Effects of fault geometry uncertainty on tsunami inversion: a study based on the 2011 Tohoku earthquake

Abstract Finite-fault inversion of tsunami waveforms requires prior knowledge of fault geometry, which is often subject to considerable uncertainty. Quantifying effects of such uncertainty is essential for evaluating the reliability of inversion results derived from inadequately constrained fault models. Although previous studies have examined tsunami sensitivity to fault geometry, they typically relied on forward modeling and assumed planar faults. Using the 2011 Tohoku earthquake as a case study, this paper investigates the impact of fault geometry uncertainty on the inverted slip distribution and tsunami predictions. We consider 2D non-planar fault geometry, defined by depth as a function of distance from the trench. 9592 fault depth profiles are generated by densely sampling the region of historical seismicity, representing the maximum uncertainty in the absence of other geophysical constraints. For each sampled geometry, we invert near-field tsunami observations for slip distribution and evaluate the waveform fitting quality. The results show that these fault samples introduce moderate (20–30%) relative variability in slip amount across most of the rupture area, and the overall slip pattern remains robust. Owing to the trade-off between fault slip and geometry, variations in fault geometry only slightly affect the tsunami predictions at most stations. Nevertheless, inaccurate fault geometry leads to noticeable misfits at several coastal stations, which cannot be compensated for by adjusting fault slip. In addition, fault samples yielding better waveform fits are generally closer to the true geometry. These findings suggest that, for shallowly dipping plate interfaces, fault geometry uncertainty has a limited impact on tsunami inversion. High-quality near-field tsunami observations may further help reduce this uncertainty when only historical seismicity is available as a constraint. More studies are needed for tsunamigenic faults with significant along-strike geometric variations and in distinct tectonic settings. Graphical Abstract