Mansour Ioualalen’s research while affiliated with Institute of Research for Development and other places

Three-dimensional short-crested water waves are known to host harmonic resonances (HRs). Their existence depends on their sporadicity versus their persistency. Previous studies, using a unique yet hybrid solution, suggested that HRs exhibit sporadic instability, with the domain of instability exhibiting a bubble-like structure which experiences a loss of stability followed by a re-stabilization. Through the calculation of their complete multiple solution structures and normal forms, we discuss the particular harmonic resonance (2,6). The (2,6) resonance was chosen, not only because it is of lower order, and thus more likely to be significant, but also because it is representative of a fully developed three-dimensional water wave field. Its appearance, growth rate and persistency are discussed. On our converged solutions, we show that, at an incidence angle for which HR (2,6) occurs, the associated superharmonic instability is no longer sporadic. It was also found that the multiple solution operates a subcritical pitchfork bifurcation, so regardless of the value of the control parameter, the wave steepness, a stable branch of the solution always exists. As a result, the analysis reveals two competing processes that either provoke and enhance HRs, or inhibit their appearance and development.

The tsunami hazard in Martinique due to local, regional and distant seismicity is investigated. Former deterministic scenarios are processed. They were proposed elsewhere based on historical records and the analysis of the morpho-tectonics of the area, i.e., two local subduction scenarios of magnitudes Mw 7.5 and Mw 8.0 representative of the local 1839 earthquake, the regional Mw 7.2–7.5, 1867 Virgin Island event, along with the distant Mw ~8.5, 1755 Lisbon earthquake. The tsunami runup and currents are simulated and analyzed. Then, a recent tsunami intensity index is applied and discussed. The modeling of the regional 1867 scenario shows negligible impact on the coast of Martinique which could explain the lack of direct observations in Martinique. Despite a difference of one unit magnitude between the local Mw 7.5 and the distant Mw ~8.5 earthquakes, it is found that their tsunami signature for Martinique is nearly equivalent. The local Mw 8.0 event thus controls the tsunami hazard in the island. In the case of this tsunami, whose source is at the east, the western side of the island is not impacted because of wave refraction. Eastward, the coral reef barrier also inhibits tsunami impacts in some sites. Interestingly, it is shown that most of the potentially inundated areas are mangroves that are already subject to daily tidal flooding and seasonal cyclonic activity. They represent an efficient protection against tsunamis. Four occupied locations are however exposed to severe inundation (horizontal penetration of hundreds of meters): Pointe Marin, Le Vauclin, Le Robert and, for the largest, Baie de la Trinité. In terms of current hazard, we have applied two simple procedures to identify sheltered areas for watercrafts: an integrated one, that is easier to use but less accurate, and a direct one, derived from the mapping, which is more complex to implement by end-users but of better precision and efficiency. A large tsunami intensity is obtained at the 4 sites exposed to inundation, possibly yielding severe damages on land and at sea, and which would be extreme at Baie de la Trinité. Even a distant tsunami could impact that site although to a lower extent compared to a strong local earthquake.

In this article, we estimate the tsunami hazard in Martinique due to tsunamis generated by earthquakes associated with the Lesser Antilles subduction zone. Using a deterministic approach based on reliable earthquake scenarios, we use high-resolution bathymetric and topographic data to model tsunami propagation and inundation with Cornell Multi-grid Coupled Tsunami model. An extreme earthquake subduction scenario of magnitude Mw 8.0 is tested, and a further realistic scenario of lower magnitude Mw 7.5, thus of different tsunami frequency content, is also processed to test the possible appearance of bay resonances. We find that the western coast of the island is relatively sheltered, because it represents a shadow area to diffraction, in particular, for the major city of Fort de France. Because of its very gentle slope, the eastern coast is prone to numerous floodings with meter scale wave amplitudes; most of the inundated zones consist of mangroves and geological depressions t are naturally regularly flooded by tides or storm surges. Hence such areas are often not exploited, the mangroves being let in their natural state, enhancing the protection of the surrounding communities. However, some strategic inhabited areas are subject to severe inundation. Finally, comparing our results with studies of the 1755 Lisbon transoceanic tsunami reveals a tsunami hazard close to our local Mw 7.5 scenario. It suggests the possibility to generalize our local tsunami hazard assessment in Martinique to other tsunami contexts and enlarge its validity. This issue is crucial for minimizing the efforts and increasing the efficiency of tsunami preparedness.

