Abstract
The amplitudes of high-frequency seismic waves generated by local and/or regional earthquakes vary from site to site, even at similar hypocentral distances. It had been suggested that, in addition to local site effects (e.g., variable attenuation and amplification in surficial layers), complex wave propagation in inhomogeneous crustal media is responsible for this observation. To quantitatively investigate this effect, we performed observational, theoretical, and numerical studies on the characteristics of seismic amplitude fluctuations in inhomogeneous crust. Our observations of P-wave amplitude for small to moderately sized crustal earthquakes revealed that fluctuations in P-wave amplitude increase with increasing frequency and hypocentral distance, with large fluctuations showing up to ten-times difference between the largest and the smallest P-wave amplitudes. Based on our theoretical investigation, we developed an equation to evaluate the amplitude fluctuations of time-harmonic waves that radiated isotropically from a point source and propagated spherically in acoustic von Karman-type random media. Our equation predicted relationships between amplitude fluctuations and observational parameters (e.g., wave frequency and hypocentral distance). Our numerical investigation, which was based on the finite difference method, enabled us to investigate the characteristics of wave propagation in both acoustic and elastic random inhomogeneous media using a variety of source time functions. The numerical simulations indicate that amplitude fluctuation characteristics differ a little between medium types (i.e., acoustic or elastic) or source time function durations. These results confirm the applicability of our analytical equation to practical seismic data analysis.
Highlights
To predict the strong ground motion of future destructive earthquakes, empirical ground motion prediction equations have been developed and applied to many regions of the world (e.g., Si and Midorikawa 1999; Douglas 2003)
To better understand the amplitude fluctuations of highfrequency seismic waves, we carried out observational, theoretical, and numerical analyses of waves propagating in random inhomogeneous media
Our seismic observations of the P-wave amplitudes of small to moderately sized crustal earthquakes revealed that the fluctuation of P-wave amplitude increased with increasing hypocentral distance, with up to ten-times difference between the largest and the smallest P-wave amplitudes
Summary
To predict the strong ground motion of future destructive earthquakes, empirical ground motion prediction equations have been developed and applied to many regions of the world (e.g., Si and Midorikawa 1999; Douglas 2003). These empirical equations are useful for estimating the average characteristics of seismic amplitude variation with hypocentral distance; the observed ground motions (e.g., peak ground velocity) show large scatter around the predictions (e.g., Strasser et al 2009). In order to investigate the stochastic properties of the Few studies have tackled this issue (e.g., Nikolaev 1975), mainly because of the difficulties in acquiring high-quality observational data.
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