2004年新潟県中越地震 (<i>M</i><sub>w</sub>6.6): スペクトル比法を用いた余震系列の解析

Abstract

The Mid Niigata Prefecture Earthquake (October 23, 2004, 17:56 (JST), Mw 6.6) that was preceded by a couple of foreshocks, produced a number of aftershocks including six events with a moment magnitude (Mw) of 5.5 or greater. We evaluated the relationship between seismic moment (M0) and corner frequency (fc), and identified spectral characteristics in the aftershock area. A spectral ratio method was employed to determine fc of 57 smaller events (3.5≤Mw≤4.0). These events with similar focal mechanisms are located within 20km from the mainshock epicenter, and were recorded at four common stations. In the spectral ratio method, the mainshock spectrum is divided by the spectrum of a foreshock or an aftershock. By taking the averages of the spectral ratios calculated at four common stations, propagation path effects can be reduced. The obtained fc-M0 relation shows substantial scatter relative to a straight line of M0∝fc-3. The scatter in the narrow range of Mw suggests the variation of radiated seismic energy ER of small earthquakes, and deviation from an ω-2 model based on the assumption of constant rupture velocity. The moment-rate spectra become more complex in the vicinity of fc than those expected from a typical ω-2 model. We also classified the aftershock area into five zones based on the estimated faults by a previous study, and investigated spectral ratio characteristics in the frequency band from 1 to 7Hz. In particular, the slopes of spectral ratios vary in the high frequency band, which may reflect the difference in relative quantity of the radiated energy among the zones. Our results suggest clues that could be useful in evaluating the seismic energy release in a complex manner during the aftershock sequence.