Doctor Thesis, Kyoto University, 2003 

Frequency-Dependent Source Heterogeneities for Broadband Ground Motion Simulation

Hiroe Miyake

(Kyoto University)


Contents

  • Chapter 1. Intoroduction


  • Chapter 2. Source Characterization for Broadband Ground Motion Simulation:
                  Kinematic Heterogeneous Source Model and Strong Motion Generation Area


  • Chapter 3. Frequency-Dependent Source Processes for the 1989 Loma Prieta Earthquake using Complex Spectral Inversion


  • Chapter 4. Slip Velocity Time Function for Simulating Broadband Ground Motions:
                  Application to the 1995 Hyogo-ken Nanbu (Kobe) Earthquake


  • Chapter 5 Conclusions



  • Abstract

    Source processes inverted from low- and high-frequency waveforms are generally different each other. The low- (< 1 Hz) and high-frequency (> 2 Hz) wave radiations contain significant information for interpretation of earthquake source physics and it is important to try to clarify the frequency-dependence of the generation of seismic waves from the source over as wide a frequency range as possible. Three different techniques are applied to the broadband ground motion analyses of recent earthquakes. This thesis addresses the general scaling of strong motion generation area, estimates the frequency-dependent heterogeneous source processes, and proposes appropriate slip velocity time functions as a key for controlling the rupture dynamics.

    First demonstration is that near-source ground motions are controlled by not total rupture area but strong motion generation areas with large slip velocities. The strong motion generation area obtained from broadband ground motions coincides with the area of asperities characterized from heterogeneous slip distributions estimated by low-frequency waveform inversions. The size and rise time of the strong motion generation area show the self-similar scalings with respect to seismic moment. Characterized source model for the broadband ground motion simulation is proposed. This model consists of strong motion generation areas with large slip velocities and a background slip area with a relatively small slip velocity. Applicability of the proposed model is confirmed by ground motion simulation for the 1997 Kagoshima-ken Hokuseibu earthquake.

    The frequency-dependent heterogeneous source processes for the 1989 Loma Prieta earthquake are investigated by complex spectral inversions. An approach of fitting the complex source spectra has been developed with frequency-dependent phase weighting that models both the coherent and stochastic summation of waveforms using empirical Green's functions. Large low-frequency wave radiations are made from two asperities seen in previous time-domain waveform inversions. High-frequency wave radiations are mainly made at the breaking points of these asperities. The distributions of maximum slip-velocity intensity indicate a progression of wave generation from the center to edge of each asperity with increasing frequency.

    Appropriate slip velocity time functions for simulating broadband ground motions are examined for the 1995 Hyogo-ken Nanbu (Kobe) earthquake. The slip velocity time function is expressed as the combination of the function obtained by low-frequency waveform inversion and impulsive function whose amplitude is proportional to the high-frequency amplitude spectral level. The velocity waveform inversion in the frequency range of 1 to 3 Hz indicates that maximum slip velocities required for broadband ground motions are 2-3 times larger and that effective pulse widths of peak velocities on the asperities should be shorter than those for low-frequency ground motions. Strong ground motion generation is featured by both spiky slip velocity generating high-frequencies at the breaking point of asperities and Kostrov-like slip velocity corresponding to the size of asperities where coherent summation of the seismic waves are made effectively around 1 Hz.

    The above analyses suggest that heterogeneous source processes are not expressed by randomness but controlled by systematic wave radiations corresponding the rupture dynamics of asperity.




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