Hydrographic and Oceanographic Department,
Japan Coast Guard
* Contributed on June 22, 2023

A. Surrounded on all sides by the wide expanses of ocean, Japan is a maritime nation that enjoys the benefits of the sea in the forms of maritime trade and fisheries. However, these waters are also plagued by various problems, including maritime accidents, marine crime such as smuggling and illegal migration, and international disputes over the sovereignty of territorial possessions and maritime resources. Since its establishment in May 1948, the Japan Coast Guard (JCG) has been engaged day and night in a variety of activities, including criminal investigations, maritime security operations, search and rescue work, marine environment preservation, disaster mitigation, oceanographic research, and maritime safety operations, and also working to strengthen collaboration and cooperation with other countries, all so that people of Japan can use and enjoy the various blessings of the ocean environment.

A. For our country, surrounded by the sea on all sides, it is crucial to secure maritime interests such as territorial sea and Exclusive Economic Zones (EEZ), and the development of marine information is essential as a basis for this. The JCG is intensively promoting bathymetric surveys of the seafloor, geotectonic surveys, and sediment sampling and analysis in the waters surrounding Japan using survey vessels with multibeam echo sounders and Autonomous Underwater Vehicles (AUV). In addition, we conduct surveys of the low-water line, which is the basis for the outer limits of the territorial sea and EEZ, using Autonomous Ocean Vehicle (AOV) and Airborne Lidar Hydrography mounted aircraft.

A. The Japanese government's continental shelf delimitation surveys conducted from 1983 to 2008 have provided Japanese and other global researchers with a detailed understanding of the geological and geophysical structure of the Philippine Sea Plate. One of the most remarkable undersea features is the Godzilla Megamullion Province, discovered in 2001 on the seafloor approximately 2,000 km south to Tokyo and 600 km southeast to Okinotorishima Island.
Megamullion, also called an oceanic core complex, is a dome-shaped undersea feature characterized by exposure of the lower crust and upper-most mantle materials on its surface, caused by the large-scale normal faults associated with seafloor spreading. The most distinctive feature of megamullions is that they have a ridged structure on their surface parallel to the direction of seafloor spreading. Mullion is an architectural term called "HouDate" in Japanese, which refers to a member that supports a window in the vertical direction.
The seafloor around the Godzilla Megamullion Province is spreading in a northeast-southwest direction, and shows linear structure composed of extrusion of basaltic magma, arranged orthogonal to the seafloor spreading direction. This linear structure of the extruded magma was formed by seafloor movement farther away from the seafloor spreading center. On the other hand, the Godzilla Megamullion Province is unique in that it is characterized by a ridged structure parallel to the direction of seafloor spreading because this dome-shaped undersea feature was formed by the fault movement associated with seafloor spreading and the exposure of lower crustal and upper-most mantle materials on its surface. Furthermore, at approximately 125 km by 55 km, it is about three times the size of Tokyo and about ten times larger than any other megamullion, making it the largest one confirmed on earth. The Godzilla Megamullion Province is the area with these distinctive undersea features.

A. The Godzilla Megamullion Province is a significant research target in marine science. It is considered extremely valuable for elucidating global environmental changes and the earth's internal structure, as the an international research group led by Japan have yielded significant scientific findings on the formation and structure of the Philippine Sea Plate through the research on it. Researchers studying this very large and highly distinctive undersea feature named it after "Godzilla", a giant monster thought to be on the seafloor and a major character in the world-famous movie “Godzilla" by Toho Co., Ltd. because of its enormous size. Later, the name came to be widely used in the scientific world, including in many international scientific papers.
In recent years, improvements of bathymetric survey technology have made it possible to find many undersea features. The Japan’s continental shelf surveys mentioned above had used the latest survey equipment, however, there is a concern that a situation may arise where different names call the same undersea feature or different undersea features are called by similar names, which may lead to confusion. To resolve the concerns about the names, the naming in accordance with the guidelines for undersea feature names (IHO-IOC Publication B-6 " Standardization of Undersea Feature Names") and promoting the name's usage is necessary.
Since 1966, the JCG has held the "Meeting on Undersea Feature Names", inviting experts from oceanographic research institutes and related academic societies, and has named discovered undersea features. Since 2001, it has been constructively renamed the "Japanese Committee on Undersea Feature Names (JCUFN)."The JCG has invited academic experts (experts in geography and ocean bottom geoscience) and related organizations (Fisheries Agency, National Institute of Advanced Industrial Science and Technology, and Japan Agency for Marine-Earth Science and Technology) to participate as committee members and discuss names of undersea features. Furthermore, to ensure that the undersea feature names approved by JCUFN are widely used internationally, the JCG has submitted a proposal to the "Sub-Committee on Undersea Feature Names (SCUFN)," which was established to standardize the names of undersea features internationally under the International Hydrographic Organization (IHO) and the Intergovernmental Oceanographic Commission of UNESCO (IOC).
The Godzilla Megamullion Province was also proposed to the SCUFN in 2021 after review and approval by the JCUFN, and its name was approved after review by SCUFN. In addition, the JCG regards the Godzilla Megamullion Province as the body of Godzilla and gives the names of Godzilla's body parts, such as head, arms, and tail, as well as other body parts to the distinctive undersea features on the Godzilla Megamullion, which the SCUFN approved as well. Upon registration, we received prior approval from Toho, the movie company, which enabled an undersea feature name bearing Godzilla on the seabed. Of all the Megamullions distributed worldwide, the Godzilla Megamullion Province is the first to have its name registered internationally.

A. The surface of undersea features in the Godzilla Megamullion Province exposes the material in the lower part of the earth's crust, such as gabbro, and the upper mantle material, such as peridotite. This scientific information of the materials provides essential insights into the structure of the Philippine Sea Plate and composition of its original magma, , making it an extremely crucial research target for our understanding of the evolution of the earth. We expect that such undersea features named after the world-famous movie character "Godzilla" will draw the public's attention to the Godzilla Megamullion which is a vast and unique undersea feature and a vital object to study for better understanding the earth we live on. In addition, we expect this name to spread domestically and internationally.

A. This research was triggered by the detailed topographic data around Minamitorishima Island revealed by the JCG's bathymetric surveys until 2008. The research was conducted with researchers of universities and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). In 2010, JAMSTEC's crewed research submersible "Shinkai 6500" was used to conduct a submersible survey of the northwestern slope of Minamitorishima Island at a depth of 3,400 meters and, for the first time, successfully sampled the basalt rocks from the volcanic body of the island.
Minamitorishima Island is an isolated island in the far-off sea with no land within a 1,000-kilometer perimeter. Only the top of the mountain with a relative elevation of 5000m broke above the sea's surface, and it is the only land. Minamitorishima Island, similar to many of the surrounding seamounts, was believed to have been formed by volcanic activities more than 100 million years ago (Cretaceous), however, we had not collected any basalt to prove it. The age of the basalt rocks collected in 2010 revealed that Minamitorishima Island reactivated as a volcano 40 million years ago. This achievement also provides a clue to solving the mystery of why Minamitorishima Island is the only island in this area.

A. The Hydrographic and Oceanographic Department, JCG, will continue to promote systematic and efficient oceanographic surveys with the latest oceanographic survey technologies, properly manage acquired oceanographic data, and enhance the provision of useful oceanographic information to society.