Analytical and semi-empirical synthesis of near-field seismic waveforms for investigating the rupture mechanism of major earthquakes.

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Abstract
For predicting strong ground motions from major earthquakes and for close investigations into complex rupture processes, two different analytical and semi-empirical approaches are used to synthesize seismic waves from a nearby fault with a large linear dimension. The former technique is to calculate Green's functions for a horizontally layered structure by the discrete wave-number/finite element method, and the latter is to use the records of minor shocks as empirical Green's functions by convolving a correction function for the differences in the source functions and the receiver responses between the main and smaller events. In both cases, the phase-delayed Green's functions are integrated over the entire fault surface. The above methods have been applied to the case of the 1969 central Gifu earthquake (M=6.6) which was followed by moderate aftershocks (M=4.3-4.8) and a number of smaller events. It was found that the waveforms synthesized from the two approaches agree reasonably well with each other. The strong-motion records, particularly of body waves and the major portion of surface waves with periods longer than 5-7 s, can be satisfactorily modeled by the theoretical synthesis with a realistic structure and also by the semi-empirical analysis using four aftershock records, if reasonable rupture velocities and rise times are assumed. However, the shorter-period waves with periods 1-2 s involved in the records cannot be simulated by either of these syntheses, unless incoherent rupture propagation over the fault is included. A stochastic fault model with variable rupture velocities over large-scale fault segments is tentatively presented to account for the short-period waves

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