Articles

Séminaire d’Emile Okal, Professeur Emeritus, Northwestern University, Evanston, IL, USA

Quinze ans de tsunamis depuis Sumatra 2004: Gagnons-nous en sagesse?

 

Les deux tsunamis géants de 2004 en Indonésie (230 000 morts) et 2011 au Japon (18 000 morts) ont éveillé, à l’échelle mondiale, l’attention de la Société au risque posé par ces catastrophes. Entre temps, et depuis 2004, pas moins d’une vingtaine de tsunamis ont causé de substantielles pertes humaines et matérielles.  Nous examinons ici les progrès réalisés au cours de ces quinze dernières années, tant sur le plan scientifique qu’en termes de mitigation et de gestion d’alerte en temps réel. Nous définissons un « index de sagesse », basé sur une estimation certes subjective de la performance des systèmes d’alerte et de la réponse des populations à risque, dont nous montrons qu’il continue malheureusement à osciller dans le temps sans tendance bien définie. Les principaux progrès réalisés se situent en champ lointain, où on ne recense que deux victimes après 2004. Le principal défi demeure celui des “tsunami earthquakes », séismes à rupture lente se produisant en violation des lois de similitude sismique, qui peuvent être mal ou non ressentis en champ proche malgré leur violent potentiel de tsunami (p. ex. Java, 2006; Mentawai, 2010).  On terminera par une évaluation critique de la cascade d’incompétences grossières qui a conduit au désastre de la centrale nucléaire de Fukushima en 2011.

 

 

(avec Émile Okal à droite)

 

 

Rivage de Niuatoputapu (Tonga) après le tsunami des Samoa du 29 Sep. 2009. La palmeraie (futs de 15 m de haut) a été complètement détruite. Noter à la base de l’ile de Tafahi, à l’arrière plan, le liseré clair indiquant la destruction de la végétation côtière, jusqu’à une hauteur de 22 m

 

26 septembre 2019

Séminaire de Jennifer Montaño

Blind Testing of Shoreline Evolution Models 

Séminaire de Nadine Tisnerat-Laborde (LSCE)

Séminaire de Eiichi Asakawa (J-MARES/JGI, Tokyo, Japan)

“Cross-ministerial Strategic Innovation Promotion Program (SIP)” was launched by the Council for Science, Technology and Innovation (CSTI) in 2014. It has addressed eleven issues selected considering the critical social needs. “Next-generation technology for ocean resources exploration (Zipangu in the Ocean) “is one of the SIP issues. In this project, we aim to establish the survey protocol for seafloor mineral resources and have been developing technologies for ocean resources exploration at efficient and low cost.

The primary target is seafloor massive sulphide (SMS). Many hydrothermal activities have been found in submarine volcanic areas distributed along the Izu-Bonin Arc and the Okinawa Trough in Japan. SMS ore deposits exist in deep water (>1500m) and the target depth is less than 50m below the sea bottom. Therefore the survey platforms that can reach close to the sea bottom, such as AUV and ROV, are important. For example, the technology of multiple AUVs operation using ASV enables us the efficient and minute bathymetry measurement that is the first step of SMS exploration in the deep-sea floor. Figure 1 shows the fleet of AUVs and the specifications developed by National Maritime Research Institute (NMRI) in SIP. Development of ROVs equipped with multipoint coring system and sonar imaging system enables efficient, low cost sampling.

In my presentation, I particularly focus on seismic surveys. High resolution seismic technologies are essential to investigate the concealed SMS ore deposit. Our patented technology, developed for high resolution seismic survey systems are shown in the Figure below. ACS is a survey system using a deep-towed streamer with surface and/or deep- towed seismic sources. We carry out ACS at the first stage of the exploration to extract possible hydrothermally active areas ranging from tens square kilometers. ZVCS consists of a deep-towed vertical receiver cable with surface and/or deep-towed seismic sources. There are two kinds of ZVCS, using deep-tow(ZVCS-DT) or ROV(ZVCS-R). Compared to ZVCS-DT, ZVCS-R enables high lateral density shooting and very flexible operation. As a second stage survey, ZVCS-DT and ZVCS-R are planned to narrow down the prospective areas which has been extracted by ACS. In 3DVCS, multiple vertical cables are moored on the seafloor. A circular shooting at the sea surface can clearly visualize detailed 3D sub-seafloor images. To obtain a precise depth image, a 3D pre-stack depth migration (3D-PSDM) is applied to the 3DVCS data. The velocity model for 3D-PSDM can be built by the velocity analysis of common scattering point gathers obtained by the equivalent offset migration and the result of refraction tomography.

 

21 juin 2019

Séminaire de Geoffroy Lamarche (NIWA, Wellington, New Zealand)

National Institute of Water and Atmospheric Research (NIWA), Private bag 14-901, Wellington, New Zealand. School of Environment, University of Auckland, Auckland, NZ Email: pacific@seabed2030.org

Recognizing the poor overall resolution of the world ocean’s bathymetry, GEBCO and the Nippon Foundation have joined forces to establish the Seabed 2030 Project, an international effort with the objective of facilitating the complete mapping of the world ocean by 2030. The aim of Seabed2030 is to empower the world to make policy decisions, use the ocean sustainably and undertake scientific research based on detailed bathymetric information of the Earth’s seabed. It supports the United Nations Sustainable Development Goal (SDG) 14 which is « to conserve and sustainably use the world’s oceans, seas and marine resources ».

The Seabed 2030 Project has established a strong governance and strategy to insure the success of its mandate. Four regional data assembly and coordination centers (RDACCs) are responsible for assembling databases; develop protocols for data collection and tools to assemble and attribute appropriate metadata. The Centres are responsible for identifying data gaps and opportunities for new data collection, including the facilitation of new mapping endeavours through coordination of ongoing activities. Regional Mapping Committees, groups of regional experts, were established to work with the Centre in identifying data sources, including those that are not currently in publicly available databases.

Seabed 2030 encourages and supports the development of new and innovative technologies that can increase the efficiency of seafloor mapping, including crowd sourcing, and thus make the ambitious goals of Seabed 2030 more likely to be achieved.

The GEBCO 2019 release presents a huge improvement on the 2014 version, but many bathymetric datasets remain unidentified and one major task is to complete a gap analysis which will require to include information on all academic, government, navy and industry data acquired, regardless of whether these data are readily accessible.

7 juin 2019