As with earthquakes, river floods, water waves, and wind intensities, a tsunami intensity has to be synthetic and comprehensive to be efficient. Tsunami impact is complex because the effects can be felt on the beach, on inundated areas and also at berths and anchors. Within the same local area, a tsunami may severely impact the population on the coast, while its effects may be negligible on marine bodies (boats). Most existing tsunami intensity scales are based either on water elevation or on induced currents. However, it is commonly admitted that both variables should be considered simultaneously. Several existing intensity scales were integrated and were made consistent with each other. An original intensity scale is then derived based on analysis of the interdependency between the maxima of tsunami amplitude and induced current: The dimension of the couple composed by two variables is analyzed, in particular through the derivation of a linear relationship using the long wave theory and the use of a fully nonlinear numerical experiment. Our intensity scale is particularly well adapted to numerical studies, for which the two variables are naturally derived within an entire computational grid. Once the tsunami intensity scale was set up, it was briefly applied to a particular case study: the impact of the Sumatra tsunami, dated December 26, 2004, on the coast of Sri Lanka. Indeed, the tsunami scales proposed herein represent an initial framework of study and can be further improved through new or revisited tsunami observations.

The French–Italian Riviera faces several geophysical hazards, including recurrent earthquakes and underwater landslides that can be tsunamigenic. The stakes are high since this is a densely populated and touristic area. Several studies have already been carried out, in particular to map tsunami hazard resulting from the near-field seismicity of the North Ligurian Faults System, which is located a short distance off the coast. In our most recent study, runup maps were developed together with local analyses of tsunami-induced current fields. However, no conclusions were drawn, based on these results, as of the associated tsunami risk along the coast. Here, to this effect, we apply a recently proposed tsunami intensity scale to the simulation results obtained in our previous work (maximum values of tsunami depths and currents). This intensity scale (7 levels) is mapped over the entire coastal area, and its site-specific values are discussed. The scale allows quantifying the potential damage inland and at sea, based on a standard coastal vulnerability that has been assessed through different records. It thus represents a useful tool to help improving our preparedness to tsunami hazard.

The French Riviera is a densely populated and touristic coast. It is also one of the most seismically active areas of the Western Mediterranean. This is evidenced by the Mw 6.7–6.9, 1887 earthquake and tsunami, that was triggered nearshore, rupturing the easternmost 40 km of the 80-km-long Ligurian fault system, which runs parallel to and offshore of the Riviera. Here, coastal hazard from co-seismic tsunamis is assessed along the French and part of the Italian Riviera by simulating three Ligurian earthquake scenarios: (1) the 1887 event offshore Genoa, Italy; (2) a similar event transposed to the westernmost 40-km segment of the fault, offshore Nice, France; and (3) the rupture of the entire 80-km fault, which constitutes an extreme case scenario for the region. Simulations of tsunami propagation and coastal impact are performed by one-way coupling with the Boussinesq model FUNWAVE-TVD, in a series of nested grids, using new high-resolution bathymetric and topographic data. Results obtained in 10-m coastal grids provide the highest resolution predictions to date for this section of the French Riviera of co-seismic tsunami coastal hazard, in terms of inundation, runup, and current velocity. In general, the most impacted areas are bays (near Cap d’Antibes and Cap Ferrat), due to wave buildup and shoaling within semi-enclosed shallow areas, enhanced by possible resonances. In contrast to earlier work, which was based on coarser resolution grids, the area of Nice harbor is found to be rather well sheltered. It should be noted that uniform fault slip was used in the ruptures and runup estimates could locally be enhanced in case of more complex ruptures, such as segmented and heterogeneous ruptures.

The devastating April 16, 2016, Pedernales, Ecuador, Mw 7.8 earthquake was among a sequence of ruptures that occurred along the seismically segmented North Andean subduction zone. It caused 700 fatalities, andmore than 7,000 people were injured. The magnitude and location of it were similar to those of theMay 14, 1942, earthquake, relaxing some of the strain accumulation that had built up over more than 74 years. A weak tsunami was detected in four nearby tide gauges. Analysis of the records provides useful information on the earthquake and tsunami. An estimate of 6.5 cm of coseismic subsidence was reported at the Bahía de Caráquez andManta locations, while no movement was detected at Esmeraldas and La Libertad. The analysis also supports the existence of two major seismic asperities and a weaker one farther south. A tsunami resonance occurred at the Bay of Salinas, the location of La Libertad, even though the bay is very wide. It was fortunate that it was inconsequential, this time, because the incoming wave was substantially dissipated.

The devastating April 16, 2016, Pedernales, Ecuador, Mw 7.8 earthquake was among a sequence of ruptures that occurred along the seismically segmented North Andean subduction zone. It caused 700 fatalities, and more than 7,000 people were injured. The magnitude and location of it were similar to those of the May 14, 1942, earthquake, relaxing some of the strain accumulation that had built up over more than 74 years. A weak tsunami was detected in four nearby tide gauges. Analysis of the records provides useful information on the earthquake and tsunami. An estimate of 6.5 cm of coseismic subsidence was reported at the Bahía de Caráquez and Manta locations, while no movement was detected at Esmeraldas and La Libertad. The analysis also supports the existence of two major seismic asperities and a weaker one farther south. A tsunami resonance occurred at the Bay of Salinas, the location of La Libertad, even though the bay is very wide. It was fortunate that it was inconsequential, this time, because the incoming wave was substantially dissipated.