Simulation of High-Frequency Ground Motion Based on the Relationships Between Low- and High-Frequency Acceleration Envelopes

Abstract:

We present a new method to simulate high-frequency (HF) (> ~1 Hz) ground motion using the relationships between the characteristics of acceleration envelopes of high- and low-frequency (LF) ground motions. It presupposes that LF ground motion has been already obtained by other deterministic method, and takes advantage of the LF ground motion to simulate HF ground motion. The relationship of acceleration envelopes among different frequency bands are investigated using the observed ground motion data of M5-6 earthquakes that occurred near the target earthquake for which we aim to synthesize the ground motion, by simply taking their ratios in time domain. We show that the envelope ratios can be expressed by three parameters that are site-specific and independent of magnitudes of the earthquakes, and that the envelope ratios enable us to synthesize HF envelope, once LF envelope is available up to near 1 Hz. To confirm the method, we synthesize HF (> 1 Hz) ground motion for two M7 target earthquakes that occurred near Kanto area, Japan, using observed LF (< 1 Hz) ground motion. The synthesized HF ground motion well reproduces the observed ground motion of the target earthquakes, in terms of the S-wave amplitudes and the envelope shapes. Furthermore, we extended the method for M8 class earthquakes, and apply it to a broadband ground motion simulation for the 2003 Tokachi-oki, Japan, earthquake (M_W8.0), using LF ground motion computed by a finite-difference method based on the source and velocity structure models of Aoi et al. (2008).

The method has the potential to be used as a practical method for broadband ground motion prediction that is consistent with observation and computationally costless, on the condition that LF ground motion is computed by a deterministic approach based on a realistic underground velocity structure model and appropriate source models.