Development of multiscale slip inversion method and its application to the 2004 mid‐Niigata Prefecture earthquake

Abstract

We present a multiscale slip inversion method for earthquakes as a first attempt to analyze an initial rupture process on a small scale and a whole one on a large scale. A multiscale source model is constructed by renormalizing slip rate distributions on different scales. A multiscale observation equation includes a renormalized kernel matrix. Using the new method and employing theoretical and empirical Green's functions with two small events (Mw 2.3 and 3.3), we analyze the 2004 mid‐Niigata Prefecture earthquake (Mw 6.6). Following a preliminary deconvolution analysis suggesting a complexity of the initial phase, the multiscale analysis reveals details of the initial stage of the rupture process (the first 1 s) successfully. In the multiscale model, the estimated source process is consistent for all scales, while independent slip inversion analyses on three scales (monoscale analysis) result in inconsistent slip distributions with large errors. The maximum slip rate is about 1.0 m/s and the rupture velocity is 2.5–3.0 km/s in the initial rupture process. Four stages of rupture growth with different rupture directivities are found: the first 0.4 s with northeastward directivity; from 0.6 s to 1.0 s with southward directivity; until 2.0 s with northeastward directivity again; and after 2.0 s with southwestward directivity. These stages may represent cascading ruptures, evolving into a large earthquake. The image of the whole rupture process implies self‐similarity of the dynamic rupture process and is a breakthrough in the complete realization of earthquake source scaling.