10:45 AM - 12:15 PM
[SSS10-P08] Source spectra and seismic radiated energy of regular and LFEs in and around the focal area of the 2008 Iwate-Miyagi inland earthquake
Keywords:source spectra, sacled energy, low-frequency earthquake
Deep low-frequency earthquakes (Deep LFEs) mainly occur beneath volcanoes around the Moho. Compared with regular earthquakes with similar magnitudes, deep LFEs have seismic waves with lower dominant frequency and with characteristic long-trailing parts. The details of the seismic wave radiation process are not as well understood as those of regular earthquakes. It is important to quantify the seismic radiated energy of regular earthquake and deep LFEs. In and around the focal area of the 2008 Iwate-Miyagi inland earthquake, many regular and deep LFEs have been observed. In this area, Nakajima and Hasegawa (2021) reported that shallow low-frequency earthquakes (shallow LFEs) also occur in the upper crust. We here estimate source spectra of them and systematically investigate their scaled energy.
We determined source spectra from observed waveforms. First, we estimated site characteristic and attenuation factor Q-1 based on the coda normalization method (Aki and Chouet, 1975; Aki, 1982). Second, we obtained source spectra by correcting site characteristic and attenuation factor Q-1 from the observed seismic waveforms (Takahashi et al., 2005; Yoshida et al., 2017). Finally, we calculated scaled energy using the estimated source spectra.
We estimated source spectra and scaled energy eR for 1464 regular earthquakes, 169 deep LFEs and 52 shallow LFEs. The scaled energy for regular earthquakes is about 10-5 (the mean and standard deviation of log10eR are -4.3 and 0.39 respectively.), which is consistent with the previous studies (e.g., Kanamori and Brodsky, 2004). On the other hand, for deep LFEs, eR is about 10-8 to 10-7 (the mean and standard deviation of log10eR are -7.0 and 0.43 respectively.) and for shallow LFEs, eR is about 10-6 (the mean and standard deviation of log10eR are -5.4 and 0.5 respectively.), which indicated that eR for LFEs is one to three orders of magnitude systematically lower than for regular earthquakes.
We assumed that attenuation factor Q-1 is spatially homogeneous in and around the focal area of the 2008 Iwate-Miyagi inland earthquake. However, since deep LFEs occur at greater depth than regular earthquakes, the representative Q-1 value for deep LFEs may be different from that for regular earthquakes, which may lead to systematical misestimation of source spectra and scaled energy . To evaluate the effect of depth variation of Q-1 on eR , we assumed a two-layer structure of Q-1 bounded at 15 km depth. We compared eR based on the two-layer of Q-1 to eR based on the spatially homogeneous Q-1 structure and found that the difference in log10eR is only by about 0.1, with no significant difference. Thus, we conclude that the difference in eR between regular earthquakes and LFEs is not due to the assumption of spatial homogeneous Q-1
We estimated moment magnitude Mw and energy magnitude ME that is based on radiated energy and compared them to JMA (Japan Meteorological Agency) magnitude Mjma . The discrepancy between Mw and Mjma for LFEs is significantly larger than that for regular earthquakes. On the other hand, ME and Mjma are in general agreement for both regular and LFEs. These results may indicate that Mjma reflect radiated energy rather than seismic moment and that the high-frequency energy excitation of LFEs is smaller than that of regular earthquakes with similar seismic moment. These results may suggest that LFEs are associated with lower stress drops and with slower rupture velocity and deformation than regular earthquakes.
We determined source spectra from observed waveforms. First, we estimated site characteristic and attenuation factor Q-1 based on the coda normalization method (Aki and Chouet, 1975; Aki, 1982). Second, we obtained source spectra by correcting site characteristic and attenuation factor Q-1 from the observed seismic waveforms (Takahashi et al., 2005; Yoshida et al., 2017). Finally, we calculated scaled energy using the estimated source spectra.
We estimated source spectra and scaled energy eR for 1464 regular earthquakes, 169 deep LFEs and 52 shallow LFEs. The scaled energy for regular earthquakes is about 10-5 (the mean and standard deviation of log10eR are -4.3 and 0.39 respectively.), which is consistent with the previous studies (e.g., Kanamori and Brodsky, 2004). On the other hand, for deep LFEs, eR is about 10-8 to 10-7 (the mean and standard deviation of log10eR are -7.0 and 0.43 respectively.) and for shallow LFEs, eR is about 10-6 (the mean and standard deviation of log10eR are -5.4 and 0.5 respectively.), which indicated that eR for LFEs is one to three orders of magnitude systematically lower than for regular earthquakes.
We assumed that attenuation factor Q-1 is spatially homogeneous in and around the focal area of the 2008 Iwate-Miyagi inland earthquake. However, since deep LFEs occur at greater depth than regular earthquakes, the representative Q-1 value for deep LFEs may be different from that for regular earthquakes, which may lead to systematical misestimation of source spectra and scaled energy . To evaluate the effect of depth variation of Q-1 on eR , we assumed a two-layer structure of Q-1 bounded at 15 km depth. We compared eR based on the two-layer of Q-1 to eR based on the spatially homogeneous Q-1 structure and found that the difference in log10eR is only by about 0.1, with no significant difference. Thus, we conclude that the difference in eR between regular earthquakes and LFEs is not due to the assumption of spatial homogeneous Q-1
We estimated moment magnitude Mw and energy magnitude ME that is based on radiated energy and compared them to JMA (Japan Meteorological Agency) magnitude Mjma . The discrepancy between Mw and Mjma for LFEs is significantly larger than that for regular earthquakes. On the other hand, ME and Mjma are in general agreement for both regular and LFEs. These results may indicate that Mjma reflect radiated energy rather than seismic moment and that the high-frequency energy excitation of LFEs is smaller than that of regular earthquakes with similar seismic moment. These results may suggest that LFEs are associated with lower stress drops and with slower rupture velocity and deformation than regular earthquakes.