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[SCG54-P02] Scaling relationship of low frequency earthquakes (LFEs) between duration and radiated seismic energy
★Invited Papers
Keywords:deep low-frequency earthquakes, scaling law
Ordinary earthquake is a self-similar phenomenon that is characterized by the seismic moment proportional to the cube of the duration (M0∝T3) (e.g., Aki 1967). On the other hand, scaling law of both tectonic and volcanic deep low-frequency earthquakes (LFEs) is under debate. Ide et al., (2007) proposed a model in which the moment is proportional to the duration (M0∝T) by compiling the source parameter of slow events observed in broad frequencies. Bostock et al., (2015) estimated the source parameter of LFEs and found that the seismic moment is independent of the duration (M0∝T10). As such, it has remained unclear whether LFE is a self-similar phenomenon same as ordinary earthquakes. Especially, the scaling law of volcanic LFEs has not been investigated yet. Therefore, we investigated the scaling relationship of LFEs between duration and radiated seismic energy related to the seismic moment.
Although half value width of envelope waveform is often used to estimate the duration of slow events such as tremor (e.g., Ide 2010), this method overestimates the duration because the duration of LFEs is shorter than that of tremor. Thus, we estimated the duration of LFEs by fitting the box-car function to the square of envelope waveforms of LFEs. In this process, we optimized the noise amplitude, the radiated energy, the amplitude of coda wave, the onset of envelope, and the duration. The advantage of this method is that we can determine not only the duration but also the energy including the background noise. For each station, we calculated the optimized value of five unknown parameters which minimize the L-1 norm misfit between squared envelope and box-car function by using the differential evolution method. We defined the event duration as the median value of obtained duration at all station. In addition, since the estimated energy is affected by propagation path, we estimated the relative site effects of each station including geometrical spreading and intrinsic attenuation. We regarded the event energy as the median of the radiated energy corrected for the obtained site effects.
We performed above analysis for volcanic LFEs occurring at Zao volcano and tectonic LFEs at western Ehime. We obtained the scaling relationship characterized by the radiated energy proportional to 1.5 power of the duration (E∝T1.5) for both type of LFEs. Assuming that scaled energy is constant, this result indicates that LFE has a different scaling from ordinary earthquakes.
Although half value width of envelope waveform is often used to estimate the duration of slow events such as tremor (e.g., Ide 2010), this method overestimates the duration because the duration of LFEs is shorter than that of tremor. Thus, we estimated the duration of LFEs by fitting the box-car function to the square of envelope waveforms of LFEs. In this process, we optimized the noise amplitude, the radiated energy, the amplitude of coda wave, the onset of envelope, and the duration. The advantage of this method is that we can determine not only the duration but also the energy including the background noise. For each station, we calculated the optimized value of five unknown parameters which minimize the L-1 norm misfit between squared envelope and box-car function by using the differential evolution method. We defined the event duration as the median value of obtained duration at all station. In addition, since the estimated energy is affected by propagation path, we estimated the relative site effects of each station including geometrical spreading and intrinsic attenuation. We regarded the event energy as the median of the radiated energy corrected for the obtained site effects.
We performed above analysis for volcanic LFEs occurring at Zao volcano and tectonic LFEs at western Ehime. We obtained the scaling relationship characterized by the radiated energy proportional to 1.5 power of the duration (E∝T1.5) for both type of LFEs. Assuming that scaled energy is constant, this result indicates that LFE has a different scaling from ordinary earthquakes.