17:15 〜 18:30
[SCG39-P04] Attenuation effect on observed frequency characteristics of broadband slow earthquakes
キーワード：スロー地震、広帯域スロー地震、低周波地震、微動、超低周波地震
Slow earthquakes have been classified according to the observation frequency, or period, bands: such as low frequency earthquake (LFE) and tremor in 1-10 Hz, very low frequency earthquake (VLFE) in 20 to 100 s, and much longer slow slip event (SSE). This classification was popular because it is difficult to detect signals of slow earthquakes in 0.1-1 Hz due to large microseism noise. However, recent studies confirmed continuous signals from LFE and tremor to VLFE frequency bands, i.e., 0.01-10 Hz, by near-field observation at the shallow part of the subduction zone during low microseism activity (Kaneko et al., 2018), and by stacking records of many LFEs at the deep part of the subduction zone (Masuda et al., 2020). Therefore, it is time to redefine slow earthquakes as phenomena in a continuous broad frequency band, and investigate the characteristic of broadband slow earthquakes.
An important quantity characterizing a broadband slow earthquake is relative magnitude of seismic radiation among different frequency bands, which we investigate by using the maximum amplitude of the stacked waveforms aligned relative to the timing of LFEs. First, we detected LFEs using a two-step matched-filter analysis in three regions, Shikoku, Kii, and Tokai. The data used for this analysis is the velocity seismograms of Hi-net. The first step adopts templates from the observed waveforms of the selected LFE events in the JMA catalog referring the result of Kato & Nakagawa (2020). The stacked waveforms of detected events in the first step is used as templates for the second step. Then, we aligned broadband seismograms, which is the data of high sensitivity accelerometers of Hi-net, relative to the timing of the detected LFEs, stacked, integrated them to convert to velocity data, and normalized by the maximum amplitude. The maximum amplitude of stacked waves bandpass filtered in low frequency bands generally increases as distance from the hypocenter. This is due to the attenuation of seismic waves, i.e., low frequency waves are less attenuated than high frequency waves, which control the total amplitude in broad frequency band.
An important quantity characterizing a broadband slow earthquake is relative magnitude of seismic radiation among different frequency bands, which we investigate by using the maximum amplitude of the stacked waveforms aligned relative to the timing of LFEs. First, we detected LFEs using a two-step matched-filter analysis in three regions, Shikoku, Kii, and Tokai. The data used for this analysis is the velocity seismograms of Hi-net. The first step adopts templates from the observed waveforms of the selected LFE events in the JMA catalog referring the result of Kato & Nakagawa (2020). The stacked waveforms of detected events in the first step is used as templates for the second step. Then, we aligned broadband seismograms, which is the data of high sensitivity accelerometers of Hi-net, relative to the timing of the detected LFEs, stacked, integrated them to convert to velocity data, and normalized by the maximum amplitude. The maximum amplitude of stacked waves bandpass filtered in low frequency bands generally increases as distance from the hypocenter. This is due to the attenuation of seismic waves, i.e., low frequency waves are less attenuated than high frequency waves, which control the total amplitude in broad frequency band.