11:45 AM - 12:00 PM
[SSS11-11] Seismic radiated energy and source spectra of the regular and deep low-frequency earthquakes in and around the source area of the 2008 Iwate-Miyagi inland Earthquake: Their similarities and differences
Keywords:source spectrum, low-frequency earthquake
Deep low-frequency earthquakes (Deep LFEs) are unusual earthquakes that occur deeper than regular earthquakes, mainly around volcanoes. Compared to regular earthquakes of similar magnitude, deep LFEs have a small dominant frequency of the seismic wave and a characteristic trailing part. The occurrence mechanism of deep LFEs is not as well understood as those of regular earthquakes, which are known to be fault motions. In order to understand the generation mechanism of deep LFEs, it is important to understand the radiation process of seismic waves.
Source spectra of many regular earthquakes appear to follow the omega-square model (Aki, 1967; Brune, 1970). However, LFEs occurring at plate boundaries are estimated not to follow the omega-square model (Ide et al., 2007). Whether deep LFEs occurring in crust obey the omega-square model is important for understanding the physics of their occurrence and radiation processes.
In this study, we systematically investigated the characteristics of source spectra and radiated energy of regular earthquakes and deep LFEs occurred around the source area of the 2008 Iwate-Miyagi inland Earthquake. First, we investigatde the characteristics of the frequency dependence of the source spectra and then estimated radiated energy and scaled energy. In this region, shallow LFEs were estimated to occur close to regular earthquakes (Nakajima and Hasegawa, 2021).
We determined source spectra of regular earthquakes and shallow and deep LFEs using spectra of observed seismic waves. We first estimate the seismic attenuation Q-1 and site amplification by the coda normalization method (Aki, 1982; Aki and Chouet, 1975) as following Takahashi et al. (2005) and Yoshida et al., (2017). This method can evaluate relative values of the site amplifications among stations but cannot evaluate the absolute values. To estimate the absolute value of the site amplification factor, we used earthquakes that are independently validated to follow the omega-square model by an empirical method. By comparing the theoretical source spectra with the observed ones, we estimated the absolute values of site-amplifications (Yoshida et al.,2017).
We obtained the source spectra of 3371 regular earthquakes, 642 deep LFEs, and 76 shallow LFEs. From each source spectra, we estimated the corner frequency and exponent of high-frequency decay (n) by fitting the theoretical spectra using grid-search. We estimated seismic moment as low frequency levels of the source spectrum.
We did not find any significant difference in n between regular earthquakes and LFEs. For regular earthquakes, the mean value is 1.6, with a standard deviation of 0.47. For deep LFEs, the mean is 1.6, with a standard deviation of 0.27. For shallow LFEs, the mean is 1.6, with a standard deviation of 0.43. Many earthquakes in the study area showed a gradual decay of the high-frequency amplitudes compared to the omega-square model, which may reflect that some earthquakes have relatively complex rupture processes.
We fixed n at mean value 1.6 and estimated the radiated energy and scaled energy of each earthquake by using the method of Snoke (1967). Scaled energy was about 10-5 for regular earthquakes, consistent with previous studies. In the case of deep LFEs, however, scaled energy was about 10-7, and in the case of shallow LFEs detected by Nakajima and Hasegawa (2021), scaled energy was about 10-6. In summary, the results of this study show that the frequency decay rate of the source spectrum amplitude does not change between regular earthquakes and LFEs, but the scaled energy is significantly different.
In this study, we assumed spatially constant seismic attenuation Q-1 in the study area. It is important to evaluate n, radiated energy and scaled energy more precisely by considering the spatial variation of Q-1.
Source spectra of many regular earthquakes appear to follow the omega-square model (Aki, 1967; Brune, 1970). However, LFEs occurring at plate boundaries are estimated not to follow the omega-square model (Ide et al., 2007). Whether deep LFEs occurring in crust obey the omega-square model is important for understanding the physics of their occurrence and radiation processes.
In this study, we systematically investigated the characteristics of source spectra and radiated energy of regular earthquakes and deep LFEs occurred around the source area of the 2008 Iwate-Miyagi inland Earthquake. First, we investigatde the characteristics of the frequency dependence of the source spectra and then estimated radiated energy and scaled energy. In this region, shallow LFEs were estimated to occur close to regular earthquakes (Nakajima and Hasegawa, 2021).
We determined source spectra of regular earthquakes and shallow and deep LFEs using spectra of observed seismic waves. We first estimate the seismic attenuation Q-1 and site amplification by the coda normalization method (Aki, 1982; Aki and Chouet, 1975) as following Takahashi et al. (2005) and Yoshida et al., (2017). This method can evaluate relative values of the site amplifications among stations but cannot evaluate the absolute values. To estimate the absolute value of the site amplification factor, we used earthquakes that are independently validated to follow the omega-square model by an empirical method. By comparing the theoretical source spectra with the observed ones, we estimated the absolute values of site-amplifications (Yoshida et al.,2017).
We obtained the source spectra of 3371 regular earthquakes, 642 deep LFEs, and 76 shallow LFEs. From each source spectra, we estimated the corner frequency and exponent of high-frequency decay (n) by fitting the theoretical spectra using grid-search. We estimated seismic moment as low frequency levels of the source spectrum.
We did not find any significant difference in n between regular earthquakes and LFEs. For regular earthquakes, the mean value is 1.6, with a standard deviation of 0.47. For deep LFEs, the mean is 1.6, with a standard deviation of 0.27. For shallow LFEs, the mean is 1.6, with a standard deviation of 0.43. Many earthquakes in the study area showed a gradual decay of the high-frequency amplitudes compared to the omega-square model, which may reflect that some earthquakes have relatively complex rupture processes.
We fixed n at mean value 1.6 and estimated the radiated energy and scaled energy of each earthquake by using the method of Snoke (1967). Scaled energy was about 10-5 for regular earthquakes, consistent with previous studies. In the case of deep LFEs, however, scaled energy was about 10-7, and in the case of shallow LFEs detected by Nakajima and Hasegawa (2021), scaled energy was about 10-6. In summary, the results of this study show that the frequency decay rate of the source spectrum amplitude does not change between regular earthquakes and LFEs, but the scaled energy is significantly different.
In this study, we assumed spatially constant seismic attenuation Q-1 in the study area. It is important to evaluate n, radiated energy and scaled energy more precisely by considering the spatial variation of Q-1.