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

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.