Fast evaluation of tsunami scenarios: uncertainty assessment for a Mediterranean Sea database

Abstract

Abstract. We present a database of pre-calculated tsunami waveforms for the entire Mediterranean Sea, obtained by numerical propagation of uniformly spaced Gaussian-shaped elementary sources for the sea level elevation. Based on any initial sea surface displacement, the database allows the fast calculation of full waveforms at the 50 m isobath offshore of coastal sites of interest by linear superposition. A computationally inexpensive procedure is set to estimate the coefficients for the linear superposition based on the potential energy of the initial elevation field. The elementary sources size and spacing is fine enough to satisfactorily reproduce the effects of M > = 6.0 earthquakes. Tsunami propagation is modelled by using the Tsunami-HySEA code, a GPU finite volume solver for the non-linear shallow water equations. Like other existing methods based on the initial sea level elevation, the database is independent on the faulting geometry and mechanism, which makes it applicable in any tectonic environment. We model a large set of synthetic tsunami test scenarios, selected to explore the uncertainty introduced when approximating tsunami waveforms and their maxima by fast and simplified linear combination. This is the first time to our knowledge that the uncertainty associated to such a procedure is systematically analysed and that relatively small earthquakes are considered, which may be relevant in the near-field of the source in a complex tectonic setting. We find that non-linearity of tsunami evolution affects the reconstruction of the waveforms and of their maxima by introducing an almost unbiased (centred at zero) error distribution of relatively modest extent. The uncertainty introduced by our approximation can be in principle propagated to forecast results. The resulting product then is suitable for different applications such as probabilistic tsunami hazard analysis, tsunami source inversions and tsunami warning systems.

From regional to local SPTHA: efficient computation of probabilistic tsunami inundation maps addressing near-field sources

“…Indeed, heterogeneous earthquake slip is known to strongly influence the tsunami run-up (Geist, 2002;Løvholt et al, 2012b;Geist and Oglesby, 2014;Davies et al, 2015;Murphy et al, 2016) and not only in the near-field of the source (Li et al, 2016). Among the first attempts to quantify tsunami hazard uncertainty related to heterogeneous earthquake slip, Mueller et al (2014) and Griffin et al (2017) should be mentioned. Recently, Goda and De Risi (2018) proposed a multi-hazard approach including stochastic slip distributions and cascading earthquake-tsunami risk evaluation; however, they considered a limited number of tsunami scenarios without fully characterizing the epistemic uncertainties associated with the key model components.…”

Answer to Reviewers and Revised Manuscript

“…In this study, we assume that the coseismic displacement can be described by the half space elastic approach following Okada (1985). We use nine independent parameters: fault width W , fault length L, fault azimuth (strike) ϕ, dip angle δ, average slip along the fault u, slip angle (rake) λ, and depth below seafloor h to describe each seismic event of the synthetic catalogue.…”

“…In situations where the effects of smaller tsunami events are also important (e.g. interaction with harbours or other coastal structures), probabilistic hazard assessments can give a better insight into expected amplitudes and recurrence times (Geist and Parsons, 2006;Power et al, 2007;González et al, 2009;Sørensen et al, 2012;Omira et al, 2015).…”

“…We consider the simplest case of a rectangular fault with constant slip because of the location of the Gloria Fault in relation to the points of interest (POIs), and we use scaling laws to infer the fault parameters. These parameters are then used to compute the seafloor deformation using the half space elastic approach (Okada, 1985). Assuming the incompressibility of the seawater, we can directly transfer the sea bottom deformation to the water surface and use it as the initial condition for the tsunami propagation and use standard shallow water numerical modelling to provide an accurate assessment of the potential impacts close to the coasts.…”

Synthetic tsunami waveform catalogs with kinematic constraints

“…The computation of tsunami waveforms along the coast is a time-consuming process that requires substantial computational resources. To overcome this difficulty, we use the empirical Green's functions to compute the tsunami waveforms (see Molinari et al, 2016 and references therein) with rectangular prisms as described in Miranda et al (2014). The use of a representative earthquake catalogue together with the fast computation of tsunami waveforms constitutes an efficient method to evaluate tsunami impact at discrete locations along the coast.…”