Source process of the 2004 Mid-Niigata earthquake-Application of source inversion for broadband ground motions-
Heterogeneous rupture process on the fault plane during the 2004 Mid-Niigata earthquake is investigated by using waveform inversion technique based on the empirical Green’s function method and simulated annealing with near-field strong-motion records. The fault plane of the main shock is considered to be a west-dipping reverse fault, and its minute location is determined from the precisely relocated hypocenter distribution of the main and aftershocks. Inversion algorithm developed in the former study is improved by adding new constraints that force spatial smoothness for source parameter distributions in order to suppress the instability during the inversion procedure. Akaike’s Bayesian Information Criterion (ABIC) is introduced to determine suitable weight factors for the smoothing constraints objectively.Moment densities, effective stresses, rise times, and rupture times on 12*8 sub-faults of the main shock are searched by using very fast simulated annealing. Ten inversion procedures are performed with different initial values for random number generator, and eventually almost similar optimal solutions are obtained through all trials for both the displacement and velocity waveform inversions. Estimated source models display several distinct asperities of large released moment and high effective stress regions on the main-shock fault. Observed ground motion records and synthetics calculated from the optimal solution show good agreement in the frequency range from 0.2 to 5Hz, whose upper limit is much higher than that used in other conventional source inversion schemes. Comparison with near-field ground motions exhibits that two stations at Ojiya city, where observed peak ground accelerations exceeded 1,000 cm/s/s, are located near one of the high effective stress regions. Furthermore Kawaguchi town, where instrumental seismic intensity reached 7 and several stations observed peak ground velocities of more than 100 cm/s, is above the largest asperity on the main-shock fault. These results suggest that not only the seismic moment but also the effective stress controls the near-field strong ground motions, particularly in the high frequency range.
|the 2004 Mid-Niigata earthquake
|empirical Green's function