Tsunami amplitude decay on source aspect ratio investigation

Abstract:

One of the most significant uncertainties in tsunami wave height prediction comes from the difficulty of accurate estimation of source parameters. Tsunami source parameters are responsible for initial surface elevation and define wave characteristics in the near and far fields, in particular, they affect how far from the origin tsunami remains dangerous.

In the current research tsunami sensitivity to the source fault aspect ratio (AR) was investigated numerically by using the MOST model based on the splitting method. Two different idealized initial wave shapes were considered: the finite-crested waveform and the more realistic N-wave driven instantaneously by Okada finite fault model. Fault parameters and AR range were selected within realistic geophysical values.

Normalized maximum amplitude decays from idealized sources with different AR in a basin with constant depth were numerically computed and approximated with exponential function y = b + ae^-λx, y – the normalized wave amplitude, x – the distance from the source), which allowed for the quantification of a decay rate parameter, λ. The smaller AR the faster wave decays. The more elongated the source is, the stronger the directivity and the slower amplitude decay is.

The short-rupture and long-rupture tsunamis propagation in the Pacific Ocean was investigated. The initial total energy for both tsunami types was taken the same. It was shown that concentrated short-rupture tsunamis are likely to be more destructive in the near field than long-rupture events. At the same time the faster decay rate of short-rupture tsunamis degrade their larger, initial amplitude and will make it drop below that of long-rupture tsunamis.

Tsunami decay rates were correlated with historical events Tohoku (Japan, 2011) and Maule (Chile, 2010). Comparison of decay rates with measurements from DART buoys was presented. It was shown that the real amplitude decay along the streamlines with most energy concentration corresponds well with the decay from an idealized source of the same AR.

The current investigations is intending to bridge the gap between the idealized scenarios used in parametric studies and the real world, by identifying evidence of those effects in real events. The results can help to understand the specificities of real tsunamis behavior.