The geometry, size, and slip distribution of earthquake faults control the generation of strong ground motions around earthquake source regions. Fast determination, which is inevitably made by automated methods, of such source characteristics is important for estimating characteristics of strong ground motions. This study suggests that determination of the source parameters can be automated using the waveform data from modern strong-motion instruments. The method consists of three separate linear inversions, which provide data on moment tensor solutions, fault planes and their length, and distributions of moment release on the finite faults, respectively. Tests for five inland earthquakes (M>6) beneath Japan show that the fault planes and distributions of moment release, which were obtained in the present method without human inspections, are in good agreement with the aftershock distributions and the results from other studies. The successful application of this method is produced by the broadband responses of modern strong-motion instruments which are deployed at a regular spacing. The method works best for shallow strike-slip earthquakes. Success with shallow dip-slip earthquakes is supported by a numerical test and the result for a reverse-fault earthquake. The second inversion, which models waveforms using aligned point sources to determine fault planes and their length, can contribute to speeding up the analyses of source processes. The present results suggest that using realtime data from strong-motion instruments will enable fast determination of source processes, and consequently lead to the rapid estimation of ground motions.