Doctoral Dissertation 2008

Estimation of Broadband Source Process Based on Strong Motion Modeling

Wataru Suzuki

Revealing the source process during large earthquake from broadband strong motions develops a better understanding of the source physics. It is also important for advancing strong motion prediction methodology for the earthquake disaster estimation. This thesis discusses this broadband source process from the two source modeling approaches using strong motion records. Firstly, the rupture characteristics of the recurrence earthquakes are assessed from the viewpoint of the broadband strong motion generation. Secondly, the detailed broadband wave radiation process is estimated in a consecutive and unified way across frequency bands.

For the first analysis, the source model of the 2005 Miyagi-Oki earthquake is estimated from broadband strong motion simulation. This model consists of the two distinct source patches or strong motion generation areas (SMGA), whose location is estimated independently of the slip distribution from waveform inversion. The first SMGA near the hypocenter is carefully located using the phase arrival times in the P-wave portion. The location of the second deeper SMGA and the other source parameters, such as rise time and stress drop, are determined by fitting the synthetic broadband waveforms into the observed ones using the genetic algorithm. Estimated two SMGAs correspond to the large slip areas obtained from the low-frequency strong motion inversion, which implies that the broadband strong motions of the 2005 Miyagi-Oki earthquake are generated mainly from inside of the asperities. The location of the two SMGAs is found to be different from that of the previous 1978 Miyagi-Oki earthquake whereas the waveform inversions show that southern part of the rupture area of the 1978 event is re-ruptured during the 2005 event. It is observed that subduction-zone interplate earthquakes, including this event, have smaller SMGAs than inland crustal earthquakes of the same seismic moment, which is different from the empirical relationship derived for the asperities of interplate earthquakes. These results propose the possible broadband source model for subduction-zone interplate earthquakes, in which SMGAs are localized within the asperities that exist in the total rupture area.

For the second analysis, a new inversion method to obtain the low- and high-frequency wave radiation processes whose border is 1 Hz in a consecutive procedure is developed. The wavelet coefficients of velocity waveforms are used to evaluate the fitness depending on frequency-dependent characteristics of seismic waves. For the broadband waveform simulation, the hybrid Green's function is constructed from the theoretical and the empirical Green's functions for low- and high-frequency ranges, respectively. The low-frequency inversion is conducted using multi-scale approach that estimates the detailed source process by increasing the number of the model parameters in several steps. This new inversion method is applied to the 2000 Western Tottori earthquake. From the low-frequency inversion (0.0625-1 Hz), the areas of the large wave radiation intensity or asperities extend from the southeast of the hypocenter to the shallow part of the fault above the hypocenter. The wave of 1-2 Hz is intensely generated from the rupture starting area of the southeast asperity. On the other hand, the wave of 2-4 Hz most strongly radiates from the edge of the southeast asperity. These results indicate that the high-frequency waves seem to be generated in relation to the initiation and termination of the asperity rupture. There is also a possibility that the areas of the high-frequency wave radiation are localized within the asperities, representing the heterogeneity inside of the asperities.

The broadband strong motions are confirmed to be generated mainly from the large slip areas within the total rupture area while there is a possibility that the area of the high-frequency wave radiation is localized in the asperities observed from low-frequency inversion. The two analyses in this thesis can possibly illuminate the smaller scale heterogeneities inside of the asperities, which are responsible for broadband strong motion generation. These source processes involve the details of the initiation, evolution, and termination of the asperity rupture, which could provide a clue for developing the source physics further.