Systematic Differences in Energy Radiation Processes Between Regular and Intraplate Low-Frequency Earthquakes Around the Focal Area of the 2008 M w ${\boldsymbol{M}}_{\boldsymbol{w}}$ 6.9 Iwate-Miyagi, Japan, Earthquake

Abstract

Many unknowns exist regarding the energy radiation processes of intraplate low-frequency earthquakes (LFEs), which are frequently observed beneath volcanoes. To evaluate their energy radiation characteristics, we estimated scaled energy ($e_R$) for LFEs and regular earthquakes around the focal area of the 2008 $M_w$ 6.9 Iwate-Miyagi, Japan, earthquake. Their source spectra were first obtained by correcting for the site and path effects from direct S-waves. We then estimated the radiated energy and seismic moment and obtained the $e_R$ for the 1,421 regular earthquakes, 62 deep LFEs, and 46 shallow LFEs. The $e_R$ for the regular earthquakes is in the order of ${10}^{-5}$ to ${10}^{-4}$, typical for crustal earthquakes and tends to be smaller near volcanoes and the shallow LFEs. In contrast, the $e_R$ is in the order of ${10}^{-7}$ and ${10}^{-6}$ for the deep and shallow LFEs, respectively, one to three orders of magnitude smaller than that for the regular earthquakes. The systematically lower $e_R$ suggests that LFEs are associated with a much lower stress drop and (or) slower deformation than regular earthquakes. While the energy magnitudes derived from radiated energy generally show good agreement with the local magnitudes for the three types of earthquakes, the moment and local magnitudes show a large discrepancy for the LFEs. This suggests that the local magnitude based only on the maximum amplitude of the observed seismic records may not provide good information on the static sizes of LFEs whose $e_R$ is substantially different from that for regular earthquakes.