Strategy to design the sea-level monitoring networks for small tsunamigenic oceanic basins: the Western Mediterranean case

The 26 December 2004 Indian Ocean tsunami triggered a number of international and national initiatives aimed at establishing modern, reliable and robust tsunami warning systems. In addition to the seismic network for initial warning, the main component of the monitoring system is the sea level network. Networks of coastal tide gages and tsunameters are implemented to detect the tsunami after the occurrence of a large earthquake, to confirm or refute the tsunami occurrence. Large oceans tsunami monitoring currently in place in the Pacific and in implementation in the Indian Ocean will be able to detect tsunamis in 1 h. But due to the very short time of waves propagation, in general less than 1 h, a tsunami monitoring system in a smaller basin requires a denser network located close to the seismic zones. A methodology is proposed based on the modeling of tsunami travel time and waveform, and on the estimation of the delay of transmission to design the location and the spacing of the stations. In the case of Western Mediterranean, we demonstrate that a network of around 17 coastal tide gages and 13 tsunameters located at 50 km along the shore is required to detect and measure nearly all tsunamis generated on the Northern coasts of Africa

Evaluation of far-field tsunami hazard in French Polynesia based on historical data and numerical simulations

International audienceThe first tsunami hazard map of French Polynesia is presented here on the basis of historical data, and numerical simulations. French Polynesia, because of its central position in the most tsunami prone ocean, the Pacific, is strongly exposed to far-field tsunamis. As no previous study on the area had been conducted, we compiled catalogues of all the historical observations (14 events), and tide gauges records (69 marigrams). The catalogues emphasise the higher hazard associated to the Marquesas archipelago, but also the deficiency of robust data in most other parts of French Polynesia. The recourse to numerical simulations allowed us to complement the existing records, and to test tsunami scenarii over different bathymetry and topography configurations, representative of the diversity of islands in French Polynesia. The tsunami hazard map assigns a high exposure level to the Marquesas and the island of Rurutu. Other islands of the Austral, and the Gambier archipelago have a elevated level of exposure, as well as three islands of Society: Tahiti, Moorea, and Huahine. All other islands of French Polynesia are considered as moderately exposed

Polynesia) through numerical modeling of generic far-field events

Abstract. Earthquakes occurring at the Pacific Rim can trigger tsunamis that propagate across the ocean and can produce significant damages far away from the source. In French Polynesia, the Marquesas Islands are the most exposed to the far-field tsunami hazards, since they are not protected by any outer coral reef and since submarine slopes are less steep than in other islands. Between 1994 and 1996, four tsunamis have reached the bays of the archipelago, among them, the tsunami initiated by the Chilean Mw 8.1 earthquake, produced up to 3 m high waves in Tahauku Bay. Numerical modeling of these recent events has already allowed us to validate our method of resolution of hydrodynamics laws through a finite-difference scheme that simulates the propagation of the tsunamis across the ocean and computes the inundation heights (run-up) in remote bays

A catalog of tsunamis in New Caledonia from 28 March 1875 to 30 September 2009

Comptes Rendus Geoscience, vol. 342, pp. 437-444, 2010International audienceIn order to establish a tsunami alert system in New Caledonia in April 2008, the French Secretary of State for Overseas Affairs, with the aid of the UNESCO French Commission, mandated an investigation to build a more complete record of the most recent tsunamis. To complete this task, a call for witnesses was broadcast through various media and in public locations. These witnesses were then interviewed onsite about the phenomenon they had observed. Previous witness reports that had been obtained in the last few years were also used. For the most recent events, various archives were consulted. In total, 18 events were documented, of which 12 had not been previously mentioned in past work. These results confirm an exposure to a hazard of: (1) local origin (the southern part of the Vanuatu arc) with a very short post-seismic delay (< 30 min) before the arrival of wave trains; (2) regional origin (Solomon Islands arc, northern part of the Vanuatu arc) with a delay of several hours; and (3) an exposure to trans-oceanic tsunamis (Kamchatka 1952, South Chile 1960, Kuril Islands 2006, North Tonga 2009), unknown until today. These results highlight the necessity for New Caledonia to adopt an alert system, coupled with ocean tide gauges, that liaises with the main alert system for the Pacific (Pacific Tsunami Warning Center), and brings to light the importance of establishing a prevention campaign

The 2006 July 17 Java (Indonesia) tsunami from satellite imagery and numerical modelling: a single or complex source?

International audienc

The Tsunami Triggered by the 21 May 2003 Boumerdès-Zemmouri (Algeria) Earthquake: Field Investigations on the French Mediterranean Coast and Tsunami Modelling

International audienceA field survey was organized on the French Mediterranean coasts to investigate the effects of the tsunami induced by the 21 May 2003 Boumerdès-Zemmouri (Algeria) earthquake (Mw=6.9). The results show that eight harbours were affected by important sea level disturbances that caused material loss. Unfortunately, the low sampling rate of the French tide gage records (10 min) does not allow for a proper evaluation of the tsunami wave amplitudes since these amplitudes were probably underestimated in the harbours where these sensors are installed. The survey brings to light regional and local contrasts among the harbours' hydrological responses to the tsunami.To better understand these contrasts, a numerical simulation of the sea level elevations induced by the tsunami was conducted. The simulation showed a certain correlation between the field results and the wave amplification along the coast; however it underestimated the observed phenomena. Another simulation was then conducted using high resolution bathymetric grids (space step of 3 m) centred more specifically on 3 neighbouring harbours, however, again the simulation results did not match the amplitudes recorded through the observations. In order to better understand the wave amplification mechanisms inside each grid, a Gaussian signal was virtually broadcasted from the source to the harbours. Virtual sensors identified the periods which are stimulated – or not – by the arrival of the signal in each grid. Comparing these periods with those previously recorded emphasizes the proper period of each waterbody.This paper evaluates the limitations of such a study, focusing specifically on (1) the importance of having accurate and precise data about the source (the lack of information about the signal amplitude leads to an underestimation of the tsunami, thus reproducing only a fourth to a third of the observed phenomenon), (2) the need for networked tide gages with high resolution records and short sampling rates, and (3) the importance of conducting field studies immediately after a tsunami occurs

THE FRENCH TSUNAMI WARNING CENTER FOR THE MEDITERRANEAN AND NORTHEAST ATLANTIC: CENALT

CENALT (CENtre d’ALerte aux Tsunamis) is responsible for the French National Tsunami Warning Centre (NTWC). The CENALT is established in the framework of the Unesco/IOC/ICG/NEAMTWS. Its objective is to transmit a warning message in less than fifteen minutes for any events that could trigger a tsunami in the Western Mediterranean Sea and the North- Eastern Atlantic Ocean. The data collected from French installations and from institutions of European and North African countries is processed with software that permits early epicenter location of seismic events and measurements of expected tsunami impacts on the shore. On-duty analysts revise interactively all the generated information and use references of historical tsunami and earthquake databases - as well as computed tsunami scenarios – in order to disseminate the more comprehensive message possible