Appropriate slip velocity time function of on-/off- asperity for broadband ground motion simulation

Source modeling including heterogeneous slip distribution is important for estimating near-source ground motions. The slip distributions for most of large earthquakes are obtained from the kinematic waveform inversions using the low frequency ground motions (<1Hz), and characterized by asperity areas with large slip and a background slip area with less slip. For simulating broadband ground motions, we propose an appropriate slip velocity time function which consists of the lower frequency part obtained by the waveform inversion and higher frequency part (>1Hz) constrained by the high frequency amplitude spectral level. The goal of this study is to examine whether the slip velocity time function are expressed as Kostrov's type proposed by the dynamic simulation, and how different are the slip velocity time function between on- and off- asperities. We target the 1995 Hyogo-ken Nanbu (Kobe) earthquake, where the slip velocity time function for the lower frequency range is obtained deterministically by the multiple time window waveform inversion with convolution technique (Sekiguchi et al., 2001). This method is performed using subfault waveforms including the rupture propagation effect inside each subfault. We made a convolution between the empirical Green's function (records of element earthquake) and the slip velocity time function added the higher frequency part, and validate this modification by broadband ground motion simulation. The slip velocity time function on each subfault for the target earthquake is explained as a convolution between that for an element earthquake and a filtering function to adjust difference in slip velocity time function between target and element earthquake. We adopted an impulse function as a higher frequency part of the filtering function, because scaling studies on the acceleration source amplitude spectra which have a flat level at high frequencies for any size of earthquake. The amplitude and position of the impulse function control the higher frequency characteristics of the slip velocity time function for the target earthquake in time domain. From the simulation, in order to get agreement of the waveform in the lower frequency range simulated by a theoretical Green's function and the empirical one, we found that it is essential to correct radiation pattern coefficient of empirical Green's function to match subfault-station configuration. We also confirmed that the impulse function whose amplitude is proportional to the maximum value of the slip velocity at each subfault works well, where the direction of the maximum slip velocity vector agrees with that of the slip vector. It means that the higher frequency strong motions are mainly controlled by the asperity areas. Aiming at inverting precise amplitude and position of the impulse function on each subfault, we solve the linearized equation in the complex frequency domain. >http://sms.dpri.kyoto-u.ac.jp/miyake/WelcomeE.html</a>