Earth Planets Space, Vol. 60 (No. 2), pp. 145-150, 2008LETTER
Shinshu University, 4-17-1 Wakasato, Nagano, Japan
(Received June 20, 2007; Revised August 17, 2007; Accepted August 28, 2007; Online published February 19, 2008)
The Noto Hanto earthquake of March 25, 2007, is a shallow reverse-faulting event with Mw =6.7. We analyze transverse component accelerograms of the main shock and an aftershock (Mw =3.6) recorded at 10 strong motion observation stations. The hypocenter of the aftershock is located very close to that of the main shock and the epicentral distances of the stations are between 15 and 65 km. We calculate spectral ratio between records of the main shock and the aftershock in order to obtain source spectral ratio by empirically removing the effects of wave propagation path and surface layers at the stations. Thus obtained source spectral ratio is inconsistent with that expected for the self-similar omega-square source model assuming constant stress drop. Fitting omega-square source spectral ratio with variable corner frequency to the observed spectral ratio, the radiated energy and the corner frequency for the main shock and the aftershock are estimated. The radiated energy is 6.8 × 1014 J for the main shock and 6.7 × 109 J for the aftershock. The energy-to-moment ratios for the main shock and the aftershock are 5.0 × 10-5 and 2.1 × 10-5. This scale-dependence of the energy-to-moment ratio is similar to that obtained previously for shallow strike-slip earthquakes and reverse-faulting earthquakes in Japan. The relationship between the seismic moment, M0, and the corner frequency, f0, is M0 ∝ f0-3.3± 0.2.
Key words: 2007 Noto Hanto earthquake, radiated energy, corner frequency, similarity of earthquake source.