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'Plafker rule of thumb' reloaded: experimental insights into the scaling and variability of local tsunamis triggered by great subduction megathrust earthquakes

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Rosenau,  Matthias
Deutsches GeoForschungsZentrum;

Nehrlich,  R.
External Organizations;

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Brune,  Sascha
2.5 Geodynamic Modelling, 2.0 Physics of the Earth, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum;

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Oncken,  Onno
Deutsches GeoForschungsZentrum;

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Zitation

Rosenau, M., Nehrlich, R., Brune, S., Oncken, O. (2009): 'Plafker rule of thumb' reloaded: experimental insights into the scaling and variability of local tsunamis triggered by great subduction megathrust earthquakes, (EOS, Transactions, American Geophysical Union, Fall Meeting Suppl. 90, 52), AGU 2009 Fall Meeting (San Francisco 2009).


https://gfzpublic.gfz-potsdam.de/pubman/item/item_240433
Zusammenfassung
Great subduction megathrust earthquakes pose a significant tsunami risk in coastal regions. In order to constrain natural tsunamigenic source heterogeneity and its effect on tsunami variability in different subduction settings (accretive, erosive), we here analyze a sequence of experimentally simulated great megathrust earthquakes. We use an elastoplastic wedge overlying a rate- and state-dependent frictional interface as an analog model of the subduction forearc overlying a seismogenic megathrust. Near-field (local) tsunami heights are derived by means of analytical versus numerical hydrodynamic calculations from the surface deformation of the analog model in comparison to predictions of an elastic dislocation model. The cumulative slip distribution over the simulated earthquake sequence resembles widely used skewed parameterizations supporting their use in worst case scenarios. Due to body forces and strain localization, tsunamis predicted by the analog model are enriched in kinetic energy compared to EDM predictions. The tsunami height-to-slip-ratio (“Plafker rule of thumb”) and its variance scale inversely to forearc slope according to a power law from ~ 1 at accretionary to ~ 1/4 in erosive settings (Cv ~ 0.6). Tsunami height scales exponentially with earthquake magnitude, has a power-law dependence on forearc slope and a variability characterized by Cv ~ 0.5 (see Figure). In terms of predicting tsunami scale and variability the analog model outperforms the elastic dislocation model which tends to overestimate local tsunami height and underscore its variability when tested against empirical data. Based on the experimentally derived earthquake-tsunami scaling law we infer the distribution of tsunami hazard on a global scale. Because of the exponential scaling of tsunamis with earthquake magnitude, its sensitivity to forearc slope and the possible non-linear kinetic enrichment of tsunamis triggered by giant earthquakes, disaster hotspots occur preferentially along accretionary margins. Three out of the top-five tsunami hotspots we identify had giant earthquakes in the last decades (Chile 1960, Alaska 1964, Sumatra-Andaman 2004) and one (Sumatra-Mentawai) started in 2005 releasing strain in a possibly moderate mode of sequential large earthquakes. This leaves Cascadia as the major active tsunami hotspot in the focus of tsunami hazard assessment. Visualization of preliminary versions of the experimentally-derived scaling laws for peak nearshore tsunami heigth (PNTH) as functions of forearc slope, peak earthquake slip (left panel) and moment magnitude (right panel). Note that wave breaking is not considered yet. This renders the extrem peaks > 20 m unrealistic.