Okada Forward Modeling of the 2011 Tohoku-Oki Earthquake: Segmented Residual Diagnostics and Tsunami-Source Consistency Testing

Published source models for the 2011 Tohoku-Oki earthquake differ in shallow slip, northern rupture contribution, and peak-slip location, producing different predictions of coseismic deformation and tsunami-relevant seafloor motion. This study does not construct a new full slip inversion. Instead, it develops a reproducible residual-diagnostic and consistency-testing workflow for selected USGS-type and Satake/Fujii-type models with Okada forward modeling. Modeled displacements are compared with a 20-station subset of published mainshock-isolated GEONET offsets derived from 30 s solutions, followed by segmented slip-scaling sensitivity analysis, uncertainty weighting and complete leave-one-out robustness tests, offshore GNSS-A validation, and controlled JAGURS waveform simulations for the final candidate sources. For the revised USGS-type analysis, the horizontal-magnitude and up-component RMSE values decrease from 0.792 and 0.355 m to 0.204 and 0.141 m, respectively. The selected North-Iwate multiplier reaches the non-negative lower bound and is therefore interpreted only as a diagnostic indication of source over-contribution. The revised Satake/Fujii grid substantially improves the horizontal fit while retaining low vertical error. Offshore GNSS-A validation changes the ranking: the Satake baseline best reproduces vertical offshore displacement, whereas the Satake constrained-best candidate gives the best horizontal-vector and three-dimensional fit. The JAGURS waveform comparison is treated as a common-setting consistency test, not as independent validation of the Satake model, because DART21418 and relevant GPS wave-gauge observations were among the data classes used in developing that source. Thus, onshore GEONET residual reduction is necessary but insufficient; tsunami-source selection requires offshore geodetic evaluation and carefully bounded waveform consistency testing.