Tsunami Resonance in Palma Bay and Harbor, Majorca Island, as induced by the 2003 Western Mediterranean Earthquake

ABSTRACT: he tsunami induced by the May 21, 2003, Algerian Boumerde`s-Zemmouri earthquake (moment magnitude Mw=6.9) propagated across the western Mediterranean Basin, thereby causing material damages in some harbors and coastal areas. This was the case in the Balearic Islands and particularly the Palma harbor. Attempts to simulate the 2003 tsunami event found discrepancies between tsunami arrival times and wave amplitude when comparing tide gauge records with results from numerical models. To date, all published model results of the amplitude of the tsunami are underestimations, attributed to numerical limitations due to the lack of high-resolution bathymetry and poor definition of harbor geometry. Other views suggest the nappropriateness of the available seismic source parameterizations and the possible occurrence of submarine landsliding triggered by the earthquake that has not been included in the numerical simulations. In this article we present the results of a numerical study aimed at better understanding the response of the interacting Palma bay and harbor under the impact of the 2003 western Mediterranean tsunami. The transference of the tsunami energy from the generation area to the continental shelf, the bay, and the harbor has been studied and compared with the natural oscillation modes of the bay and harbor water bodies. Our work includes a sensitivity analysis of the source parameterization and the bathymetry grid size for the bay and harbor as a way to explain the discrepancies between simulations and observations. The Palma harbor tide gauge shows that energy from the tsunami concentrated mainly in periods that fitted to the natural modes of oscillations of the bay. Therefore, the significant wave amplification observed inside the harbor, mainly in its northern basin, was generated by a resonance effect induced by Palma bay. The improvement of the bathymetry grid resolution in the bay and harbor domains and the inclusion in the simulations of the exact harbor geometry and internal configuration result in a slight wave-high increment that is much below the wave height recorded in the tide gauge. Our results strongly point to a necessary revision of the tsunami seismic source parameters

Impact of a 1755-like tsunami in Huelva, Spain

Abstract. Coastal areas are highly exposed to natural hazards associated with the sea. In all cases where there is historical evidence for devastating tsunamis, as is the case of the southern coasts of the Iberian Peninsula, there is a need for quantitative hazard tsunami assessment to support spatial planning. Also, local authorities must be able to act towards the population protection in a preemptive way, to inform “what to do” and “where to go” and in an alarm, to make people aware of the incoming danger. With this in mind, we investigated the inundation extent, run-up and water depths, of a 1755-like event on the region of Huelva, located on the Spanish southwestern coast, one of the regions that was affected in the past by several high energy events, as proved by historical documents and sedimentological data. Modelling was made with a slightly modified version of the COMCOT (Cornell Multi-grid Coupled Tsunami Model) code. Sensitivity tests were performed for a single source in order to understand the relevance and influence of the source parameters in the inundation extent and the fundamental impact parameters. We show that a 1755-like event will have a dramatic impact in a large area close to Huelva inundating an area between 82 and 92 km2 and reaching maximum run-up around 5 m. In this sense our results show that small variations on the characteristics of the tsunami source are not too significant for the impact assessment. We show that the maximum flow depth and the maximum run-up increase with the average slip on the source, while the strike of the fault is not a critical factor as Huelva is significantly far away from the potential sources identified up to now. We also show that the maximum flow depth within the inundated area is very dependent on the tidal level, while maximum run-up is less affected, as a consequence of the complex morphology of the area

The BIG’95 Submarine Landslide–Generated Tsunami: A Numerical Simulation

Abstract: This article presents a reasonable present-day, sea-level highstand numerical simulation and scenario for a potential tsunami generated by a landslide with the characteristics of the BIG’95 debris flow, which occurred on the Ebro margin in the western Mediterranean Sea in prehistoric times (11,500 cal yr BP). The submarine landslide deposit covers an area of 2200 km2 of the slope and base of slope (200–1800-m water depth), involving a volume of 26 km3. A leapfrog finite difference model, COMCOT (Cornell multigrid coupled tsunami model), is used to simulate the propagation of the debris-flow-generated tsunami and its associated impact on the nearby Balearic Islands and Iberian coastlines. As a requisite of the model, reconstruction of the bathymetry before the landslide occurrence and seafloor variation during landsliding have been developed based on the conceptual and numerical model of Lastras et al. (2005). We have also taken into account all available multibeam bathymetry of the area and high-resolution seismic profiles of the debris flow deposit. The results of the numerical simulation are displayed using plots of snapshots at consecutive times, marigrams of synthetic stations, maximum amplitude plots, and spectral analyses. The obtained outputs show that the nearest shoreline, the Iberian coast, would not be the first one hit by the tsunami. The eastward, outgoing wave would arrive at Eivissa Island 18 min after the triggering of the slide and at Mallorca Island 9 min later, whereas the westward-spreading wave would hit the Iberian Peninsula 54 min after the slide was triggered. This noticeable delay in the arrival times at the peninsula is produced by the asymmetric bathymetry of the Catalano-Balearic Sea and the shoaling effect due to the presence of the wide Ebro continental shelf, which in addition significantly amplifies the tsunami wave (19 m). The wave amplitudes attain 8 m in Eivissa, and waves up to 3 m high would arrive to Palma Bay. Resonance effects produced in the narrow Santa Ponc¸a Bay in Mallorca Island could produce waves up to 9 m high. A similar event occurring today would have catastrophic consequences, especially in summer when human use of these tourist coasts increases significantly

The unperceived risk to Europe’s coasts: tsunamis and the vulnerability of Cadiz, Spain

Abstract. The development of appropriate risk and vulnerability reduction strategies to cope with tsunami risks is a major challenge for countries, regions, and cities exposed to potential tsunamis. European coastal cities such as Cadiz are exposed to tsunami risks. However, most official risk reduction strategies as well as the local population are not aware of the probability of such a phenomenon and the potential threat that tsunami waves could pose to their littoral. This paper outlines how tsunami risks, and particularly tsunami vulnerability, could be assessed and measured. To achieve this, a vulnerability assessment framework was applied focusing on the city of Cadiz as a case study in order to highlight the practical use and the challenges and gaps such an assessment has to deal with. The findings yield important information that could assist with the systematic improvement of societal response capacities of cities and their inhabitants to potential tsunami risks. Hazard and vulnerability maps were developed, and qualitative data was obtained through, for example, focused group discussions. These maps and surveys are essential for the development of a people-centred early warning and response system. Therefore, in this regard, the Tsunami Early Warning and Mitigation System in the North Eastern Atlantic, the Mediterranean, and connected seas promoted by the UNESCO-Intergovernmental Oceanographic Commission (IOC) should encompass these assessments to ensure that action is particularly intensified and fostered by those potentially exposed. That means that besides the necessary technical infrastructure for tsunami detection, additional response and adaptation measures need to be promoted – particularly those that reduce the vulnerability of people and regions exposed – in terms of national systems. In addition, it is important to develop emergency preparedness and awareness plans in order to create an integrated regional Tsunami EarlyWarning System (TEWS) by 2011. The findings of the paper are based on research conducted within the framework of the EC funded project TRANSFER: “Tsunami Risk ANd Strategies For the European Region”, a project that aims to improve the understanding of tsunami processes in the Euro- Mediterranean region, to develop methods and tools to assess vulnerability and risk, and to identify strategies for the reduction of tsunami risks