Probabilistic earthquake- and tsunami-source studies

This work develops probabilistic non-linear inversion methods to study the spatio-temporal evolution of earthquake rupture and tsunami sources including rigorous uncertainty estimationfor rupture parameters. Spatio-temporal source models are typically inferred by finite faultinversion techniques which estimate the slip (magnitude and direction) on a fault as a function of time. Such models can lead to answering questions about the physics of fault rupture: Where and why does rupture nucleate, propagate, and terminate, what is the nature of asperities, how does fault slip relate to tsunami generation? However, the fundamental problem of estimating slip on an unknown fault surface from incomplete and noisy data is highly non-linear, non-unique, and challenging to address without making substantial assumptions/parametrization choices about many of the unknowns. Such choices can significantly change parameter and uncertainty estimates. Historically, this has lead to inconclusive and sometimes contradictory results in the literature. To date, uncertainties of rupture parameters are poorly understood, and the effect of choices such as fault discretisation on uncertainties has not been studied. We show that model choice is closely linked to uncertainty estimation and can have profound effects on results. The inversion developed here is based on a trans-dimensional fault model, does not require regularisation, and provides rigorous uncertainty estimation that accounts for model-selection ambiguity associated with the fault discretisation (for details see Dettmer et al. 2014 GJI, in press). Results are presented for W-phase waveforms of two tsunamigenic megathrust earthquakes (Maule and Tohoku). In both cases, the probabilistic approach allows new insights into the spatio-temporal rupture.A similar method can be applied to directly infer the initial sea state due to tsunamigenicmegathrust earthquakes (the tsunami source). Tsunami waveform data are considered from ocean-bottom pressure and GPS wave gauges. Data predictions are based on Green-function libraries computed with ocean-basin scale tsunami models for elementary waves of Gaussian shape and grid spacing which is below the resolution of the data. The inversionis applied to tsunami waveforms from the great Tohoku (Japan) earthquake. The resultsshow a strongly elongated tsunami source along the Japan trench, as obtained in previous studies. However, we find that the tsunami data is fit with a source that is generally simpler than obtained in other studies, with a maximum amplitude less than 5 m.