the effects of a 3D velocity model and a finite‐source model.
Online Material: Figures comparing simulated and observed low‐frequency waveforms and root mean square acceleration envelopes in different frequency ranges.